flux_refineck/

checker.rs

use std::{collections::hash_map::Entry, iter, vec};

use flux_common::{
    bug, dbg, dbg::SpanTrace, index::IndexVec, iter::IterExt, span_bug, tracked_span_bug,
    tracked_span_dbg_assert_eq,
};
use flux_config::{self as config, InferOpts};
use flux_infer::{
    infer::{
        ConstrReason, GlobalEnvExt as _, InferCtxt, InferCtxtRoot, InferResult, SubtypeReason,
    },
    projections::NormalizeExt as _,
    refine_tree::{Marker, RefineCtxtTrace},
};
use flux_middle::{
    PanicReason, PanicSpec,
    global_env::GlobalEnv,
    pretty::PrettyCx,
    queries::{QueryResult, try_query},
    query_bug,
    rty::{
        self, AdtDef, BaseTy, Binder, Bool, Clause, Constant, CoroutineObligPredicate, EarlyBinder,
        Expr, FnOutput, FnSig, FnTraitPredicate, GenericArg, GenericArgsExt as _, Int, IntTy,
        Mutability, Path, PolyFnSig, PtrKind, RefineArgs, RefineArgsExt,
        Region::ReErased,
        Ty, TyKind, Uint, UintTy, VariantIdx,
        fold::{TypeFoldable, TypeFolder, TypeSuperFoldable},
        refining::{Refine, Refiner},
    },
};
use flux_rustc_bridge::{
    self, ToRustc,
    mir::{
        self, AggregateKind, AssertKind, BasicBlock, Body, BodyRoot, BorrowKind, CastKind,
        ConstOperand, Location, NonDivergingIntrinsic, Operand, Place, Rvalue, START_BLOCK,
        Statement, StatementKind, Terminator, TerminatorKind, UnOp,
    },
    ty::{self, GenericArgsExt as _},
};
use itertools::{Itertools, izip};
use rustc_data_structures::{
    graph::dominators::Dominators,
    unord::{UnordMap, UnordSet},
};
use rustc_hash::FxHashMap;
use rustc_hir::{
    LangItem,
    def_id::{DefId, LocalDefId},
};
use rustc_index::{IndexSlice, bit_set::DenseBitSet};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_middle::{
    mir::{Promoted, SwitchTargets},
    ty::{TyCtxt, TypeSuperVisitable as _, TypeVisitable as _, TypingMode},
};
use rustc_span::{
    Span, Symbol,
    sym::{self},
};

use self::errors::{CheckerError, ResultExt};
use crate::{
    ghost_statements::{CheckerId, GhostStatement, GhostStatements, Point},
    primops,
    queue::WorkQueue,
    rty::Char,
    type_env::{BasicBlockEnv, BasicBlockEnvShape, PtrToRefBound, TypeEnv, TypeEnvTrace},
};

type Result<T = ()> = std::result::Result<T, CheckerError>;

pub(crate) struct Checker<'ck, 'genv, 'tcx, M> {
    genv: GlobalEnv<'genv, 'tcx>,
    /// [`CheckerId`] of the function-like item being checked.
    checker_id: CheckerId,
    inherited: Inherited<'ck, M>,
    body: &'ck Body<'tcx>,
    /// The type used for the `resume` argument if we are checking a generator.
    resume_ty: Option<Ty>,
    fn_sig: FnSig,
    /// A marker to the node in the refinement tree at the end of the basic block after applying
    /// the effects of the terminator.
    markers: IndexVec<BasicBlock, Option<Marker>>,
    visited: DenseBitSet<BasicBlock>,
    queue: WorkQueue<'ck>,
    default_refiner: Refiner<'genv, 'tcx>,
    /// The templates for the promoted bodies of the current function
    promoted: &'ck IndexSlice<Promoted, Ty>,
}

/// Fields shared by the top-level function and its nested closure/generators
struct Inherited<'ck, M> {
    /// [`Expr`]s used to instantiate the early bound refinement parameters of the top-level function
    /// signature
    ghost_stmts: &'ck UnordMap<CheckerId, GhostStatements>,
    mode: &'ck mut M,

    /// This map has the "templates" generated for the closures constructed (in [`Checker::check_rvalue_closure`]).
    /// The [`PolyFnSig`] can have free variables (inside the scope of kvars), so we need to be
    /// careful and only use it in the correct scope.
    closures: &'ck mut UnordMap<DefId, PolyFnSig>,
}

#[derive(Debug)]
struct ResolvedCall {
    output: Ty,
    /// The refine arguments given to the call
    _early_args: Vec<Expr>,
    /// The refine arguments given to the call
    _late_args: Vec<Expr>,
}

impl<'ck, M: Mode> Inherited<'ck, M> {
    fn new(
        mode: &'ck mut M,
        ghost_stmts: &'ck UnordMap<CheckerId, GhostStatements>,
        closures: &'ck mut UnordMap<DefId, PolyFnSig>,
    ) -> Self {
        Self { ghost_stmts, mode, closures }
    }

    fn reborrow(&mut self) -> Inherited<'_, M> {
        Inherited { ghost_stmts: self.ghost_stmts, mode: self.mode, closures: self.closures }
    }
}

pub(crate) trait Mode: Sized {
    #[expect(dead_code)]
    const NAME: &str;

    fn enter_basic_block<'ck, 'genv, 'tcx>(
        ck: &mut Checker<'ck, 'genv, 'tcx, Self>,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        bb: BasicBlock,
    ) -> TypeEnv<'ck>;

    fn check_goto_join_point<'genv, 'tcx>(
        ck: &mut Checker<'_, 'genv, 'tcx, Self>,
        infcx: InferCtxt<'_, 'genv, 'tcx>,
        env: TypeEnv,
        terminator_span: Span,
        target: BasicBlock,
    ) -> Result<bool>;

    fn clear(ck: &mut Checker<Self>, bb: BasicBlock);
}

pub(crate) struct ShapeMode {
    bb_envs: FxHashMap<CheckerId, FxHashMap<BasicBlock, BasicBlockEnvShape>>,
}

pub(crate) struct RefineMode {
    bb_envs: FxHashMap<CheckerId, FxHashMap<BasicBlock, BasicBlockEnv>>,
}

/// The result of running the shape phase.
pub(crate) struct ShapeResult(FxHashMap<CheckerId, FxHashMap<BasicBlock, BasicBlockEnvShape>>);

/// A `Guard` describes extra "control" information that holds at the start of a successor basic block
#[derive(Debug)]
enum Guard {
    /// No extra information holds, e.g., for a plain goto.
    None,
    /// A predicate that can be assumed, e.g., in the branches of an if-then-else.
    Pred(Expr),
    /// The corresponding place was found to be of a particular variant.
    Match(Place, VariantIdx),
}

impl<'genv, 'tcx> Checker<'_, 'genv, 'tcx, ShapeMode> {
    pub(crate) fn run_in_shape_mode<'ck>(
        genv: GlobalEnv<'genv, 'tcx>,
        local_id: LocalDefId,
        ghost_stmts: &'ck UnordMap<CheckerId, GhostStatements>,
        closures: &'ck mut UnordMap<DefId, PolyFnSig>,
        opts: InferOpts,
        poly_sig: &PolyFnSig,
    ) -> Result<ShapeResult> {
        let def_id = local_id.to_def_id();
        dbg::shape_mode_span!(genv.tcx(), local_id).in_scope(|| {
            let span = genv.tcx().def_span(local_id);
            let mut mode = ShapeMode { bb_envs: FxHashMap::default() };

            let body = genv.mir(local_id).with_span(span)?;

            // In shape mode we don't care about kvars
            let mut root_ctxt = try_query(|| {
                genv.infcx_root(&body.infcx, opts)
                    .with_dummy_kvars()
                    .identity_for_item(def_id)?
                    .build()
            })
            .with_span(span)?;

            let inherited = Inherited::new(&mut mode, ghost_stmts, closures);

            let infcx = root_ctxt.infcx(def_id, &body.infcx);
            Checker::run(infcx, local_id, inherited, poly_sig.clone())?;

            Ok(ShapeResult(mode.bb_envs))
        })
    }
}

impl<'genv, 'tcx> Checker<'_, 'genv, 'tcx, RefineMode> {
    pub(crate) fn run_in_refine_mode<'ck>(
        genv: GlobalEnv<'genv, 'tcx>,
        local_id: LocalDefId,
        ghost_stmts: &'ck UnordMap<CheckerId, GhostStatements>,
        closures: &'ck mut UnordMap<DefId, PolyFnSig>,
        bb_env_shapes: ShapeResult,
        opts: InferOpts,
        poly_sig: &PolyFnSig,
    ) -> Result<InferCtxtRoot<'genv, 'tcx>> {
        let def_id = local_id.to_def_id();
        let span = genv.tcx().def_span(def_id);

        let body = genv.mir(local_id).with_span(span)?;
        let mut root_ctxt = try_query(|| {
            genv.infcx_root(&body.infcx, opts)
                .identity_for_item(def_id)?
                .build()
        })
        .with_span(span)?;
        let bb_envs = bb_env_shapes.into_bb_envs(&mut root_ctxt, &body.body);

        dbg::refine_mode_span!(genv.tcx(), def_id, bb_envs).in_scope(|| {
            // Check the body of the function def_id against its signature
            let mut mode = RefineMode { bb_envs };
            let inherited = Inherited::new(&mut mode, ghost_stmts, closures);
            let infcx = root_ctxt.infcx(def_id, &body.infcx);
            Checker::run(infcx, local_id, inherited, poly_sig.clone())?;

            Ok(root_ctxt)
        })
    }
}

/// `SubFn` lets us reuse _most_ of the same code for `check_fn_subtyping` for both the case where
/// we have an early-bound function signature (e.g., for a trait method???) and versions without,
/// e.g. a plain closure against its FnTraitPredicate obligation.
#[derive(Debug)]
pub enum SubFn {
    Poly(DefId, EarlyBinder<rty::PolyFnSig>, rty::GenericArgs),
    Mono(rty::PolyFnSig),
}

impl SubFn {
    pub fn as_ref(&self) -> &rty::PolyFnSig {
        match self {
            SubFn::Poly(_, sig, _) => sig.skip_binder_ref(),
            SubFn::Mono(sig) => sig,
        }
    }
}

/// The function `check_fn_subtyping` does a function subtyping check between
/// the sub-type (T_f) corresponding to the type of `def_id` @ `args` and the
/// super-type (T_g) corresponding to the `oblig_sig`. This subtyping is handled
/// as akin to the code
///
///   T_f := (S1,...,Sn) -> S
///   T_g := (T1,...,Tn) -> T
///   T_f <: T_g
///
///  fn g(x1:T1,...,xn:Tn) -> T {
///      f(x1,...,xn)
///  }
fn check_fn_subtyping(
    infcx: &mut InferCtxt,
    sub_sig: SubFn,
    super_sig: &rty::PolyFnSig,
    span: Span,
) -> InferResult {
    let mut infcx = infcx.branch();
    let mut infcx = infcx.at(span);
    let tcx = infcx.genv.tcx();

    let super_sig = super_sig
        .replace_bound_vars(
            |_| rty::ReErased,
            |sort, _, kind| Expr::fvar(infcx.define_bound_reft_var(sort, kind)),
        )
        .deeply_normalize(&mut infcx)?;

