[ICE]: delayed bug in fully_perform + broken MIR when AsyncFnMut(&()) is cast to FnMut(&())

Code

struct Wrapper<F>(F);

fn wrap<F: AsyncFnMut(&())>(f: F) -> Wrapper<F> {
    Wrapper(f)
}

trait Trait {
    type Assoc;
    fn method(self) -> Self::Assoc;
}

trait Trait2 {}

impl<F: Trait2> Trait for F {
    type Assoc = ();
    fn method(self) -> Self::Assoc {
        loop {}
    }
}

impl<F: FnMut(&()) -> Fut, Fut> Trait for Wrapper<F> {
    type Assoc = ();
    fn method(self) -> Self::Assoc {
        loop {}
    }
}

fn main() {
    wrap(async |_| {}).method();
}

Playground

It errors out in fully_perform without an associated type as well, but the broken MIR error doesn't appear, so I kept it just in case.

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (3659db0d3 2026-07-05)
binary: rustc
commit-hash: 3659db0d3e2cd634c766fcda79ed118eca31a9fd
commit-date: 2026-07-05
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Reproduces on stable as well.

Error output

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: error performing operation: fully_perform
  --> ydb/examples/topic-read-in-transaction-example.rs:29:5
   |
29 |     wrap(async |_| {}).method();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: delayed at /rustc-dev/3659db0d3e2cd634c766fcda79ed118eca31a9fd/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:126:26 - disabled backtrace
  --> ydb/examples/topic-read-in-transaction-example.rs:29:5
   |
29 |     wrap(async |_| {}).method();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: broken MIR in DefId(0:22 ~ topic_read_in_transaction_example[dee8]::main) (_1 = <Wrapper<{async closure@ydb/examples/topic-read-in-transaction-example.rs:29:10: 29:19}> as Trait>::method(move _2) -> [return: bb2, unwind: bb3]): call dest mismatch (() <- Alias(No, Alias { kind: Projection { def_id: DefId(0:10 ~ topic_read_in_transaction_example[dee8]::Trait::Assoc) }, args: [Wrapper<CoroutineClosure(DefId(0:23 ~ topic_read_in_transaction_example[dee8]::main::{closure#0}), [i16, Binder { value: extern "RustCall" fn(std::future::ResumeTy, (&'^0 (),)) -> ((), ()), bound_vars: [Region(BrAnon)] }, (), Binder { value: fn(), bound_vars: [Region(BrEnv)] }])>], .. })): NoSolution
  --> ydb/examples/topic-read-in-transaction-example.rs:29:24
   |
29 |     wrap(async |_| {}).method();
   |                        ^^^^^^
   |
note: delayed at /rustc-dev/3659db0d3e2cd634c766fcda79ed118eca31a9fd/compiler/rustc_borrowck/src/type_check/mod.rs:1989:17 - disabled backtrace
  --> ydb/examples/topic-read-in-transaction-example.rs:29:24
   |
29 |     wrap(async |_| {}).method();
   |                        ^^^^^^

Originally found by @liferooter, reduced by me.

Start by compiling the reduced Rust example from the issue and confirm both the delayed fully_perform ICE and broken MIR error. Read compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs and compiler/rustc_borrowck/src/type_check/mod.rs around the reported locations, then trace the associated-type projection and async FnMut cast. Done means the example no longer produces an internal compiler error or broken MIR.