Running Flux

You can run flux on a single file or entire crate.

Running on a File: rustc-flux

You can use rustc-flux as you would use rustc. For example, the following command checks the file

rustc-flux path/to/

The flux binary accepts the same flags as rustc. You could for example check a file as a library instead of a binary like so

rustc-flux --crate-type=lib path/to/

Running on a Crate: cargo-flux

You can use cargo-flux as you would use cargo. For the most part this means instead of running cargo check, you should run

cargo-flux check

in order to get flux to check your code.

Developing locally

You can set the FLUX_DRIVER_PATH environment variable to ./target/debug/flux-driver if you want cargo-flux and rustc-flux to use the version of flux-driver that is built when you run cargo build. This is useful if you want to run cargo build instead of cargo xtask install every time you make a change.

A tiny example

The following example declares a function inc that returns an integer greater than the input. We use the nightly feature register_tool to register the flux tool in order to add refinement annotations to functions.


fn main() {
#[flux::sig(fn(x: i32) -> i32{v: x < v})]
pub fn inc(x: i32) -> i32 {
    x - 1

You can save the above snippet in say and then run

rustc-flux --crate-type=lib path/to/

you should see in your output

error[FLUX]: postcondition might not hold
6 |     x - 1
  |     ^^^^^

as indeed x - 1 is not greater than x as required by the output refinement i32{v: x < v}.

If you fix the error by replacing x - 1 with x + 1, you should get no errors in the output (the output may be empty, but in this case no output is a good thing).

Read these chapters to learn more about what you specify and verify with flux.

A note about the flux-driver binary

The flux-driver binary is a rustc driver (similar to how clippy works) meaning it uses rustc as a library to "drive" compilation performing additional analysis along the way. Running the binary requires dynamically linking a correct version of librustc. Thus, to avoid the hassle you should never execute it directly. Instead, use rustc-flux or cargo-flux.

Editor Support

This section assumes you have installed flux, cargo-flux, and rustc-flux.

Rust-Analyzer in VSCode

Add this to the workspace settings i.e. .vscode/settings.json

  "rust-analyzer.check.overrideCommand": [

If you want to change the flux-driver used by cargo-flux, then also specify the FLUX_PATH like in the example below, which uses the version generated when you run cargo build.

  "rust-analyzer.check.extraEnv": {
    "FLUX_DRIVER_PATH": "/path/to/flux-repo/target/debug/flux-driver",

Note: Make sure to edit the paths in the above snippet to point to the correct locations on your machine.


Environment Variables

You can set various env variables to customize the behavior of flux.

  • FLUX_CONFIG tells flux where to find a config file for these settings.
    • By default, flux searches its directory for a flux.toml or .flux.toml.
  • FLUX_DRIVER_PATH=path/to/flux-driver tells cargo-flux and rustc-flux where to find the flux binary.
    • Defaults to the default flux-driver installation (typically found in ~/.cargo/bin).
  • FLUX_LOG_DIR=path/to/log/ with default ./log/
  • FLUX_DUMP_CONSTRAINT=1 sets the directory where constraints, timing and cache are saved.
  • FLUX_DUMP_CHECKER_TRACE=1 saves the checker's trace (useful for debugging!)
  • FLUX_DUMP_TIMINGS=1 saves the profile information
  • FLUX_DUMP_MIR=1 saves the low-level MIR for each analyzed function
  • FLUX_POINTER_WIDTH=N the size of (either 32 or 64), used to determine if an integer cast is lossy (default 64).
  • FLUX_CHECK_DEF=name only checks definitions containing name as a substring
  • FLUX_CACHE=1" switches on query caching and saves the cache in FLUX_CACHE_FILE
  • FLUX_CACHE_FILE=file.json customizes the cache file, default FLUX_LOG_DIR/cache.json
  • FLUX_CHECK_OVERFLOW=1 checks for over and underflow on arithmetic integer operations, default 0. When set to 0, it still checks for underflow on unsigned integer subtraction.

Config file

The config file is a .toml file that contains on each line the lowercase name of a flux command line flag without the FLUX_ prefix. Set environment variables take priority over the config file.

The config file should be in the project root.

For example, suppose your project root contains the following flux.toml.

log_dir = "./test"
dump_timings = true
dump_mir = true
cache = true

and you run in the project root

FLUX_DUMP_MIR=0 cargo-flux check

then flux will create the directory ./test/ and write ./test/timings, a file containing profiling information. It will not dump the MIR because that setting was overridden by setting the environment variable FLUX_DUMP_MIR=0.

Crate Config

Some flags can be configured on a per-crate basis using the custom inner attribute #![flux::cfg]. This can only be used in nightly and requires enabling the feature custom_inner_attributes For example, to enable overflow checking:

#![flux:cfg(check_overflow = true)]
fn main() {

The only flag supported now is overflow checking.

Query Caching

FLUX_CACHE=1 persistently caches the safe fixpoint queries for each DefId in FLUX_LOG_DIR/FLUX_CACHE_FILE, and on subsequent runs, skips queries that are already in the cache, which considerably speeds up cargo-flux check on an entire crate.