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Fix ICE in transmutability error reporting when type aliases are normalized #151703

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Fix ICE in transmutability error reporting when type aliases are normalized #151703

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29 changes: 24 additions & 5 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -257,6 +257,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
match self.get_safe_transmute_error_and_reason(
obligation.clone(),
main_trait_predicate,
root_obligation,
span,
) {
GetSafeTransmuteErrorAndReason::Silent => {
Expand Down Expand Up @@ -2797,6 +2798,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
&self,
obligation: PredicateObligation<'tcx>,
trait_pred: ty::PolyTraitPredicate<'tcx>,
root_obligation: &PredicateObligation<'tcx>,
span: Span,
) -> GetSafeTransmuteErrorAndReason {
use rustc_transmute::Answer;
Expand Down Expand Up @@ -2913,11 +2915,28 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
safe_transmute_explanation: Some(safe_transmute_explanation),
}
}
// Should never get a Yes at this point! We already ran it before, and did not get a Yes.
Answer::Yes => span_bug!(
span,
"Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
),
// The normalized types pass, but the original check failed.
// This can happen when type aliases were normalized for diagnostics.
// Retry with root_obligation's types to get the real error.
Answer::Yes => {
if obligation.predicate != root_obligation.predicate {
if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(root_pred)) =
root_obligation.predicate.kind().skip_binder()
&& root_pred.def_id() == trait_pred.trait_ref.def_id
Comment on lines +2922 to +2925
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@Kivooeo Kivooeo Jan 26, 2026

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Suggested change
if obligation.predicate != root_obligation.predicate {
if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(root_pred)) =
root_obligation.predicate.kind().skip_binder()
&& root_pred.def_id() == trait_pred.trait_ref.def_id
if obligation.predicate != root_obligation.predicate
&& let ty::PredicateKind::Clause(ty::ClauseKind::Trait(root_pred)) =
root_obligation.predicate.kind().skip_binder()
&& root_pred.def_id() == trait_pred.trait_ref.def_id

{
return self.get_safe_transmute_error_and_reason(
root_obligation.clone(),
root_obligation.predicate.kind().rebind(root_pred),
root_obligation,
span,
);
}
}
span_bug!(
span,
"Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
)
}
// Reached when a different obligation (namely `Freeze`) causes the
// transmutability analysis to fail. In this case, silence the
// transmutability error message in favor of that more specific
Expand Down
31 changes: 31 additions & 0 deletions tests/ui/transmutability/type-alias-normalization.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,31 @@
//! regression test for https://github.com/rust-lang/rust/issues/151462
//@compile-flags: -Znext-solver=globally
#![feature(lazy_type_alias, transmutability)]
#![allow(incomplete_features)]
mod assert {
use std::mem::{Assume, TransmuteFrom};

pub fn is_maybe_transmutable<Src, Dst>()
where
Src: TransmuteFrom<
Src,
{
Assume {
alignment: true,
lifetimes: true,
safety: true,
validity: true,
}
},
>,
{
}
}

fn test() {
type JustUnit = ();
assert::is_maybe_transmutable::<JustUnit, ()>();
//~^ ERROR `JustUnit` cannot be safely transmuted into `JustUnit`
}

fn main() {}
29 changes: 29 additions & 0 deletions tests/ui/transmutability/type-alias-normalization.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,29 @@
error[E0277]: `JustUnit` cannot be safely transmuted into `JustUnit`
--> $DIR/type-alias-normalization.rs:27:37
|
LL | assert::is_maybe_transmutable::<JustUnit, ()>();
| ^^^^^^^^ analyzing the transmutability of `JustUnit` is not yet supported
|
note: required by a bound in `is_maybe_transmutable`
--> $DIR/type-alias-normalization.rs:10:14
|
LL | pub fn is_maybe_transmutable<Src, Dst>()
| --------------------- required by a bound in this function
LL | where
LL | Src: TransmuteFrom<
| ______________^
LL | | Src,
LL | | {
LL | | Assume {
... |
LL | | },
LL | | >,
| |_________^ required by this bound in `is_maybe_transmutable`
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
|
LL | fn test() where (): TransmuteFrom<(), Assume { alignment: true, lifetimes: true, safety: true, validity: true }> {
| ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
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