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Replace #[rustc_do_not_implement_via_object] with #[rustc_dyn_incompatible_trait], which makes the marked trait dyn-incompatible.

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Removes the attribute from `MetaSized` and `PointeeSized`, with a special case in the trait solvers for `MetaSized`.

`dyn MetaSized` is a perfectly cromulent type, and seems to only have had #[rustc_do_not_implement_via_object] so the builtin object
candidate does not overlap with the builtin MetaSized impl that all `dyn` types get.
Resolves this with a special case by checking `is_sizedness_trait` where the trait solvers previously checked `implement_via_object`.

`dyn PointeeSized` alone is rejected for other reasons (since `dyn PointeeSized` is considered to have no principal trait because `PointeeSized`
is removed at an earlier stage of the compiler), but `(dyn PointeeSized + Send)` is valid and equivalent to `dyn Send`.

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Update compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs and tests

Co-authored-by: lcnr <rust@lcnr.de>
This commit is contained in:
Zachary S 2025-11-07 00:22:14 -06:00
parent 5c49c4f7c8
commit c3205f98dd
35 changed files with 387 additions and 154 deletions
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4
compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
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@ -792,7 +792,9 @@ where
candidates: &mut Vec<Candidate<I>>,
) {
let cx = self.cx();
if !cx.trait_may_be_implemented_via_object(goal.predicate.trait_def_id(cx)) {
if cx.is_sizedness_trait(goal.predicate.trait_def_id(cx)) {
// `dyn MetaSized` is valid, but should get its `MetaSized` impl from
// being `dyn` (SizedCandidate), not from the object candidate.
return;
}