    // 1. Unpack `T_g` input types
    let actuals = super_sig
        .inputs()
        .iter()
        .map(|ty| infcx.unpack(ty))
        .collect_vec();

    let mut env = TypeEnv::empty();
    let actuals = unfold_local_ptrs(&mut infcx, &mut env, sub_sig.as_ref(), &actuals)?;
    let actuals = infer_under_mut_ref_hack(&mut infcx, &actuals[..], sub_sig.as_ref());

    let output = infcx.ensure_resolved_evars(|infcx| {
        // 2. Fresh names for `T_f` refine-params / Instantiate fn_def_sig and normalize it
        // in subtyping_mono, skip next two steps...
        let sub_sig = match sub_sig {
            SubFn::Poly(def_id, early_sig, sub_args) => {
                let refine_args = infcx.instantiate_refine_args(def_id, &sub_args)?;
                early_sig.instantiate(tcx, &sub_args, &refine_args)
            }
            SubFn::Mono(sig) => sig,
        };
        // ... jump right here.
        let sub_sig = sub_sig
            .replace_bound_vars(
                |_| rty::ReErased,
                |sort, mode, _| infcx.fresh_infer_var(sort, mode),
            )
            .deeply_normalize(infcx)?;

        // 3. INPUT subtyping (g-input <: f-input)
        for requires in super_sig.requires() {
            infcx.assume_pred(requires);
        }
        infcx.check_pred(
            Expr::implies(super_sig.no_panic(), sub_sig.no_panic()),
            ConstrReason::Subtype(SubtypeReason::Input),
        );
        for (actual, formal) in iter::zip(actuals, sub_sig.inputs()) {
            let reason = ConstrReason::Subtype(SubtypeReason::Input);
            infcx.subtyping_with_env(&mut env, &actual, formal, reason)?;
        }
        // we check the requires AFTER the actual-formal subtyping as the above may unfold stuff in
        // the actuals
        for requires in sub_sig.requires() {
            let reason = ConstrReason::Subtype(SubtypeReason::Requires);
            infcx.check_pred(requires, reason);
        }

        Ok(sub_sig.output())
    })?;

    let output = infcx
        .fully_resolve_evars(&output)
        .replace_bound_refts_with(|sort, _, kind| {
            Expr::fvar(infcx.define_bound_reft_var(sort, kind))
        });

    // 4. OUTPUT subtyping (f_out <: g_out)
    infcx.ensure_resolved_evars(|infcx| {
        let super_output = super_sig
            .output()
            .replace_bound_refts_with(|sort, mode, _| infcx.fresh_infer_var(sort, mode));
        let reason = ConstrReason::Subtype(SubtypeReason::Output);
        infcx.subtyping(&output.ret, &super_output.ret, reason)?;

        // 6. Update state with Output "ensures" and check super ensures
        env.assume_ensures(infcx, &output.ensures, span);
        fold_local_ptrs(infcx, &mut env, span)?;
        env.check_ensures(
            infcx,
            &super_output.ensures,
            ConstrReason::Subtype(SubtypeReason::Ensures),
        )
    })
}

/// Trait subtyping check, which makes sure that the type for an impl method (def_id)
/// is a subtype of the corresponding trait method.
pub(crate) fn trait_impl_subtyping<'genv, 'tcx>(
    genv: GlobalEnv<'genv, 'tcx>,
    def_id: LocalDefId,
    opts: InferOpts,
    span: Span,
) -> InferResult<Option<InferCtxtRoot<'genv, 'tcx>>> {
    let tcx = genv.tcx();

    // Skip the check if this is not an impl method
    let Some((impl_trait_ref, trait_method_id)) = find_trait_item(genv, def_id)? else {
        return Ok(None);
    };
    let impl_method_id = def_id.to_def_id();
    // Skip the check if either the trait-method or the impl-method are marked as `trusted_impl`
    if genv.has_trusted_impl(trait_method_id) || genv.has_trusted_impl(impl_method_id) {
        return Ok(None);
    }

    let impl_id = tcx.impl_of_assoc(impl_method_id).unwrap();
    let impl_method_args = GenericArg::identity_for_item(genv, impl_method_id)?;
    let trait_method_args = impl_method_args.rebase_onto(&tcx, impl_id, &impl_trait_ref.args);
    let trait_refine_args = RefineArgs::identity_for_item(genv, trait_method_id)?;

    let rustc_infcx = genv
        .tcx()
        .infer_ctxt()
        .with_next_trait_solver(true)
        .build(TypingMode::non_body_analysis());

    let mut root_ctxt = genv
        .infcx_root(&rustc_infcx, opts)
        .with_const_generics(impl_id)?
        .with_refinement_generics(trait_method_id, &trait_method_args)?
        .build()?;

    let mut infcx = root_ctxt.infcx(impl_method_id, &rustc_infcx);

    let trait_fn_sig =
        genv.fn_sig(trait_method_id)?
            .instantiate(tcx, &trait_method_args, &trait_refine_args);
    let impl_sig = genv.fn_sig(impl_method_id)?;
    let sub_sig = SubFn::Poly(impl_method_id, impl_sig, impl_method_args);

    check_fn_subtyping(&mut infcx, sub_sig, &trait_fn_sig, span)?;
    Ok(Some(root_ctxt))
}

fn find_trait_item(
    genv: GlobalEnv<'_, '_>,
    def_id: LocalDefId,
) -> QueryResult<Option<(rty::TraitRef, DefId)>> {
    let tcx = genv.tcx();
    let def_id = def_id.to_def_id();
    if let Some(impl_id) = tcx.trait_impl_of_assoc(def_id) {
        let impl_trait_ref = genv.impl_trait_ref(impl_id)?.instantiate_identity();
        let trait_item_id = tcx.associated_item(def_id).trait_item_def_id().unwrap();
        return Ok(Some((impl_trait_ref, trait_item_id)));
    }
    Ok(None)
}

/// Temporarily (around a function call) convert an `&mut` to an `&strg` to allow for the call to be
/// checked. This is done by unfolding the `&mut` into a local pointer at the call-site and then
/// folding the pointer back into the `&mut` upon return.
/// See also [`fold_local_ptrs`].
///
/// ```text
///             unpack(T) = T'
/// ---------------------------------------[local-unfold]
/// Γ ; &mut T => Γ, l:[<: T] T' ; ptr(l)
/// ```
fn unfold_local_ptrs(
    infcx: &mut InferCtxt,
    env: &mut TypeEnv,
    fn_sig: &PolyFnSig,
    actuals: &[Ty],
) -> InferResult<Vec<Ty>> {
    // We *only* need to know whether each input is a &strg or not
    let fn_sig = fn_sig.skip_binder_ref();
    let mut tys = vec![];
    for (actual, input) in izip!(actuals, fn_sig.inputs()) {
        let actual = if let (
            TyKind::Indexed(BaseTy::Ref(re, bound, Mutability::Mut), _),
            TyKind::StrgRef(_, _, _),
        ) = (actual.kind(), input.kind())
        {
            let loc = env.unfold_local_ptr(infcx, bound)?;
            let path1 = Path::new(loc, rty::List::empty());
            Ty::ptr(PtrKind::Mut(*re), path1)
        } else {
            actual.clone()
        };
        tys.push(actual);
    }
    Ok(tys)
}

/// Fold local pointers implements roughly a rule like the following (for all local pointers)
/// that converts the local pointers created via [`unfold_local_ptrs`] back into `&mut`.
///
/// ```text
///       T1 <: T2
/// --------------------- [local-fold]
/// Γ, l:[<: T2] T1 => Γ
/// ```
fn fold_local_ptrs(infcx: &mut InferCtxt, env: &mut TypeEnv, span: Span) -> InferResult {
    let mut at = infcx.at(span);
    env.fold_local_ptrs(&mut at)
}

fn promoted_fn_sig(ty: &Ty) -> PolyFnSig {
    let safety = rustc_hir::Safety::Safe;
    let abi = rustc_abi::ExternAbi::Rust;
    let requires = rty::List::empty();
    let inputs = rty::List::empty();
    let output =
        Binder::bind_with_vars(FnOutput::new(ty.clone(), rty::List::empty()), rty::List::empty());
    let fn_sig = crate::rty::FnSig::new(safety, abi, requires, inputs, output, Expr::tt(), false);
    PolyFnSig::bind_with_vars(fn_sig, crate::rty::List::empty())
}

impl<'ck, 'genv, 'tcx, M: Mode> Checker<'ck, 'genv, 'tcx, M> {
    fn new(
        genv: GlobalEnv<'genv, 'tcx>,
        checker_id: CheckerId,
        inherited: Inherited<'ck, M>,
        body: &'ck Body<'tcx>,
        fn_sig: FnSig,
        promoted: &'ck IndexSlice<Promoted, Ty>,
    ) -> QueryResult<Self> {
        let root_id = checker_id.root_id();

        let resume_ty = if let CheckerId::DefId(def_id) = checker_id
            && genv.tcx().is_coroutine(def_id.to_def_id())
        {
            Some(fn_sig.inputs()[1].clone())
        } else {
            None
        };

        let bb_len = body.basic_blocks.len();
        Ok(Self {
            checker_id,
            genv,
            inherited,
            body,
            resume_ty,
            visited: DenseBitSet::new_empty(bb_len),
            fn_sig,
            markers: IndexVec::from_fn_n(|_| None, bb_len),
            queue: WorkQueue::empty(bb_len, &body.dominator_order_rank),
            default_refiner: Refiner::default_for_item(genv, root_id.to_def_id())?,
            promoted,
        })
    }

    fn check_body(
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        checker_id: CheckerId,
        inherited: Inherited<'ck, M>,
        body: &'ck Body<'tcx>,
        poly_sig: PolyFnSig,
        promoted: &'ck IndexSlice<Promoted, Ty>,
    ) -> Result {
        let span = body.span();

        let fn_sig = poly_sig
            .replace_bound_vars(
                |_| rty::ReErased,
                |sort, _, kind| {
                    let name = infcx.define_bound_reft_var(sort, kind);
                    Expr::fvar(name)
                },
            )
            .deeply_normalize(&mut infcx.at(span))
            .with_span(span)?;
        let mut env = TypeEnv::new(infcx, body, &fn_sig);

        let mut ck = Checker::new(infcx.genv, checker_id, inherited, body, fn_sig, promoted)
            .with_span(span)?;
        ck.check_ghost_statements_at(infcx, &mut env, Point::FunEntry, span)?;

        ck.check_goto(infcx.branch(), env, body.span(), START_BLOCK)?;

        while let Some(bb) = ck.queue.pop() {
            let visited = ck.visited.contains(bb);

            if visited {
                M::clear(&mut ck, bb);
            }

            let marker = ck.marker_at_dominator(bb);
            let mut infcx = infcx.move_to(marker, visited);
            let mut env = M::enter_basic_block(&mut ck, &mut infcx, bb);
            env.unpack(&mut infcx);
            ck.check_basic_block(infcx, env, bb)?;
        }
        Ok(())
    }

    /// Assign a template with fresh kvars to each promoted constant in `body_root`.
    fn promoted_tys(
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        def_id: LocalDefId,
        body_root: &BodyRoot<'tcx>,
    ) -> QueryResult<IndexVec<Promoted, Ty>> {
        let hole_refiner = Refiner::with_holes(infcx.genv, def_id.into())?;

        body_root
            .promoted
            .iter()
            .map(|body| {
                Ok(body
                    .return_ty()
                    .refine(&hole_refiner)?
                    .replace_holes(|binders, kind| infcx.fresh_infer_var_for_hole(binders, kind)))
            })
            .collect()
    }

    fn run(
        mut infcx: InferCtxt<'_, 'genv, 'tcx>,
        def_id: LocalDefId,
        mut inherited: Inherited<'_, M>,
        poly_sig: PolyFnSig,
    ) -> Result {
        let genv = infcx.genv;
        let span = genv.tcx().def_span(def_id);
        let body_root = genv.mir(def_id).with_span(span)?;

        // 1. Generate templates for promoted consts
        let promoted_tys = Self::promoted_tys(&mut infcx, def_id, &body_root).with_span(span)?;

