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Don't FCW assoc consts in patterns #140467

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Apr 30, 2025
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Don't FCW assoc consts in patterns #140467

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Don't FCW assoc consts in patterns
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@BoxyUwU
BoxyUwU committed Apr 29, 2025
commit be906131de0079173048194c368ebefaa5e426d4
19 changes: 10 additions & 9 deletions compiler/rustc_middle/src/mir/interpret/queries.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,15 +115,16 @@ impl<'tcx> TyCtxt<'tcx> {
// @lcnr believes that successfully evaluating even though there are
// used generic parameters is a bug of evaluation, so checking for it
// here does feel somewhat sensible.
if !self.features().generic_const_exprs() && ct.args.has_non_region_param() {
let def_kind = self.def_kind(instance.def_id());
assert!(
matches!(
def_kind,
DefKind::InlineConst | DefKind::AnonConst | DefKind::AssocConst
),
"{cid:?} is {def_kind:?}",
);
if !self.features().generic_const_exprs()
&& ct.args.has_non_region_param()
// We only FCW for anon consts as repeat expr counts with anon consts are the only place
// that we have a back compat hack for. We don't need to check this is a const argument
// as only anon consts as const args should get evaluated "for the type system".
//
// If we don't *only* FCW anon consts we can wind up incorrectly FCW'ing uses of assoc
// consts in pattern positions. #140447
&& self.def_kind(instance.def_id()) == DefKind::AnonConst
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I think there should be a comment explaining why we check the def_kind here.

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doh.... yeah, why would I put it only in the test 😆

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updated. let me know if the comment doesn't make sense to you and it needs elaborating somewhere :)

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I barely know what this entire function does, let alone what the FCW is for, so I'm afraid I am mostly lost here, but this way at least there are some breadcrumbs to follow for someone who stumbles upon this curious extra check in the future. :)

{
let mir_body = self.mir_for_ctfe(instance.def_id());
if mir_body.is_polymorphic {
let Some(local_def_id) = ct.def.as_local() else { return };
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23 changes: 23 additions & 0 deletions tests/ui/const-generics/const_eval_unchecked_doesnt_fire_patterns.rs
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@@ -0,0 +1,23 @@
//@ check-pass

// Previously the `CONST_EVALUATABLE_UNCHECKED` FCW would fire on const evaluation of
// associated consts. This is unnecessary as the FCW only needs to apply for repeat expr
// counts which are anon consts with generic parameters provided. #140447

pub struct Foo<const N: usize>;

impl<const N: usize> Foo<N> {
const UNUSED_PARAM: usize = {
let _: [(); N];
3
};

pub fn bar() {
match 1 {
Self::UNUSED_PARAM => (),
_ => (),
}
}
}

fn main() {}
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