Rollup merge of #123302 - compiler-errors:sized-bound-first, r=estebank

Make sure to insert `Sized` bound first into clauses list

#120323 made it so that we don't insert an implicit `Sized` bound whenever we see an *explicit* `Sized` bound. However, since the code that inserts implicit sized bounds puts the bound as the *first* in the list, that means that it had the **side-effect** of possibly meaning we check `Sized` *after* checking other trait bounds.

If those trait bounds result in ambiguity or overflow or something, it may change how we winnow candidates. (**edit: SEE** #123303) This is likely the cause for the regression in https://github.com/rust-lang/rust/issues/123279#issuecomment-2028899598, since the impl...

```rust
impl<T: Job + Sized> AsJob for T { // <----- changing this to `Sized + Job` or just `Job` (which turns into `Sized + Job`) will FIX the issue.
}
```

...looks incredibly suspicious.

Fixes [after beta-backport] #123279.

Alternative is to revert #120323. I don't have a strong opinion about this, but think it may be nice to keep the diagnostic changes around.

1 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_hir_analysis/src/bounds.rs b/compiler/rustc_hir_analysis/src/bounds.rs
index d5465bb5dd5..d3f51195dfb 100644
--- a/compiler/rustc_hir_analysis/src/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/bounds.rs
@@ -54,14 +54,20 @@ impl<'tcx> Bounds<'tcx> {
         span: Span,
         polarity: ty::PredicatePolarity,
     ) {
-        self.clauses.push((
+        let clause = (
             trait_ref
                 .map_bound(|trait_ref| {
                     ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity })
                 })
                 .to_predicate(tcx),
             span,
-        ));
+        );
+        // FIXME(-Znext-solver): We can likely remove this hack once the new trait solver lands.
+        if tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
+            self.clauses.insert(0, clause);
+        } else {
+            self.clauses.push(clause);
+        }
     }
 
     pub fn push_projection_bound(