        // 2. Check the body of all promoted
        for (promoted, ty) in promoted_tys.iter_enumerated() {
            let body = &body_root.promoted[promoted];
            let poly_sig = promoted_fn_sig(ty);
            Checker::check_body(
                &mut infcx,
                CheckerId::Promoted(def_id, promoted),
                inherited.reborrow(),
                body,
                poly_sig,
                &promoted_tys,
            )?;
        }

        // 3. Check the main body
        Checker::check_body(
            &mut infcx,
            CheckerId::DefId(def_id),
            inherited,
            &body_root.body,
            poly_sig,
            &promoted_tys,
        )
    }

    fn check_basic_block(
        &mut self,
        mut infcx: InferCtxt<'_, 'genv, 'tcx>,
        mut env: TypeEnv,
        bb: BasicBlock,
    ) -> Result {
        dbg::basic_block_start!(bb, infcx, env);

        self.visited.insert(bb);
        let data = &self.body.basic_blocks[bb];
        let mut last_stmt_span = None;
        let mut location = Location { block: bb, statement_index: 0 };
        for stmt in &data.statements {
            let span = stmt.source_info.span;
            self.check_ghost_statements_at(
                &mut infcx,
                &mut env,
                Point::BeforeLocation(location),
                span,
            )?;
            bug::track_span(span, || {
                dbg::statement!("start", stmt, &infcx, &env, span, &self);
                self.check_statement(&mut infcx, &mut env, stmt)?;
                dbg::statement!("end", stmt, &infcx, &env, span, &self);
                Ok(())
            })?;
            if !stmt.is_nop() {
                last_stmt_span = Some(span);
            }
            location = location.successor_within_block();
        }

        if let Some(terminator) = &data.terminator {
            let span = terminator.source_info.span;
            self.check_ghost_statements_at(
                &mut infcx,
                &mut env,
                Point::BeforeLocation(location),
                span,
            )?;

            bug::track_span(span, || {
                dbg::terminator!("start", terminator, infcx, env);

                let successors = self.check_terminator(
                    &mut infcx,
                    &mut env,
                    terminator,
                    location,
                    last_stmt_span,
                )?;
                dbg::terminator!("end", terminator, infcx, env);

                self.markers[bb] = Some(infcx.marker());
                let term_span = last_stmt_span.unwrap_or(span);
                self.check_successors(infcx, env, bb, term_span, successors)
            })?;
        }
        Ok(())
    }

    fn check_assign_ty(
        &mut self,
        infcx: &mut InferCtxt,
        env: &mut TypeEnv,
        place: &Place,
        ty: Ty,
        span: Span,
    ) -> InferResult {
        let ty = infcx.hoister(true).hoist(&ty);
        env.assign(&mut infcx.at(span), place, ty)
    }

    fn check_statement(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt: &Statement<'tcx>,
    ) -> Result {
        let stmt_span = stmt.source_info.span;
        match &stmt.kind {
            StatementKind::Assign(place, rvalue) => {
                let ty = self.check_rvalue(infcx, env, stmt_span, rvalue)?;
                self.check_assign_ty(infcx, env, place, ty, stmt_span)
                    .with_span(stmt_span)?;
            }
            StatementKind::SetDiscriminant { .. } => {
                // TODO(nilehmann) double check here that the place is unfolded to
                // the correct variant. This should be guaranteed by rustc
            }
            StatementKind::FakeRead(_) => {
                // TODO(nilehmann) fake reads should be folding points
            }
            StatementKind::AscribeUserType(_, _) => {
                // User ascriptions affect nll, but no refinement type checking.
                // Maybe we can use this to associate refinement type to locals.
            }
            StatementKind::PlaceMention(_) => {
                // Place mentions are a no-op used to detect uses of unsafe that would
                // otherwise be optimized away.
            }
            StatementKind::Nop => {}
            StatementKind::Intrinsic(NonDivergingIntrinsic::Assume(op)) => {
                // Currently, we only have the `assume` intrinsic, which if we're to trust rustc should be a NOP.
                // TODO: There may be a use-case to actually "assume" the bool index associated with the operand,
                // i.e. to strengthen the `rcx` / `env` with the assumption that the bool-index is in fact `true`...
                let _ = self
                    .check_operand(infcx, env, stmt_span, op)
                    .with_span(stmt_span)?;
            }
        }
        Ok(())
    }

    fn is_exit_block(&self, bb: BasicBlock) -> bool {
        let data = &self.body.basic_blocks[bb];
        let is_no_op = data.statements.iter().all(Statement::is_nop);
        let is_ret = match &data.terminator {
            None => false,
            Some(term) => term.is_return(),
        };
        is_no_op && is_ret
    }

    /// For `check_terminator`, the output `Vec<BasicBlock, Guard>` denotes,
    /// - `BasicBlock` "successors" of the current terminator, and
    /// - `Guard` are extra control information from, e.g. the `SwitchInt` (or `Assert`) you can assume when checking the corresponding successor.
    fn check_terminator(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        terminator: &Terminator<'tcx>,
        location: Location,
        last_stmt_span: Option<Span>,
    ) -> Result<Vec<(BasicBlock, Guard)>> {
        let source_info = terminator.source_info;
        let terminator_span = source_info.span;
        match &terminator.kind {
            TerminatorKind::Return => {
                self.check_ret(infcx, env, last_stmt_span.unwrap_or(terminator_span))?;
                Ok(vec![])
            }
            TerminatorKind::Unreachable => Ok(vec![]),
            TerminatorKind::CoroutineDrop => Ok(vec![]),
            TerminatorKind::Goto { target } => Ok(vec![(*target, Guard::None)]),
            TerminatorKind::Yield { resume, resume_arg, .. } => {
                if let Some(resume_ty) = self.resume_ty.clone() {
                    self.check_assign_ty(infcx, env, resume_arg, resume_ty, terminator_span)
                        .with_span(terminator_span)?;
                } else {
                    bug!("yield in non-generator function");
                }
                Ok(vec![(*resume, Guard::None)])
            }
            TerminatorKind::SwitchInt { discr, targets } => {
                let discr_ty = self
                    .check_operand(infcx, env, terminator_span, discr)
                    .with_span(terminator_span)?;
                if discr_ty.is_integral() || discr_ty.is_bool() || discr_ty.is_char() {
                    Ok(Self::check_if(&discr_ty, targets))
                } else {
                    Ok(self.check_match(infcx, env, &discr_ty, targets, terminator_span))
                }
            }
            TerminatorKind::Call { kind, args, destination, target, .. } => {
                let actuals = self
                    .check_operands(infcx, env, terminator_span, args)
                    .with_span(terminator_span)?;
                let ret = match kind {
                    mir::CallKind::FnDef { resolved_id, resolved_args, .. } => {
                        let fn_sig = self.genv.fn_sig(*resolved_id).with_span(terminator_span)?;
                        let generic_args = instantiate_args_for_fun_call(
                            self.genv,
                            self.checker_id.root_id().to_def_id(),
                            *resolved_id,
                            &resolved_args.lowered,
                        )
                        .with_span(terminator_span)?;
                        self.check_call(
                            infcx,
                            env,
                            terminator_span,
                            Some(location),
                            Some(*resolved_id),
                            fn_sig,
                            &generic_args,
                            &actuals,
                        )?
                        .output
                    }
                    mir::CallKind::FnPtr { operand, .. } => {
                        let ty = self
                            .check_operand(infcx, env, terminator_span, operand)
                            .with_span(terminator_span)?;
                        if let TyKind::Indexed(BaseTy::FnPtr(fn_sig), _) = infcx.unpack(&ty).kind()
                        {
                            self.check_call(
                                infcx,
                                env,
                                terminator_span,
                                Some(location),
                                None,
                                EarlyBinder(fn_sig.clone()),
                                &[],
                                &actuals,
                            )?
                            .output
                        } else {
                            bug!("TODO: fnptr call {ty:?}")
                        }
                    }
                };

                let name = destination.name(&self.body.local_names);
                let ret = infcx.unpack_at_name(name, &ret);
                infcx.assume_invariants(&ret);

                env.assign(&mut infcx.at(terminator_span), destination, ret)
                    .with_span(terminator_span)?;

                if let Some(target) = target {
                    Ok(vec![(*target, Guard::None)])
                } else {
                    Ok(vec![])
                }
            }
            TerminatorKind::Assert { cond, expected, target, msg } => {
                Ok(vec![(
                    *target,
                    self.check_assert(infcx, env, terminator_span, cond, *expected, msg)
                        .with_span(terminator_span)?,
                )])
            }
            TerminatorKind::Drop { place, target, .. } => {
                let _ = env.move_place(&mut infcx.at(terminator_span), place);
                Ok(vec![(*target, Guard::None)])
            }
            TerminatorKind::FalseEdge { real_target, .. } => Ok(vec![(*real_target, Guard::None)]),
            TerminatorKind::FalseUnwind { real_target, .. } => {
                Ok(vec![(*real_target, Guard::None)])
            }
            TerminatorKind::UnwindResume => bug!("TODO: implement checking of cleanup code"),
        }
    }

    fn check_ret(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        span: Span,
    ) -> Result {
        let obligations = infcx
            .at(span)
            .ensure_resolved_evars(|infcx| {
                let ret_place_ty = env.lookup_place(infcx, Place::RETURN)?;
                let output = self
                    .fn_sig
                    .output
                    .replace_bound_refts_with(|sort, mode, _| infcx.fresh_infer_var(sort, mode));
                let obligations =
                    infcx.subtyping_with_env(env, &ret_place_ty, &output.ret, ConstrReason::Ret)?;

                env.check_ensures(infcx, &output.ensures, ConstrReason::Ret)?;

                Ok(obligations)
            })
            .with_span(span)?;

        self.check_coroutine_obligations(infcx, obligations)
    }

    #[expect(clippy::too_many_arguments)]
    fn check_call(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        span: Span,
        location: Option<Location>,
        callee_def_id: Option<DefId>,
        fn_sig: EarlyBinder<PolyFnSig>,
        generic_args: &[GenericArg],
        actuals: &[Ty],
    ) -> Result<ResolvedCall> {
        let genv = self.genv;
        let tcx = genv.tcx();

        let actuals =
            unfold_local_ptrs(infcx, env, fn_sig.skip_binder_ref(), actuals).with_span(span)?;
        let actuals = infer_under_mut_ref_hack(infcx, &actuals, fn_sig.skip_binder_ref());
        infcx.push_evar_scope();

        // Replace holes in generic arguments with fresh inference variables
        let generic_args = infcx.instantiate_generic_args(generic_args);

        // Generate fresh inference variables for refinement arguments
        let early_refine_args = match callee_def_id {
            Some(callee_def_id) => {
                infcx
                    .instantiate_refine_args(callee_def_id, &generic_args)
                    .with_span(span)?
            }
            None => rty::List::empty(),
        };

        let clauses = match callee_def_id {
            Some(callee_def_id) => {
                genv.predicates_of(callee_def_id)
                    .with_span(span)?
                    .predicates()
                    .instantiate(tcx, &generic_args, &early_refine_args)
            }
            None => crate::rty::List::empty(),
        };

        let (clauses, fn_clauses) = Clause::split_off_fn_trait_clauses(self.genv, &clauses);
        infcx
            .at(span)
            .check_non_closure_clauses(&clauses, ConstrReason::Call)
            .with_span(span)?;

        for fn_trait_pred in &fn_clauses {
            self.check_fn_trait_clause(infcx, fn_trait_pred, span)?;
        }

