Fix ICE in pointer layout for DSTs with bogus Sized impls#155260
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GokhanKabar wants to merge 1 commit intorust-lang:mainfrom
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Fix ICE in pointer layout for DSTs with bogus Sized impls#155260GokhanKabar wants to merge 1 commit intorust-lang:mainfrom
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A bogus `impl<T: ?Sized> Sized for T {}` (rejected as E0322) was
processed by the new trait solver (`-Znext-solver=globally`), making
DSTs like `[T]` appear sized. This caused `*const [T]` to receive a
thin-pointer (Scalar) layout instead of a fat-pointer (ScalarPair)
layout. When const evaluation then performed an array-to-slice unsizing
coercion and tried to write a ScalarPair value to a Scalar-layout place,
it triggered an ICE:
"invalid immediate for given destination place: value ScalarPair(...)
does not match ABI Scalar(...)"
The fix adds a structural cross-check in the pointer layout computation:
after `is_sized` returns true, we verify via `struct_tail_for_codegen`
(which is purely structural and unaffected by bogus trait impls) that
the pointee tail is not a known DST kind (Slice/Str/Dynamic). If it is,
we skip the thin-pointer early return and fall through to compute the
correct fat-pointer layout.
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Fixes #155252.
Summary
A bogus
impl<T: ?Sized> Sized for T {}(rejected as E0322) was processed by the new trait solver (-Znext-solver=globally), making DSTs like[T]appear sized. This caused*const [T]to receive a thin-pointer (Scalar) layout instead of a fat-pointer (ScalarPair) layout inrustc_ty_utils/src/layout.rs.When const evaluation then performed an array-to-slice unsizing coercion (
unsize_into_ptr) and tried to write the resultingScalarPairvalue to aScalar-layout place, it triggered an ICE:Fix
In
compiler/rustc_ty_utils/src/layout.rs, afteris_sizedreturnstruefor the pointee, we cross-check usingstruct_tail_for_codegen— a purely structural traversal that is unaffected by bogus trait impls. If the tail is a known DST kind (Slice/Str/Dynamic), we skip the thin-pointer early return and fall through to compute the correct fat-pointer (ScalarPair) layout.This approach is safe because
struct_tail_for_codegendoes not invoke the trait solver, so it cannot be misled by the bogusSizedimpl.Test
Added
tests/ui/traits/next-solver/dont-ice-on-bogus-sized-impl-in-const-block.rswhich previously ICE'd and now produces the expected E0322 + E0210 errors.