…Trait

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus
considering `[T; 0]` to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the
feature-guard `transmute_const_generics`. However, it merely allows
comparing fixed-size types with fixed-size types, and generic types with
generic types. For generic types, it merely compares whether their
arguments match (ordering them first). Even if their exact sizes are not
known at compile time, it can ensure that they will eventually be the
same.

This patch extends this by shortcutting the size-evaluation of zero
sized arrays and thus allowing size comparisons of `()` with `[T; 0]`,
where one contains generics and the other does not.

This code is guarded by `transmute_const_generics` (rust-lang#109929), even
though it is unclear whether it should be. However, this assumes that a
separate stabilization PR is required to move this out of the feature
guard.

Initially reported in rust-lang#98104.

Feels more complete, and for ImplPolarity has the side-effect of making
sure we also handle reservation impls correctly

Fixes rust-lang#97725

…(|r| !r.iter().any(|sc| sc.chr == \'\\t\')) with edition 2021

Fixes rust-lang#105210

This query allows for sharing code between `rustc_const_eval` and
`rustc_transmutability`.

Also moves `DummyMachine` to `rustc_const_eval`.

compiler: allow transmute of ZST arrays with generics

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same.

This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not.

This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard.

Initially reported in rust-lang#98104.

Note that the caller chooses a type for type param

```
error[E0308]: mismatched types
  --> $DIR/return-impl-trait.rs:23:5
   |
LL | fn other_bounds<T>() -> T
   |                 -       -
   |                 |       |
   |                 |       expected `T` because of return type
   |                 |       help: consider using an impl return type: `impl Trait`
   |                 expected this type parameter
...
LL |     ()
   |     ^^ expected type parameter `T`, found `()`
   |
   = note: expected type parameter `T`
                   found unit type `()`
   = note: the caller chooses the type of T which can be different from ()
```

Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics.

Revives rust-lang#112088 and rust-lang#104755.

…rino,compiler-errors

Suggest `_` for missing generic arguments in turbofish

The compiler may suggest unusable generic type names for missing generic arguments in an expression context:

```rust
fn main() {
    (0..1).collect::<Vec>()
}
```

> help: add missing generic argument
>
>      (0..1).collect::<Vec<T>>()

but `T` is not a valid name in this context, and this suggestion won't compile.

I've changed it to use `_` inside method calls (turbofish), so it will suggest `(0..1).collect::<Vec<_>>()` which _may_ compile.

It's possible that the suggested `_` will be ambiguous, but there is very extensive E0283 that will help resolve that, which is more helpful than a basic "cannot find type `T` in this scope" users would get otherwise.

Out of caution to limit scope of the change I've limited it to just turbofish, but I suspect `_` could be the better choice in more cases. Perhaps in all expressions?

…r-errors

Add `tag_for_variant` query

This query allows for sharing code between `rustc_const_eval` and `rustc_transmutability`. It's a precursor to a PR I'm working on to entirely replace the bespoke layout computations in `rustc_transmutability`.

r? `@compiler-errors`

…, r=lcnr

Split out `PredicatePolarity` from `ImplPolarity`

Because having to deal with a third `Reservation` level in all the trait solver code is kind of weird.

r? `@lcnr` or `@oli-obk`

…Simulacrum

Merge my contributor emails into one using mailmap

…, r=compiler-errors

Adjust better spastorino membership to triagebot's adhoc_groups

…iler-errors

add a couple more tests