Change:
<ins> error[E0059]: type parameter to bare `FnOnce` trait must be a tuple --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:24:12 | LL | impl<A, B> FnOnce<A> for CachedFun<A, B> |            ^^^^^^^^^ the trait `Tuple` is not implemented for `A` | note: required by a bound in `FnOnce` --> $SRC_DIR/core/src/ops/function.rs:LL:COL help: consider further restricting this bound | LL |     A: Eq + Hash + Clone + std::marker::Tuple, |                          ++++++++++++++++++++ error[E0059]: type parameter to bare `FnMut` trait must be a tuple --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:38:12 | LL | impl<A, B> FnMut<A> for CachedFun<A, B> |            ^^^^^^^^ the trait `Tuple` is not implemented for `A` | note: required by a bound in `FnMut` --> $SRC_DIR/core/src/ops/function.rs:LL:COL help: consider further restricting this bound | LL |     A: Eq + Hash + Clone + std::marker::Tuple, |                          ++++++++++++++++++++ error[E0277]: functions with the "rust-call" ABI must take a single non-self tuple argument --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:31:5 | LL |     extern "rust-call" fn call_once(mut self, a: A) -> Self::Output { |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `A` | help: consider further restricting this bound | LL |     A: Eq + Hash + Clone + std::marker::Tuple, |                          ++++++++++++++++++++ error[E0277]: functions with the "rust-call" ABI must take a single non-self tuple argument --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:44:5 | LL |     extern "rust-call" fn call_mut(&mut self, a: A) -> Self::Output { |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `A` | help: consider further restricting this bound | LL |     A: Eq + Hash + Clone + std::marker::Tuple, |                          ++++++++++++++++++++ error[E0277]: `A` is not a tuple --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:33:23 | LL |         self.call_mut(a) |              -------- ^ the trait `Tuple` is not implemented for `A` |              | |              required by a bound introduced by this call | note: required by a bound in `call_mut` --> $SRC_DIR/core/src/ops/function.rs:LL:COL help: consider further restricting this bound | LL |     A: Eq + Hash + Clone + std::marker::Tuple, |                          ++++++++++++++++++++ error[E0277]: `i32` is not a tuple --> $DIR/rust-call-abi-not-a-tuple-ice-81974.rs:57:26 | LL |     cachedcoso.call_once(1); |                --------- ^ the trait `Tuple` is not implemented for `i32` |                | |                required by a bound introduced by this call | note: required by a bound in `call_once` --> $SRC_DIR/core/src/ops/function.rs:LL:COL error: aborting due to 6 previous errors Some errors have detailed explanations: E0059, E0277. For more information about an error, try `rustc --explain E0059`. </ins>

Discussion:
: Same in 1.52.0-nightly, same in 1.51.0. : <details<summary<strongBacktrace</strong</summary<p</p</detailsGood luck! <3
This happens on top of too much code for me to take the time to minimalize this issue, sorry. Though I'll try to file the fixing patch.
Wait! I made it! This issue revolves around L.31. Comment to see the ice go away. Interestingly, comment L.40 (or the whole impl L.37-45) to see ice go away as well. Commenting L.47-56 does not make ice go away. Commenting L.13-22 should raise another error (as L.31 is supposed to go there), but doesn't. Actually, the real fix is to replace L.31 with: Have fun!
Minimized with , then a bit by hand: <details<summaryDetails</summary<pIcemelter version: v0.2.0 label +S-bug-has-mcve </p</details
fixed by . we previously used to eagerly error but did not consider them as part of the cache key. So using a cache key from an evaluation with different can result in buggy behavior