        // Instantiate function signature and normalize it
        let late_refine_args = vec![];
        let fn_sig = fn_sig
            .instantiate(tcx, &generic_args, &early_refine_args)
            .replace_bound_vars(
                |_| rty::ReErased,
                |sort, mode, _| infcx.fresh_infer_var(sort, mode),
            );

        let fn_sig = fn_sig
            .deeply_normalize(&mut infcx.at(span))
            .with_span(span)?;

        let mut at = infcx.at(span);

        if let Some(callee_def_id) = callee_def_id
            && genv.def_kind(callee_def_id).is_fn_like()
        {
            let callee_no_panic = fn_sig.no_panic();

            // Recover the resolved callee `NodeKey` from the call graph by the call-site location,
            // then query the no-panic spec map.
            let body_def_id = match self.checker_id {
                CheckerId::DefId(def_id) => Some(def_id.to_def_id()),
                CheckerId::Promoted(..) => None,
            };
            let callee_inferred_spec = body_def_id
                .zip(location)
                .and_then(|(body_def_id, location)| {
                    genv.call_graph().resolved_callee(body_def_id, location)
                })
                .map(|key| genv.inferred_no_panic_key(key))
                .unwrap_or(PanicSpec::MightPanic(PanicReason::NotInCallGraph));

            let inferred_panic_expr = if callee_inferred_spec == PanicSpec::WillNotPanic {
                Expr::tt()
            } else {
                Expr::ff()
            };

            at.check_pred(
                Expr::implies(
                    self.fn_sig.no_panic(),
                    Expr::or(callee_no_panic, inferred_panic_expr),
                ),
                ConstrReason::NoPanic(callee_def_id, callee_inferred_spec),
            );
        }

        // Check requires predicates
        for requires in fn_sig.requires() {
            at.check_pred(requires, ConstrReason::Call);
        }

        // Check arguments
        for (actual, formal) in iter::zip(actuals, fn_sig.inputs()) {
            at.subtyping_with_env(env, &actual, formal, ConstrReason::Call)
                .with_span(span)?;
        }

        infcx.pop_evar_scope().with_span(span)?;
        env.fully_resolve_evars(infcx);

        let output = infcx
            .fully_resolve_evars(&fn_sig.output)
            .replace_bound_refts_with(|sort, _, kind| {
                Expr::fvar(infcx.define_bound_reft_var(sort, kind))
            });

        env.assume_ensures(infcx, &output.ensures, span);
        fold_local_ptrs(infcx, env, span).with_span(span)?;

        Ok(ResolvedCall {
            output: output.ret,
            _early_args: early_refine_args
                .into_iter()
                .map(|arg| infcx.fully_resolve_evars(arg))
                .collect(),
            _late_args: late_refine_args
                .into_iter()
                .map(|arg| infcx.fully_resolve_evars(&arg))
                .collect(),
        })
    }

    fn check_coroutine_obligations(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        obligs: Vec<Binder<CoroutineObligPredicate>>,
    ) -> Result {
        for oblig in obligs {
            // FIXME(nilehmann) we shouldn't be skipping this binder
            let oblig = oblig.skip_binder();

            #[expect(clippy::disallowed_methods, reason = "coroutines cannot be extern speced")]
            let def_id = oblig.def_id.expect_local();
            let span = self.genv.tcx().def_span(def_id);
            let body = self.genv.mir(def_id).with_span(span)?;
            Checker::run(
                infcx.change_item(def_id, &body.infcx),
                def_id,
                self.inherited.reborrow(),
                oblig.to_poly_fn_sig(),
            )?;
        }
        Ok(())
    }

    fn find_self_ty_fn_sig(
        &self,
        self_ty: rustc_middle::ty::Ty<'tcx>,
        span: Span,
    ) -> Result<PolyFnSig> {
        let tcx = self.genv.tcx();
        let mut def_id = Some(self.checker_id.root_id().to_def_id());
        while let Some(did) = def_id {
            let generic_predicates = self
                .genv
                .predicates_of(did)
                .with_span(span)?
                .instantiate_identity();
            let predicates = generic_predicates.predicates;

            for poly_fn_trait_pred in Clause::split_off_fn_trait_clauses(self.genv, &predicates).1 {
                if poly_fn_trait_pred.skip_binder_ref().self_ty.to_rustc(tcx) == self_ty {
                    return Ok(poly_fn_trait_pred.map(|fn_trait_pred| fn_trait_pred.fndef_sig()));
                }
            }
            // Continue to the parent if we didn't find a match
            def_id = generic_predicates.parent;
        }

        span_bug!(
            span,
            "cannot find self_ty_fn_sig for {:?} with self_ty = {self_ty:?}",
            self.checker_id
        );
    }

    fn check_fn_trait_clause(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        poly_fn_trait_pred: &Binder<FnTraitPredicate>,
        span: Span,
    ) -> Result {
        let self_ty = poly_fn_trait_pred
            .skip_binder_ref()
            .self_ty
            .as_bty_skipping_existentials();
        let oblig_sig = poly_fn_trait_pred.map_ref(|fn_trait_pred| fn_trait_pred.fndef_sig());
        match self_ty {
            Some(BaseTy::Closure(def_id, _, _, _)) => {
                let Some(poly_sig) = self.inherited.closures.get(def_id).cloned() else {
                    span_bug!(span, "missing template for closure {def_id:?}");
                };
                check_fn_subtyping(infcx, SubFn::Mono(poly_sig.clone()), &oblig_sig, span)
                    .with_span(span)?;
            }
            Some(BaseTy::FnDef(def_id, args)) => {
                // Generates "function subtyping" obligations between the (super-type) `oblig_sig` in the `fn_trait_pred`
                // and the (sub-type) corresponding to the signature of `def_id + args`.
                // See `tests/neg/surface/fndef00.rs`
                let sub_sig = self.genv.fn_sig(def_id).with_span(span)?;
                check_fn_subtyping(
                    infcx,
                    SubFn::Poly(*def_id, sub_sig, args.clone()),
                    &oblig_sig,
                    span,
                )
                .with_span(span)?;
            }
            Some(BaseTy::FnPtr(sub_sig)) => {
                check_fn_subtyping(infcx, SubFn::Mono(sub_sig.clone()), &oblig_sig, span)
                    .with_span(span)?;
            }

            // Some(self_ty) => {
            Some(self_ty @ BaseTy::Param(_)) => {
                // Step 1. Find matching clause and turn it into a FnSig
                let tcx = self.genv.tcx();
                let self_ty = self_ty.to_rustc(tcx);
                let sub_sig = self.find_self_ty_fn_sig(self_ty, span)?;
                // Step 2. Issue the subtyping
                check_fn_subtyping(infcx, SubFn::Mono(sub_sig), &oblig_sig, span)
                    .with_span(span)?;
            }
            _ => {}
        }
        Ok(())
    }

    fn check_assert(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        terminator_span: Span,
        cond: &Operand<'tcx>,
        expected: bool,
        msg: &AssertKind,
    ) -> InferResult<Guard> {
        let ty = self.check_operand(infcx, env, terminator_span, cond)?;
        let TyKind::Indexed(BaseTy::Bool, idx) = ty.kind() else {
            tracked_span_bug!("unexpected ty `{ty:?}`");
        };
        let pred = if expected { idx.clone() } else { idx.not() };

        let msg = match msg {
            AssertKind::DivisionByZero => "possible division by zero",
            AssertKind::BoundsCheck => "possible out-of-bounds access",
            AssertKind::RemainderByZero => "possible remainder with a divisor of zero",
            AssertKind::Overflow(mir::BinOp::Div) => "possible division with overflow",
            AssertKind::Overflow(mir::BinOp::Rem) => "possible reminder with overflow",
            AssertKind::Overflow(_) => return Ok(Guard::Pred(pred)),
        };
        infcx
            .at(terminator_span)
            .check_pred(&pred, ConstrReason::Assert(msg));
        Ok(Guard::Pred(pred))
    }

    /// Checks conditional branching as in a `match` statement. [`SwitchTargets`](https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/mir/struct.SwitchTargets.html) contains a list of branches - the exact bit value which is being compared and the block to jump to. Using the conditionals, each branch can be checked using the new control flow information.
    /// See <https://github.com/flux-rs/flux/pull/840#discussion_r1786543174>
    fn check_if(discr_ty: &Ty, targets: &SwitchTargets) -> Vec<(BasicBlock, Guard)> {
        let mk = |bits| {
            match discr_ty.kind() {
                TyKind::Indexed(BaseTy::Bool, idx) => {
                    if bits == 0 {
                        idx.not()
                    } else {
                        idx.clone()
                    }
                }
                TyKind::Indexed(bty @ (BaseTy::Int(_) | BaseTy::Uint(_) | BaseTy::Char), idx) => {
                    Expr::eq(idx.clone(), Expr::from_bits(bty, bits))
                }
                _ => tracked_span_bug!("unexpected discr_ty {:?}", discr_ty),
            }
        };

        let mut successors = vec![];

        for (bits, bb) in targets.iter() {
            successors.push((bb, Guard::Pred(mk(bits))));
        }
        let otherwise = Expr::and_from_iter(targets.iter().map(|(bits, _)| mk(bits).not()));
        successors.push((targets.otherwise(), Guard::Pred(otherwise)));

        successors
    }

    fn check_match(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        discr_ty: &Ty,
        targets: &SwitchTargets,
        span: Span,
    ) -> Vec<(BasicBlock, Guard)> {
        let (adt_def, place) = discr_ty.expect_discr();
        let idx = if let Ok(ty) = env.lookup_place(&mut infcx.at(span), place)
            && let TyKind::Indexed(_, idx) = ty.kind()
        {
            Some(idx.clone())
        } else {
            None
        };

        let mut successors = vec![];
        let mut remaining: FxHashMap<u128, VariantIdx> = adt_def
            .discriminants()
            .map(|(idx, discr)| (discr, idx))
            .collect();
        for (bits, bb) in targets.iter() {
            let variant_idx = remaining
                .remove(&bits)
                .expect("value doesn't correspond to any variant");
            successors.push((bb, Guard::Match(place.clone(), variant_idx)));
        }
        let guard = if remaining.len() == 1 {
            // If there's only one variant left, we know for sure that this is the one, so can force an unfold
            let (_, variant_idx) = remaining
                .into_iter()
                .next()
                .unwrap_or_else(|| tracked_span_bug!());
            Guard::Match(place.clone(), variant_idx)
        } else if adt_def.sort_def().is_reflected()
            && let Some(idx) = idx
        {
            // If there's more than one variant left, we can only assume the `is_ctor` holds for one of them
            let mut cases = vec![];
            for (_, variant_idx) in remaining {
                let did = adt_def.did();
                cases.push(rty::Expr::is_ctor(did, variant_idx, idx.clone()));
            }
            Guard::Pred(Expr::or_from_iter(cases))
        } else {
            Guard::None
        };
        successors.push((targets.otherwise(), guard));

        successors
    }

    fn check_successors(
        &mut self,
        mut infcx: InferCtxt<'_, 'genv, 'tcx>,
        env: TypeEnv,
        from: BasicBlock,
        terminator_span: Span,
        successors: Vec<(BasicBlock, Guard)>,
    ) -> Result {
        for (target, guard) in successors {
            let mut infcx = infcx.branch();
            let mut env = env.clone();
            match guard {
                Guard::None => {}
                Guard::Pred(expr) => {
                    infcx.assume_pred(&expr);
                }
                Guard::Match(place, variant_idx) => {
                    env.downcast(&mut infcx.at(terminator_span), &place, variant_idx)
                        .with_span(terminator_span)?;
                }
            }
            self.check_ghost_statements_at(
                &mut infcx,
                &mut env,
                Point::Edge(from, target),
                terminator_span,
            )?;
            self.check_goto(infcx, env, terminator_span, target)?;
        }
        Ok(())
    }

    fn check_goto(
        &mut self,
        mut infcx: InferCtxt<'_, 'genv, 'tcx>,
        mut env: TypeEnv,
        span: Span,
        target: BasicBlock,
    ) -> Result {
        if self.is_exit_block(target) {
            // We inline *exit basic blocks* (i.e., that just return) because this typically
            // gives us better a better error span.
            let mut location = Location { block: target, statement_index: 0 };
            for _ in &self.body.basic_blocks[target].statements {
                self.check_ghost_statements_at(
                    &mut infcx,
                    &mut env,
                    Point::BeforeLocation(location),
                    span,
                )?;
                location = location.successor_within_block();
            }
            self.check_ghost_statements_at(
                &mut infcx,
                &mut env,
                Point::BeforeLocation(location),
                span,
            )?;
            self.check_ret(&mut infcx, &mut env, span)
        } else if let Some(real_target) = self.is_dummy_join(target) {
            self.check_goto(infcx, env, span, real_target)
        } else if self.body.is_join_point(target) {
            if M::check_goto_join_point(self, infcx, env, span, target)? {
                self.queue.insert(target);
            }
            Ok(())
        } else {
            self.check_basic_block(infcx, env, target)
        }
    }

