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Improve DispatchFromDyn and CoerceUnsized impl validation #135228

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Improve DispatchFromDyn and CoerceUnsized impl validation #135228

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  • Disallow arbitrary 1-ZST fields in DispatchFromDyn -- only PhantomData, and 1-ZSTs that mention no params (which is needed to support, e.g., the Global alloctor in Box<T, U = Global>).
  • Don't allow coercing between non-ZSTs to ZSTs (since the previous check wasn't actually checking the field tys were the same before checking the layout...)
  • Normalize the field before checking it's PhantomData.

Fixes #135215
Fixes #135214
Fixes #135220

r? @BoxyUwU or reassign

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 8, 2025
Darksonn
Darksonn reviewed Jan 10, 2025
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Thank you!

compiler/rustc_hir_analysis/src/coherence/builtin.rs
@@ -460,8 +478,16 @@ pub(crate) fn coerce_unsized_info<'tcx>(
.filter_map(|(i, f)| {
let (a, b) = (f.ty(tcx, args_a), f.ty(tcx, args_b));

if tcx.type_of(f.did).instantiate_identity().is_phantom_data() {
// Ignore PhantomData fields
// Ignore PhantomData fielzds
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Suggested change
// Ignore PhantomData fielzds
// Ignore PhantomData fields

tests/ui/self/phantomdata-in-coerce-and-dispatch-impls.rs
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This test is from #135214 that this PR is listed as fixing, but that issue also has another broken example that involves feature(lazy_type_alias). Is that part of #135214 also fixed by this?

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