Function parse_atom

fn parse_atom(cx: &mut ParseCtxt<'_>, allow_struct: bool) -> ParseResult<Expr>

⟨atom⟩ := ⟨if_expr⟩
        | ⟨lit⟩
        | ( ⟨expr⟩ )
        | ⟨epath⟩
        | ⟨bounded_quant⟩
        |  <⟨ty⟩ as ⟨path⟩> :: ⟨ident⟩
        | ⟨epath⟩ { ⟨constructor_arg⟩,* }    if allow_struct
        | { ⟨constructor_arg⟩,* }            if allow_struct