    /// A dummy-join-block is a BasicBlock that has
    /// 1. MULTIPLE incoming edges,
    /// 2. SINGLE outgoing edge,
    /// 3. ONLY no-op statements, and
    /// 4. NO ghosts statements.
    ///
    /// (1) is because we want to avoid "spurious joins"
    /// but also, without it, we have problems with
    /// degenerate loops like (e.g. `const_generics/loop.rs`).
    ///
    /// We can "skip" such blocks and jump straight to
    /// the first (transitively reachable) non-dummy
    /// successor, aka the "real" successor, which allows
    /// us to avoid emitting KVars for spurious joins.
    fn is_dummy_join(&self, bb: BasicBlock) -> Option<BasicBlock> {
        if self.body.is_join_point(bb)
            && self.body.basic_blocks[bb]
                .statements
                .iter()
                .all(Statement::is_nop)
            && let Some(TerminatorKind::Goto { target: real_target }) = &self.body.basic_blocks[bb]
                .terminator
                .as_ref()
                .map(|terminator| &terminator.kind)
            && self.no_ghosts_at(bb, *real_target)
        {
            Some(*real_target)
        } else {
            None
        }
    }

    fn no_ghosts_at(&self, bb: BasicBlock, real_target: BasicBlock) -> bool {
        let Some(ghosts) = self.inherited.ghost_stmts.get(&self.checker_id) else {
            return true;
        };

        let mut res = ghosts
            .statements_at(Point::Edge(bb, real_target))
            .next()
            .is_none();

        let mut location = Location { block: bb, statement_index: 0 };
        for _ in &self.body.basic_blocks[bb].statements {
            res = res
                && ghosts
                    .statements_at(Point::BeforeLocation(location))
                    .next()
                    .is_none();
            location = location.successor_within_block();
        }
        res = res
            && ghosts
                .statements_at(Point::BeforeLocation(location))
                .next()
                .is_none();
        res
    }

    fn closure_template(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        args: &flux_rustc_bridge::ty::GenericArgs,
        operands: &[Operand<'tcx>],
    ) -> InferResult<(Vec<Ty>, PolyFnSig)> {
        let upvar_tys = self
            .check_operands(infcx, env, stmt_span, operands)?
            .into_iter()
            .map(|ty| {
                if let TyKind::Ptr(PtrKind::Mut(re), path) = ty.kind() {
                    env.ptr_to_ref(
                        &mut infcx.at(stmt_span),
                        ConstrReason::Other,
                        *re,
                        path,
                        PtrToRefBound::Infer,
                    )
                } else {
                    Ok(ty.clone())
                }
            })
            .try_collect_vec()?;

        let closure_args = args.as_closure();
        let ty = closure_args.sig_as_fn_ptr_ty();

        if let flux_rustc_bridge::ty::TyKind::FnPtr(poly_sig) = ty.kind() {
            let poly_sig = poly_sig.unpack_closure_sig();
            let poly_sig = self.refine_with_holes(&poly_sig)?;
            let poly_sig = poly_sig.hoist_input_binders();
            let poly_sig = poly_sig
                .replace_holes(|binders, kind| infcx.fresh_infer_var_for_hole(binders, kind));

            Ok((upvar_tys, poly_sig))
        } else {
            bug!("check_rvalue: closure: expected fn_ptr ty, found {ty:?} in {args:?}");
        }
    }

    fn check_closure_body(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        did: &DefId,
        upvar_tys: &[Ty],
        args: &flux_rustc_bridge::ty::GenericArgs,
        poly_sig: &PolyFnSig,
    ) -> Result {
        let genv = self.genv;
        let tcx = genv.tcx();
        #[expect(clippy::disallowed_methods, reason = "closures cannot be extern speced")]
        let closure_id = did.expect_local();
        let span = tcx.def_span(closure_id);
        let body = genv.mir(closure_id).with_span(span)?;
        let no_panic = self.genv.no_panic(*did);
        let closure_sig = rty::to_closure_sig(tcx, closure_id, upvar_tys, args, poly_sig, no_panic);
        Checker::run(
            infcx.change_item(closure_id, &body.infcx),
            closure_id,
            self.inherited.reborrow(),
            closure_sig,
        )
    }

    fn check_rvalue_closure(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        did: &DefId,
        args: &flux_rustc_bridge::ty::GenericArgs,
        operands: &[Operand<'tcx>],
    ) -> Result<Ty> {
        // (1) Create the closure template
        let (upvar_tys, poly_sig) = self
            .closure_template(infcx, env, stmt_span, args, operands)
            .with_span(stmt_span)?;
        // (2) Check the closure body against the template
        self.check_closure_body(infcx, did, &upvar_tys, args, &poly_sig)?;
        // (3) "Save" the closure type in the `closures` map
        self.inherited.closures.insert(*did, poly_sig);
        // (4) Return the closure type
        let no_panic = self.genv.no_panic(*did);
        Ok(Ty::closure(*did, upvar_tys, args, no_panic))
    }

    fn check_rvalue(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        rvalue: &Rvalue<'tcx>,
    ) -> Result<Ty> {
        let genv = self.genv;
        match rvalue {
            Rvalue::Use(operand) => {
                self.check_operand(infcx, env, stmt_span, operand)
                    .with_span(stmt_span)
            }
            Rvalue::Repeat(operand, c) => {
                let ty = self
                    .check_operand(infcx, env, stmt_span, operand)
                    .with_span(stmt_span)?;
                let arr_ty = ty
                    .with_holes()
                    .replace_holes(|binders, kind| infcx.fresh_infer_var_for_hole(binders, kind));
                infcx
                    .at(stmt_span)
                    .subtyping_with_env(env, &ty, &arr_ty, ConstrReason::Other)
                    .with_span(stmt_span)?;
                Ok(Ty::array(arr_ty, c.clone()))
            }
            Rvalue::Ref(r, BorrowKind::Mut { .. }, place) => {
                env.borrow(&mut infcx.at(stmt_span), *r, Mutability::Mut, place)
                    .with_span(stmt_span)
            }
            Rvalue::Ref(r, BorrowKind::Shared | BorrowKind::Fake(..), place) => {
                env.borrow(&mut infcx.at(stmt_span), *r, Mutability::Not, place)
                    .with_span(stmt_span)
            }

            Rvalue::RawPtr(mir::RawPtrKind::FakeForPtrMetadata, place) => {
                // see tests/tests/neg/surface/slice02.rs for what happens without unfolding here.
                env.unfold(infcx, place, stmt_span).with_span(stmt_span)?;
                let ty = env
                    .lookup_place(&mut infcx.at(stmt_span), place)
                    .with_span(stmt_span)?;
                let ty = BaseTy::RawPtrMetadata(ty).to_ty();
                Ok(ty)
            }
            Rvalue::RawPtr(kind, place) => {
                // ignore any refinements on the type stored at place
                let ty = &env.lookup_rust_ty(genv, place).with_span(stmt_span)?;
                let ty = self.refine_default(ty).with_span(stmt_span)?;
                let ty = BaseTy::RawPtr(ty, kind.to_mutbl_lossy()).to_ty();
                Ok(ty)
            }
            Rvalue::Cast(kind, op, to) => {
                let from = self
                    .check_operand(infcx, env, stmt_span, op)
                    .with_span(stmt_span)?;
                self.check_cast(infcx, env, stmt_span, *kind, &from, to)
                    .with_span(stmt_span)
            }
            Rvalue::BinaryOp(bin_op, op1, op2) => {
                self.check_binary_op(infcx, env, stmt_span, *bin_op, op1, op2)
                    .with_span(stmt_span)
            }

            Rvalue::UnaryOp(UnOp::PtrMetadata, Operand::Copy(place))
            | Rvalue::UnaryOp(UnOp::PtrMetadata, Operand::Move(place)) => {
                self.check_raw_ptr_metadata(infcx, env, stmt_span, place)
            }
            Rvalue::UnaryOp(un_op, op) => {
                self.check_unary_op(infcx, env, stmt_span, *un_op, op)
                    .with_span(stmt_span)
            }
            Rvalue::Discriminant(place) => {
                let ty = env
                    .lookup_place(&mut infcx.at(stmt_span), place)
                    .with_span(stmt_span)?;
                // HACK(nilehmann, mut-ref-unfolding) place should be unfolded here.
                let (adt_def, ..) = ty
                    .as_bty_skipping_existentials()
                    .unwrap_or_else(|| tracked_span_bug!())
                    .expect_adt();
                Ok(Ty::discr(adt_def.clone(), place.clone()))
            }
            Rvalue::Aggregate(
                AggregateKind::Adt(def_id, variant_idx, args, _, field_idx),
                operands,
            ) => {
                let actuals = self
                    .check_operands(infcx, env, stmt_span, operands)
                    .with_span(stmt_span)?;
                let sig = genv
                    .variant_sig(*def_id, *variant_idx)
                    .with_span(stmt_span)?
                    .ok_or_query_err(*def_id)
                    .with_span(stmt_span)?
                    .to_poly_fn_sig(*field_idx);

                let args = instantiate_args_for_constructor(
                    genv,
                    self.checker_id.root_id().to_def_id(),
                    *def_id,
                    args,
                )
                .with_span(stmt_span)?;
                self.check_call(infcx, env, stmt_span, None, Some(*def_id), sig, &args, &actuals)
                    .map(|resolved_call| resolved_call.output)
            }
            Rvalue::Aggregate(AggregateKind::Array(arr_ty), operands) => {
                let args = self
                    .check_operands(infcx, env, stmt_span, operands)
                    .with_span(stmt_span)?;
                let arr_ty = self.refine_with_holes(arr_ty).with_span(stmt_span)?;
                self.check_mk_array(infcx, env, stmt_span, &args, arr_ty)
                    .with_span(stmt_span)
            }
            Rvalue::Aggregate(AggregateKind::Tuple, args) => {
                let tys = self
                    .check_operands(infcx, env, stmt_span, args)
                    .with_span(stmt_span)?;
                Ok(Ty::tuple(tys))
            }
            Rvalue::Aggregate(AggregateKind::Closure(did, args), operands) => {
                self.check_rvalue_closure(infcx, env, stmt_span, did, args, operands)
            }
            Rvalue::Aggregate(AggregateKind::Coroutine(did, args), ops) => {
                let coroutine_args = args.as_coroutine();
                let resume_ty = self
                    .refine_default(coroutine_args.resume_ty())
                    .with_span(stmt_span)?;
                let upvar_tys = self
                    .check_operands(infcx, env, stmt_span, ops)
                    .with_span(stmt_span)?;
                Ok(Ty::coroutine(*did, resume_ty, upvar_tys.into(), args.clone()))
            }
            Rvalue::ShallowInitBox(operand, _) => {
                self.check_operand(infcx, env, stmt_span, operand)
                    .with_span(stmt_span)?;
                Ty::mk_box_with_default_alloc(self.genv, Ty::uninit()).with_span(stmt_span)
            }
        }
    }

