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const-eval interning: accpt interior mutable pointers in final value (but keep rejecting mutable references)

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This commit is contained in:
Ralf Jung
2024-08-02 15:34:59 +02:00
parent 304b7f801b
commit f76f128dc9
21 changed files with 177 additions and 563 deletions
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18
compiler/rustc_const_eval/src/interpret/intern.rs
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@@ -223,16 +223,20 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx, const_eval
continue;
}
// Crucially, we check this *before* checking whether the `alloc_id`
// has already been interned. The point of this check is to ensure that when
// there are multiple pointers to the same allocation, they are *all* immutable.
// Therefore it would be bad if we only checked the first pointer to any given
// allocation.
// Ensure that this is is derived from a shared reference. Crucially, we check this *before*
// checking whether the `alloc_id` has already been interned. The point of this check is to
// ensure that when there are multiple pointers to the same allocation, they are *all*
// derived from a shared reference. Therefore it would be bad if we only checked the first
// pointer to any given allocation.
// (It is likely not possible to actually have multiple pointers to the same allocation,
// so alternatively we could also check that and ICE if there are multiple such pointers.)
// See <https://github.com/rust-lang/rust/pull/128543> for why we are checking for
// "shared reference" and not "immutable", i.e., for why we are allowed interior-mutable
// shared references: they can actually be created in safe code while pointing to apparently
// "immutable" values, via promotion of `&None::<Cell<T>>`.
if intern_kind != InternKind::Promoted
&& inner_mutability == Mutability::Not
&& !prov.immutable()
&& !prov.shared_ref()
{
if ecx.tcx.try_get_global_alloc(alloc_id).is_some()
&& !just_interned.contains(&alloc_id)
@@ -245,7 +249,7 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx, const_eval
// this to the todo list, since after all it is already interned.
continue;
}
// Found a mutable pointer inside a const where inner allocations should be
// Found a mutable reference inside a const where inner allocations should be
// immutable. We exclude promoteds from this, since things like `&mut []` and
// `&None::<Cell<i32>>` lead to promotion that can produce mutable pointers. We rely
// on the promotion analysis not screwing up to ensure that it is sound to intern