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Normalize before erasing late-bound regions in equal_up_to_regions #101437

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Sep 8, 2022
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Normalize before erasing late-bound regions in equal_up_to_regions #101437

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29 changes: 15 additions & 14 deletions compiler/rustc_const_eval/src/transform/validate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -89,20 +89,21 @@ pub fn equal_up_to_regions<'tcx>(

// Normalize lifetimes away on both sides, then compare.
let normalize = |ty: Ty<'tcx>| {
let ty = ty.fold_with(&mut BottomUpFolder {
tcx,
// FIXME: We erase all late-bound lifetimes, but this is not fully correct.
// If you have a type like `<for<'a> fn(&'a u32) as SomeTrait>::Assoc`,
// this is not necessarily equivalent to `<fn(&'static u32) as SomeTrait>::Assoc`,
// since one may have an `impl SomeTrait for fn(&32)` and
// `impl SomeTrait for fn(&'static u32)` at the same time which
// specify distinct values for Assoc. (See also #56105)
lt_op: |_| tcx.lifetimes.re_erased,
// Leave consts and types unchanged.
ct_op: |ct| ct,
ty_op: |ty| ty,
});
tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty)
tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty).fold_with(
&mut BottomUpFolder {
tcx,
// FIXME: We erase all late-bound lifetimes, but this is not fully correct.
// If you have a type like `<for<'a> fn(&'a u32) as SomeTrait>::Assoc`,
// this is not necessarily equivalent to `<fn(&'static u32) as SomeTrait>::Assoc`,
// since one may have an `impl SomeTrait for fn(&32)` and
Comment on lines +95 to +98
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This FIXME can be removed now, right? Since we now normalize before erasing, so the <for<'a> fn(&'a u32) as SomeTrait>::Assoc will be normalized to the correct type.

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This still applies to the can_eq call that happens below

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Also normalizing the associated type might fail if this is in a generic context.

// `impl SomeTrait for fn(&'static u32)` at the same time which
// specify distinct values for Assoc. (See also #56105)
lt_op: |_| tcx.lifetimes.re_erased,
// Leave consts and types unchanged.
ct_op: |ct| ct,
ty_op: |ty| ty,
},
)
};
tcx.infer_ctxt().enter(|infcx| infcx.can_eq(param_env, normalize(src), normalize(dest)).is_ok())
}
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