    fn check_raw_ptr_metadata(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        place: &Place,
    ) -> Result<Ty> {
        let ty = env
            .lookup_place(&mut infcx.at(stmt_span), place)
            .with_span(stmt_span)?;
        let ty = match ty.kind() {
            TyKind::Indexed(BaseTy::RawPtrMetadata(ty), _)
            | TyKind::Indexed(BaseTy::Ref(_, ty, _), _) => ty,
            _ => tracked_span_bug!("check_metadata: bug! unexpected type `{ty:?}`"),
        };
        match ty.kind() {
            TyKind::Indexed(BaseTy::Array(_, len), _) => {
                let idx = Expr::from_const(self.genv.tcx(), len);
                Ok(Ty::indexed(BaseTy::Uint(UintTy::Usize), idx))
            }
            TyKind::Indexed(BaseTy::Slice(_), len) => {
                Ok(Ty::indexed(BaseTy::Uint(UintTy::Usize), len.clone()))
            }
            _ => Ok(Ty::unit()),
        }
    }

    fn check_binary_op(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        bin_op: mir::BinOp,
        op1: &Operand<'tcx>,
        op2: &Operand<'tcx>,
    ) -> InferResult<Ty> {
        let ty1 = self.check_operand(infcx, env, stmt_span, op1)?;
        let ty2 = self.check_operand(infcx, env, stmt_span, op2)?;

        match (ty1.kind(), ty2.kind()) {
            (TyKind::Indexed(bty1, idx1), TyKind::Indexed(bty2, idx2)) => {
                let rule =
                    primops::match_bin_op(bin_op, bty1, idx1, bty2, idx2, infcx.check_overflow);
                if let Some(pre) = rule.precondition {
                    infcx.at(stmt_span).check_pred(pre.pred, pre.reason);
                }

                Ok(rule.output_type)
            }
            _ => tracked_span_bug!("incompatible types: `{ty1:?}` `{ty2:?}`"),
        }
    }

    fn check_unary_op(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        un_op: mir::UnOp,
        op: &Operand<'tcx>,
    ) -> InferResult<Ty> {
        let ty = self.check_operand(infcx, env, stmt_span, op)?;
        match ty.kind() {
            TyKind::Indexed(bty, idx) => {
                let rule = primops::match_un_op(un_op, bty, idx, infcx.check_overflow);
                if let Some(pre) = rule.precondition {
                    infcx.at(stmt_span).check_pred(pre.pred, pre.reason);
                }
                Ok(rule.output_type)
            }
            _ => tracked_span_bug!("invalid type for unary operator `{un_op:?}` `{ty:?}`"),
        }
    }

    fn check_mk_array(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        args: &[Ty],
        arr_ty: Ty,
    ) -> InferResult<Ty> {
        let arr_ty = infcx.ensure_resolved_evars(|infcx| {
            let arr_ty =
                arr_ty.replace_holes(|binders, kind| infcx.fresh_infer_var_for_hole(binders, kind));

            let (arr_ty, pred) = arr_ty.unconstr();
            let mut at = infcx.at(stmt_span);
            at.check_pred(&pred, ConstrReason::Other);
            for ty in args {
                at.subtyping_with_env(env, ty, &arr_ty, ConstrReason::Other)?;
            }
            Ok(arr_ty)
        })?;
        let arr_ty = infcx.fully_resolve_evars(&arr_ty);

        Ok(Ty::array(arr_ty, rty::Const::from_usize(self.genv.tcx(), args.len())))
    }

    fn check_cast(
        &self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt_span: Span,
        kind: CastKind,
        from: &Ty,
        to: &ty::Ty,
    ) -> InferResult<Ty> {
        use ty::TyKind as RustTy;
        let ty = match kind {
            CastKind::PointerExposeProvenance => {
                match to.kind() {
                    RustTy::Int(int_ty) => Ty::int(*int_ty),
                    RustTy::Uint(uint_ty) => Ty::uint(*uint_ty),
                    _ => tracked_span_bug!("unsupported PointerExposeProvenance cast"),
                }
            }
            CastKind::IntToInt => {
                match (from.kind(), to.kind()) {
                    (Bool!(idx), RustTy::Int(int_ty)) => bool_int_cast(idx, *int_ty),
                    (Bool!(idx), RustTy::Uint(uint_ty)) => bool_uint_cast(idx, *uint_ty),
                    (Int!(int_ty1, idx), RustTy::Int(int_ty2)) => {
                        int_int_cast(idx, *int_ty1, *int_ty2)
                    }
                    (Uint!(uint_ty1, idx), RustTy::Uint(uint_ty2)) => {
                        uint_uint_cast(idx, *uint_ty1, *uint_ty2)
                    }
                    (Uint!(uint_ty, idx), RustTy::Int(int_ty)) => {
                        uint_int_cast(idx, *uint_ty, *int_ty)
                    }
                    (Int!(int_ty, idx), RustTy::Uint(uint_ty)) => {
                        int_uint_cast(idx, *int_ty, *uint_ty)
                    }
                    (TyKind::Discr(adt_def, _), RustTy::Int(int_ty)) => {
                        Self::discr_to_int_cast(adt_def, BaseTy::Int(*int_ty))
                    }
                    (TyKind::Discr(adt_def, _place), RustTy::Uint(uint_ty)) => {
                        Self::discr_to_int_cast(adt_def, BaseTy::Uint(*uint_ty))
                    }
                    (Char!(idx), RustTy::Uint(uint_ty)) => char_uint_cast(idx, *uint_ty),
                    (Uint!(_, idx), RustTy::Char) => uint_char_cast(idx),
                    _ => {
                        tracked_span_bug!("invalid int to int cast {from:?} --> {to:?}")
                    }
                }
            }
            CastKind::PointerCoercion(mir::PointerCast::Unsize) => {
                self.check_unsize_cast(infcx, env, stmt_span, from, to)?
            }
            CastKind::PointerCoercion(mir::PointerCast::MutToConstPointer) => {
                match from.kind() {
                    TyKind::Indexed(BaseTy::RawPtr(inner_ty, Mutability::Mut), idx) => {
                        Ty::indexed(BaseTy::RawPtr(inner_ty.clone(), Mutability::Not), idx.clone())
                    }
                    _ => self.refine_default(to)?,
                }
            }
            CastKind::FloatToInt
            | CastKind::IntToFloat
            | CastKind::FloatToFloat
            | CastKind::PtrToPtr
            | CastKind::PointerCoercion(mir::PointerCast::ClosureFnPointer)
            | CastKind::PointerWithExposedProvenance => self.refine_default(to)?,
            CastKind::PointerCoercion(mir::PointerCast::ReifyFnPointer) => {
                let to = self.refine_default(to)?;
                if let TyKind::Indexed(BaseTy::FnDef(def_id, args), _) = from.kind()
                    && let TyKind::Indexed(BaseTy::FnPtr(super_sig), _) = to.kind()
                {
                    let current_did = infcx.def_id;
                    let sub_sig =
                        SubFn::Poly(current_did, infcx.genv.fn_sig(*def_id)?, args.clone());
                    // TODO:CLOSURE:2 TODO(RJ) dicey maneuver? assumes that sig_b is unrefined?
                    check_fn_subtyping(infcx, sub_sig, super_sig, stmt_span)?;
                    to
                } else {
                    tracked_span_bug!("invalid cast from `{from:?}` to `{to:?}`")
                }
            }
        };
        Ok(ty)
    }

    fn discr_to_int_cast(adt_def: &AdtDef, bty: BaseTy) -> Ty {
        // TODO: This could be a giant disjunction, maybe better (if less precise) to use the interval?
        let vals = adt_def
            .discriminants()
            .map(|(_, idx)| Expr::eq(Expr::nu(), Expr::from_bits(&bty, idx)))
            .collect_vec();
        Ty::exists_with_constr(bty, Expr::or_from_iter(vals))
    }

    fn check_unsize_cast(
        &self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        span: Span,
        src: &Ty,
        dst: &ty::Ty,
    ) -> InferResult<Ty> {
        // Convert `ptr` to `&mut`
        let src = if let TyKind::Ptr(PtrKind::Mut(re), path) = src.kind() {
            env.ptr_to_ref(
                &mut infcx.at(span),
                ConstrReason::Other,
                *re,
                path,
                PtrToRefBound::Identity,
            )?
        } else {
            src.clone()
        };

        if let ty::TyKind::Ref(_, deref_ty, _) = dst.kind()
            && let ty::TyKind::Dynamic(..) = deref_ty.kind()
        {
            return Ok(self.refine_default(dst)?);
        }

        // `&mut [T; n] -> &mut [T]` or `&[T; n] -> &[T]`
        if let TyKind::Indexed(BaseTy::Ref(_, deref_ty, _), _) = src.kind()
            && let TyKind::Indexed(BaseTy::Array(arr_ty, arr_len), _) = deref_ty.kind()
            && let ty::TyKind::Ref(re, _, mutbl) = dst.kind()
        {
            let idx = Expr::from_const(self.genv.tcx(), arr_len);
            Ok(Ty::mk_ref(*re, Ty::indexed(BaseTy::Slice(arr_ty.clone()), idx), *mutbl))

        // `Box<[T; n]> -> Box<[T]>`
        } else if let TyKind::Indexed(BaseTy::Adt(adt_def, args), _) = src.kind()
            && adt_def.is_box()
            && let (deref_ty, alloc_ty) = args.box_args()
            && let TyKind::Indexed(BaseTy::Array(arr_ty, arr_len), _) = deref_ty.kind()
        {
            let idx = Expr::from_const(self.genv.tcx(), arr_len);
            Ok(Ty::mk_box(
                self.genv,
                Ty::indexed(BaseTy::Slice(arr_ty.clone()), idx),
                alloc_ty.clone(),
            )?)
        } else {
            Err(query_bug!("unsupported unsize cast from `{src:?}` to `{dst:?}`"))?
        }
    }

    fn check_operands(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        span: Span,
        operands: &[Operand<'tcx>],
    ) -> InferResult<Vec<Ty>> {
        operands
            .iter()
            .map(|op| self.check_operand(infcx, env, span, op))
            .try_collect()
    }

    fn check_operand(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        span: Span,
        operand: &Operand<'tcx>,
    ) -> InferResult<Ty> {
        let ty = match operand {
            Operand::Copy(p) => env.lookup_place(&mut infcx.at(span), p)?,
            Operand::Move(p) => env.move_place(&mut infcx.at(span), p)?,
            Operand::Constant(c) => self.check_constant(infcx, c)?,
        };
        Ok(infcx.hoister(true).hoist(&ty))
    }

    fn check_constant(
        &mut self,
        infcx: &InferCtxt<'_, 'genv, 'tcx>,
        constant: &ConstOperand<'tcx>,
    ) -> QueryResult<Ty> {
        use rustc_middle::mir::Const;
        match constant.const_ {
            Const::Ty(ty, cst) => self.check_ty_const(constant, cst, ty)?,
            Const::Val(val, ty) => self.check_const_val(val, ty)?,
            Const::Unevaluated(uneval, ty) => {
                self.check_uneval_const(infcx, constant, uneval, ty)?
            }
        }
        .map_or_else(|| self.refine_default(&constant.ty), Ok)
    }

