Auto merge of #90104 - spastorino:coherence-for-negative-trait, r=nikomatsakis

Implement coherence checks for negative trait impls

The main purpose of this PR is to be able to [move Error trait to core](https://github.com/rust-lang/project-error-handling/issues/3).

This feature is necessary to handle the following from impl on box.

```rust
impl From<&str> for Box<dyn Error> { ... }
```

Without having negative traits affect coherence moving the error trait into `core` and moving that `From` impl to `alloc` will cause the from impl to no longer compiler because of a potential future incompatibility. The compiler indicates that `&str` _could_ introduce an `Error` impl in the future, and thus prevents the `From` impl in `alloc` that would cause overlap with `From<E: Error> for Box<dyn Error>`. Adding `impl !Error for &str {}` with the negative trait coherence feature will disable this error by encoding a stability guarantee that `&str` will never implement `Error`, making the `From` impl compile.

We would have this in `alloc`:

```rust
impl From<&str> for Box<dyn Error> {} // A
impl<E> From<E> for Box<dyn Error> where E: Error {} // B
```

and this in `core`:

```rust
trait Error {}
impl !Error for &str {}
```

r? `@nikomatsakis`

This PR was built on top of `@yaahc` PR #85764.

Language team proposal: to https://github.com/rust-lang/lang-team/issues/96

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -854,6 +854,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                         Binder::dummy(TraitPredicate {
                             trait_ref,
                             constness: ty::BoundConstness::ConstIfConst,
+                            polarity: ty::ImplPolarity::Positive,
                         }),
                     );
 

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -162,6 +162,7 @@ impl Qualif for NeedsNonConstDrop {
             ty::Binder::dummy(ty::TraitPredicate {
                 trait_ref,
                 constness: ty::BoundConstness::ConstIfConst,
+                polarity: ty::ImplPolarity::Positive,
             }),
         );