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promote_consts experiment: do not promote !Freeze shared references #142287

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promote_consts experiment: do not promote !Freeze shared references #142287

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14 changes: 13 additions & 1 deletion compiler/rustc_mir_transform/src/promote_consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -397,7 +397,19 @@ impl<'tcx> Validator<'_, 'tcx> {
}

BorrowKind::Shared => {
let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
// Let's just see what happens if we reject anything `!Freeze`...
// (Except ZST which definitely can't have interior mut)
let ty = place.ty(self.body, self.tcx).ty;
let has_mut_interior = match ty.kind() {
// Empty arrays have no interior mutability no matter their element type.
ty::Array(_elem, count)
if count.try_to_target_usize(self.tcx).is_some_and(|v| v == 0) =>
{
false
}
// Fallback to checking `Freeze`.
_ => !ty.is_freeze(self.tcx, self.typing_env),
};
if has_mut_interior {
return Err(Unpromotable);
}
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2 changes: 1 addition & 1 deletion compiler/rustc_pattern_analysis/src/pat.rs
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Expand Up @@ -192,7 +192,7 @@ impl<'p, Cx: PatCx> PatOrWild<'p, Cx> {
}
pub(crate) fn ctor(self) -> &'p Constructor<Cx> {
match self {
PatOrWild::Wild => &Wildcard,
PatOrWild::Wild => const { &Wildcard },
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@RalfJung RalfJung Jun 10, 2025 •
edited
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Not even the compiler builds without some changes.^^

What happens here is that by making the const expression &Wildcard, the lifetime extension rules kick in, and those still do value-based reasoning and therefore allow this constant without ever involving promotion.

PatOrWild::Pat(pat) => pat.ctor(),
}
}
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