    fn check_ty_const(
        &mut self,
        constant: &ConstOperand<'tcx>,
        cst: rustc_middle::ty::Const<'tcx>,
        ty: rustc_middle::ty::Ty<'tcx>,
    ) -> QueryResult<Option<Ty>> {
        use rustc_middle::ty::ConstKind;
        match cst.kind() {
            ConstKind::Param(param) => {
                let idx = Expr::const_generic(param);
                let ctor = self
                    .default_refiner
                    .refine_ty_or_base(&constant.ty)?
                    .expect_base();
                Ok(Some(ctor.replace_bound_reft(&idx).to_ty()))
            }
            ConstKind::Value(val_tree) => {
                let val = self.genv.tcx().valtree_to_const_val(val_tree);
                Ok(self.check_const_val(val, ty)?)
            }
            _ => Ok(None),
        }
    }

    fn check_const_val(
        &mut self,
        val: rustc_middle::mir::ConstValue,
        ty: rustc_middle::ty::Ty<'tcx>,
    ) -> QueryResult<Option<Ty>> {
        use rustc_middle::{mir::ConstValue, ty};
        match val {
            ConstValue::Scalar(scalar) => self.check_scalar(scalar, ty),
            ConstValue::ZeroSized if ty.is_unit() => Ok(Some(Ty::unit())),
            ConstValue::Slice { .. } => {
                if let ty::Ref(_, ref_ty, Mutability::Not) = ty.kind()
                    && ref_ty.is_str()
                    && let Some(data) = val.try_get_slice_bytes_for_diagnostics(self.genv.tcx())
                {
                    let str = String::from_utf8_lossy(data);
                    let idx = Expr::constant(Constant::Str(Symbol::intern(&str)));
                    Ok(Some(Ty::mk_ref(ReErased, Ty::indexed(BaseTy::Str, idx), Mutability::Not)))
                } else {
                    Ok(None)
                }
            }
            _ => Ok(None),
        }
    }

    fn check_uneval_const(
        &mut self,
        infcx: &InferCtxt<'_, 'genv, 'tcx>,
        constant: &ConstOperand<'tcx>,
        uneval: rustc_middle::mir::UnevaluatedConst<'tcx>,
        ty: rustc_middle::ty::Ty<'tcx>,
    ) -> QueryResult<Option<Ty>> {
        // 1. Use template for promoted constants, if applicable
        if let Some(promoted) = uneval.promoted
            && let Some(ty) = self.promoted.get(promoted)
        {
            return Ok(Some(ty.clone()));
        }

        // 2. `Genv::constant_info` cannot handle constants with generics, so, we evaluate
        //    them here. These mostly come from inline consts, e.g., `const { 1 + 1 }`, because
        //    the generic_const_items feature is unstable.
        if !uneval.args.is_empty() {
            let tcx = self.genv.tcx();
            let param_env = tcx.param_env(self.checker_id.root_id());
            let typing_env = infcx.region_infcx.typing_env(param_env);
            if let Ok(val) = tcx.const_eval_resolve(typing_env, uneval, constant.span) {
                return self.check_const_val(val, ty);
            } else {
                return Ok(None);
            }
        }

        // 3. Try to see if we have `consant_info` for it.
        if let rty::TyOrBase::Base(ctor) = self.default_refiner.refine_ty_or_base(&constant.ty)?
            && let rty::ConstantInfo::Interpreted(idx, _) = self.genv.constant_info(uneval.def)?
        {
            return Ok(Some(ctor.replace_bound_reft(&idx).to_ty()));
        }

        Ok(None)
    }

    fn check_scalar(
        &mut self,
        scalar: rustc_middle::mir::interpret::Scalar,
        ty: rustc_middle::ty::Ty<'tcx>,
    ) -> QueryResult<Option<Ty>> {
        use rustc_middle::mir::interpret::{GlobalAlloc, Scalar};
        match scalar {
            Scalar::Int(scalar_int) => Ok(self.check_scalar_int(scalar_int, ty)),
            Scalar::Ptr(ptr, _) => {
                let alloc_id = ptr.provenance.alloc_id();
                if let GlobalAlloc::Static(def_id) = self.genv.tcx().global_alloc(alloc_id)
                    && let rty::StaticInfo::Known(ty) = self.genv.static_info(def_id)?
                    && !self.genv.tcx().is_mutable_static(def_id)
                // TODO: mutable statics!
                {
                    Ok(Some(Ty::mk_ref(ReErased, ty, Mutability::Not)))
                } else {
                    Ok(None)
                }
            }
        }
    }

    fn check_scalar_int(
        &mut self,
        scalar: rustc_middle::ty::ScalarInt,
        ty: rustc_middle::ty::Ty<'tcx>,
    ) -> Option<Ty> {
        use flux_rustc_bridge::const_eval::{scalar_to_int, scalar_to_uint};
        use rustc_middle::ty;

        let tcx = self.genv.tcx();

        match ty.kind() {
            ty::Int(int_ty) => {
                let idx = Expr::constant(Constant::from(scalar_to_int(tcx, scalar, *int_ty)));
                Some(Ty::indexed(BaseTy::Int(*int_ty), idx))
            }
            ty::Uint(uint_ty) => {
                let idx = Expr::constant(Constant::from(scalar_to_uint(tcx, scalar, *uint_ty)));
                Some(Ty::indexed(BaseTy::Uint(*uint_ty), idx))
            }
            ty::Float(float_ty) => Some(Ty::float(*float_ty)),
            ty::Char => {
                let idx = Expr::constant(Constant::Char(scalar.try_into().unwrap()));
                Some(Ty::indexed(BaseTy::Char, idx))
            }
            ty::Bool => {
                let idx = Expr::constant(Constant::Bool(scalar.try_to_bool().unwrap()));
                Some(Ty::indexed(BaseTy::Bool, idx))
            }
            // ty::Tuple(tys) if tys.is_empty() => Constant::Unit,
            _ => None,
        }
    }

    fn check_ghost_statements_at(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        point: Point,
        span: Span,
    ) -> Result {
        bug::track_span(span, || {
            for stmt in self.ghost_stmts().statements_at(point) {
                self.check_ghost_statement(infcx, env, stmt, span)
                    .with_span(span)?;
            }
            Ok(())
        })
    }

    fn check_ghost_statement(
        &mut self,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        env: &mut TypeEnv,
        stmt: &GhostStatement,
        span: Span,
    ) -> InferResult {
        dbg::statement!("start", stmt, infcx, env, span, &self);
        match stmt {
            GhostStatement::Fold(place) => {
                env.fold(&mut infcx.at(span), place)?;
            }
            GhostStatement::Unfold(place) => {
                env.unfold(infcx, place, span)?;
            }
            GhostStatement::Unblock(place) => env.unblock(infcx, place),
            GhostStatement::PtrToRef(place) => {
                env.ptr_to_ref_at_place(&mut infcx.at(span), place)?;
            }
        }
        dbg::statement!("end", stmt, infcx, env, span, &self);
        Ok(())
    }

    #[track_caller]
    fn marker_at_dominator(&self, bb: BasicBlock) -> &Marker {
        marker_at_dominator(self.body, &self.markers, bb)
    }

    fn dominators(&self) -> &'ck Dominators<BasicBlock> {
        self.body.dominators()
    }

    fn ghost_stmts(&self) -> &'ck GhostStatements {
        &self.inherited.ghost_stmts[&self.checker_id]
    }

    fn refine_default<T: Refine>(&self, ty: &T) -> QueryResult<T::Output> {
        ty.refine(&self.default_refiner)
    }

    fn refine_with_holes<T: Refine>(&self, ty: &T) -> QueryResult<<T as Refine>::Output> {
        ty.refine(&Refiner::with_holes(self.genv, self.checker_id.root_id().to_def_id())?)
    }
}

fn instantiate_args_for_fun_call(
    genv: GlobalEnv,
    caller_id: DefId,
    callee_id: DefId,
    args: &ty::GenericArgs,
) -> QueryResult<Vec<rty::GenericArg>> {
    let params_in_clauses = collect_params_in_clauses(genv, callee_id);
    let assumed_parametric_params = genv.assume_parametric_params(callee_id);

    let hole_refiner = Refiner::new_for_item(genv, caller_id, |bty| {
        let sort = bty.sort();
        let bty = bty.shift_in_escaping(1);
        let constr = if !sort.is_unit() {
            rty::SubsetTy::new(bty, Expr::nu(), Expr::hole(rty::HoleKind::Pred))
        } else {
            rty::SubsetTy::trivial(bty, Expr::nu())
        };
        Binder::bind_with_sort(constr, sort)
    })?;
    let default_refiner = Refiner::default_for_item(genv, caller_id)?;

    let callee_generics = genv.generics_of(callee_id)?;
    args.iter()
        .enumerate()
        .map(|(idx, arg)| {
            let param = callee_generics.param_at(idx, genv)?;
            let is_parametric = !params_in_clauses.contains(&idx)
                || assumed_parametric_params.contains(&(idx as u32));
            let refiner = if is_parametric { &hole_refiner } else { &default_refiner };
            refiner.refine_generic_arg(&param, arg)
        })
        .collect()
}

fn instantiate_args_for_constructor(
    genv: GlobalEnv,
    caller_id: DefId,
    adt_id: DefId,
    args: &ty::GenericArgs,
) -> QueryResult<Vec<rty::GenericArg>> {
    let params_in_clauses = collect_params_in_clauses(genv, adt_id);

    let adt_generics = genv.generics_of(adt_id)?;
    let hole_refiner = Refiner::with_holes(genv, caller_id)?;
    let default_refiner = Refiner::default_for_item(genv, caller_id)?;
    args.iter()
        .enumerate()
        .map(|(idx, arg)| {
            let param = adt_generics.param_at(idx, genv)?;
            let refiner =
                if params_in_clauses.contains(&idx) { &default_refiner } else { &hole_refiner };
            refiner.refine_generic_arg(&param, arg)
        })
        .collect()
}

fn collect_params_in_clauses(genv: GlobalEnv, def_id: DefId) -> UnordSet<usize> {
    let tcx = genv.tcx();
    struct Collector {
        params: UnordSet<usize>,
    }

    impl rustc_middle::ty::TypeVisitor<TyCtxt<'_>> for Collector {
        fn visit_ty(&mut self, t: rustc_middle::ty::Ty) {
            if let rustc_middle::ty::Param(param_ty) = t.kind() {
                self.params.insert(param_ty.index as usize);
            }
            t.super_visit_with(self);
        }
    }
    let mut vis = Collector { params: UnordSet::new() };

    let span = genv.tcx().def_span(def_id);
    for (clause, _) in all_predicates_of(tcx, def_id) {
        if let Some(trait_pred) = clause.as_trait_clause() {
            let trait_id = trait_pred.def_id();
            let ignore = [
                LangItem::MetaSized,
                LangItem::Sized,
                LangItem::Tuple,
                LangItem::Copy,
                LangItem::Destruct,
            ];
            if ignore
                .iter()
                .any(|lang_item| tcx.require_lang_item(*lang_item, span) == trait_id)
            {
                continue;
            }

            if tcx.fn_trait_kind_from_def_id(trait_id).is_some() {
                continue;
            }
            if tcx.get_diagnostic_item(sym::Hash) == Some(trait_id) {
                continue;
            }
            if tcx.get_diagnostic_item(sym::Eq) == Some(trait_id) {
                continue;
            }
        }
        if let Some(proj_pred) = clause.as_projection_clause() {
            let assoc_id = proj_pred.item_def_id();
            if genv.is_fn_output(assoc_id) {
                continue;
            }
        }
        if let Some(outlives_pred) = clause.as_type_outlives_clause() {
            // We skip outlives bounds if they are not 'static. A 'static bound means the type
            // implements `Any` which makes it unsound to instantiate the argument with refinements.
            if outlives_pred.skip_binder().1 != tcx.lifetimes.re_static {
                continue;
            }
        }
        clause.visit_with(&mut vis);
    }
    vis.params
}

fn all_predicates_of(
    tcx: TyCtxt<'_>,
    id: DefId,
) -> impl Iterator<Item = &(rustc_middle::ty::Clause<'_>, Span)> {
    let mut next_id = Some(id);
    iter::from_fn(move || {
        next_id.take().map(|id| {
            let preds = tcx.predicates_of(id);
            next_id = preds.parent;
            preds.predicates.iter()
        })
    })
    .flatten()
}

struct SkipConstr;

impl TypeFolder for SkipConstr {
    fn fold_ty(&mut self, ty: &rty::Ty) -> rty::Ty {
        if let rty::TyKind::Constr(_, inner_ty) = ty.kind() {
            inner_ty.fold_with(self)
        } else {
            ty.super_fold_with(self)
        }
    }
}

fn is_indexed_mut_skipping_constr(ty: &Ty) -> bool {
    let ty = SkipConstr.fold_ty(ty);
    if let rty::Ref!(_, inner_ty, Mutability::Mut) = ty.kind()
        && let TyKind::Indexed(..) = inner_ty.kind()
    {
        true
    } else {
        false
    }
}

/// HACK(nilehmann) This let us infer parameters under mutable references for the simple case
/// where the formal argument is of the form `&mut B[@n]`, e.g., the type of the first argument
/// to `RVec::get_mut` is `&mut RVec<T>[@n]`. We should remove this after we implement opening of
/// mutable references.
fn infer_under_mut_ref_hack(rcx: &mut InferCtxt, actuals: &[Ty], fn_sig: &PolyFnSig) -> Vec<Ty> {
    iter::zip(actuals, fn_sig.skip_binder_ref().inputs())
        .map(|(actual, formal)| {
            if let rty::Ref!(re, deref_ty, Mutability::Mut) = actual.kind()
                && is_indexed_mut_skipping_constr(formal)
            {
                rty::Ty::mk_ref(*re, rcx.unpack(deref_ty), Mutability::Mut)
            } else {
                actual.clone()
            }
        })
        .collect()
}

impl Mode for ShapeMode {
    const NAME: &str = "shape";

    fn enter_basic_block<'ck, 'genv, 'tcx>(
        ck: &mut Checker<'ck, 'genv, 'tcx, ShapeMode>,
        _infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        bb: BasicBlock,
    ) -> TypeEnv<'ck> {
        ck.inherited.mode.bb_envs[&ck.checker_id][&bb].enter(&ck.body.local_decls)
    }

    fn check_goto_join_point<'genv, 'tcx>(
        ck: &mut Checker<'_, 'genv, 'tcx, ShapeMode>,
        _: InferCtxt<'_, 'genv, 'tcx>,
        env: TypeEnv,
        span: Span,
        target: BasicBlock,
    ) -> Result<bool> {
        let bb_envs = &mut ck.inherited.mode.bb_envs;
        let target_bb_env = bb_envs.entry(ck.checker_id).or_default().get(&target);
        dbg::shape_goto_enter!(target, env, target_bb_env);

        let modified = match bb_envs.entry(ck.checker_id).or_default().entry(target) {
            Entry::Occupied(mut entry) => entry.get_mut().join(env, span),
            Entry::Vacant(entry) => {
                let scope = marker_at_dominator(ck.body, &ck.markers, target)
                    .scope()
                    .unwrap_or_else(|| tracked_span_bug!());
                entry.insert(env.into_infer(scope));
                true
            }
        };

        dbg::shape_goto_exit!(target, bb_envs[&ck.checker_id].get(&target));
        Ok(modified)
    }

    fn clear(ck: &mut Checker<ShapeMode>, root: BasicBlock) {
        ck.visited.remove(root);
        for bb in ck.body.basic_blocks.indices() {
            if bb != root && ck.dominators().dominates(root, bb) {
                ck.inherited
                    .mode
                    .bb_envs
                    .entry(ck.checker_id)
                    .or_default()
                    .remove(&bb);
                ck.visited.remove(bb);
            }
        }
    }
}

impl Mode for RefineMode {
    const NAME: &str = "refine";

    fn enter_basic_block<'ck, 'genv, 'tcx>(
        ck: &mut Checker<'ck, 'genv, 'tcx, RefineMode>,
        infcx: &mut InferCtxt<'_, 'genv, 'tcx>,
        bb: BasicBlock,
    ) -> TypeEnv<'ck> {
        ck.inherited.mode.bb_envs[&ck.checker_id][&bb].enter(infcx, &ck.body.local_decls)
    }

    fn check_goto_join_point(
        ck: &mut Checker<RefineMode>,
        mut infcx: InferCtxt,
        env: TypeEnv,
        terminator_span: Span,
        target: BasicBlock,
    ) -> Result<bool> {
        let bb_env = &ck.inherited.mode.bb_envs[&ck.checker_id][&target];
        tracked_span_dbg_assert_eq!(
            &ck.marker_at_dominator(target)
                .scope()
                .unwrap_or_else(|| tracked_span_bug!()),
            bb_env.scope()
        );

        dbg::refine_goto!(target, infcx, env, bb_env);

        env.check_goto(&mut infcx.at(terminator_span), bb_env, target)
            .with_span(terminator_span)?;

        Ok(!ck.visited.contains(target))
    }

    fn clear(_ck: &mut Checker<RefineMode>, _bb: BasicBlock) {
        bug!();
    }
}

fn bool_int_cast(b: &Expr, int_ty: IntTy) -> Ty {
    let idx = Expr::ite(b, 1, 0);
    Ty::indexed(BaseTy::Int(int_ty), idx)
}

/// Unlike [`char_uint_cast`] rust only allows `u8` to `char` casts, which are
/// non-lossy, so we can use indexed type directly.
fn uint_char_cast(idx: &Expr) -> Ty {
    let idx = Expr::cast(rty::Sort::Int, rty::Sort::Char, idx.clone());
    Ty::indexed(BaseTy::Char, idx)
}

fn char_uint_cast(idx: &Expr, uint_ty: UintTy) -> Ty {
    let idx = Expr::cast(rty::Sort::Char, rty::Sort::Int, idx.clone());
    if uint_bit_width(uint_ty) >= 32 {
        // non-lossy cast: uint[cast(idx)]
        Ty::indexed(BaseTy::Uint(uint_ty), idx)
    } else {
        // lossy-cast: uint{v: cast(idx) <= max_value => v == cast(idx) }
        guarded_uint_ty(&idx, uint_ty)
    }
}

fn bool_uint_cast(b: &Expr, uint_ty: UintTy) -> Ty {
    let idx = Expr::ite(b, 1, 0);
    Ty::indexed(BaseTy::Uint(uint_ty), idx)
}

fn int_int_cast(idx: &Expr, int_ty1: IntTy, int_ty2: IntTy) -> Ty {
    if int_bit_width(int_ty1) <= int_bit_width(int_ty2) {
        Ty::indexed(BaseTy::Int(int_ty2), idx.clone())
    } else {
        Ty::int(int_ty2)
    }
}

fn uint_int_cast(idx: &Expr, uint_ty: UintTy, int_ty: IntTy) -> Ty {
    if uint_bit_width(uint_ty) < int_bit_width(int_ty) {
        Ty::indexed(BaseTy::Int(int_ty), idx.clone())
    } else {
        Ty::int(int_ty)
    }
}

fn int_uint_cast(idx: &Expr, int_ty: IntTy, uint_ty: UintTy) -> Ty {
    let non_neg = Expr::ge(idx.clone(), Expr::zero());

    let guard: Expr = if int_bit_width(int_ty) <= uint_bit_width(uint_ty) {
        non_neg
    } else {
        // Cast is still possible if the value is known to fit.
        let fits = Expr::le(idx.clone(), Expr::uint_max(uint_ty));
        Expr::and(non_neg, fits)
    };

    let eq = Expr::eq(Expr::nu(), idx.clone());
    Ty::exists_with_constr(BaseTy::Uint(uint_ty), Expr::implies(guard, eq))
}

fn guarded_uint_ty(idx: &Expr, uint_ty: UintTy) -> Ty {
    // uint_ty2{v: idx <= max_value => v == idx }
    let max_value = Expr::uint_max(uint_ty);
    let guard = Expr::le(idx.clone(), max_value);
    let eq = Expr::eq(Expr::nu(), idx.clone());
    Ty::exists_with_constr(BaseTy::Uint(uint_ty), Expr::implies(guard, eq))
}

fn uint_uint_cast(idx: &Expr, uint_ty1: UintTy, uint_ty2: UintTy) -> Ty {
    if uint_bit_width(uint_ty1) <= uint_bit_width(uint_ty2) {
        Ty::indexed(BaseTy::Uint(uint_ty2), idx.clone())
    } else {
        guarded_uint_ty(idx, uint_ty2)
    }
}

fn uint_bit_width(uint_ty: UintTy) -> u64 {
    uint_ty
        .bit_width()
        .unwrap_or(config::pointer_width().bits())
}

fn int_bit_width(int_ty: IntTy) -> u64 {
    int_ty.bit_width().unwrap_or(config::pointer_width().bits())
}

impl ShapeResult {
    fn into_bb_envs(
        self,
        infcx: &mut InferCtxtRoot,
        body: &Body,
    ) -> FxHashMap<CheckerId, FxHashMap<BasicBlock, BasicBlockEnv>> {
        self.0
            .into_iter()
            .map(|(checker_id, shapes)| {
                let bb_envs = shapes
                    .into_iter()
                    .map(|(bb, shape)| (bb, shape.into_bb_env(infcx, body)))
                    .collect();
                (checker_id, bb_envs)
            })
            .collect()
    }
}

fn marker_at_dominator<'a>(
    body: &Body,
    markers: &'a IndexVec<BasicBlock, Option<Marker>>,
    bb: BasicBlock,
) -> &'a Marker {
    let dominator = body
        .dominators()
        .immediate_dominator(bb)
        .unwrap_or_else(|| tracked_span_bug!());
    markers[dominator]
        .as_ref()
        .unwrap_or_else(|| tracked_span_bug!())
}

pub(crate) mod errors {
    use flux_errors::{E0999, ErrorGuaranteed};
    use flux_infer::infer::InferErr;
    use flux_middle::{global_env::GlobalEnv, queries::ErrCtxt};
    use rustc_errors::Diagnostic;
    use rustc_hir::def_id::LocalDefId;
    use rustc_span::Span;

    use crate::fluent_generated as fluent;

    #[derive(Debug)]
    pub struct CheckerError {
        kind: InferErr,
        span: Span,
    }

    impl CheckerError {
        pub fn emit(self, genv: GlobalEnv, fn_def_id: LocalDefId) -> ErrorGuaranteed {
            let dcx = genv.sess().dcx().handle();
            match self.kind {
                InferErr::UnsolvedEvar(_) => {
                    let mut diag =
                        dcx.struct_span_err(self.span, fluent::refineck_param_inference_error);
                    diag.code(E0999);
                    diag.emit()
                }
                InferErr::Query(err) => {
                    let level = rustc_errors::Level::Error;
                    err.at(ErrCtxt::FnCheck(self.span, fn_def_id))
                        .into_diag(dcx, level)
                        .emit()
                }
            }
        }
    }

    pub trait ResultExt<T> {
        fn with_span(self, span: Span) -> Result<T, CheckerError>;
    }

    impl<T, E> ResultExt<T> for Result<T, E>
    where
        E: Into<InferErr>,
    {
        fn with_span(self, span: Span) -> Result<T, CheckerError> {
            self.map_err(|err| CheckerError { kind: err.into(), span })
        }
    }
}