Just like `Box<T>`, we want `ThinBox<T>` to be covariant in `T`, but the
projection in `WithHeader<<T as Pointee>::Metadata>` was making it
invariant. This is now hidden as `WithOpaqueHeader`, which we type-cast
whenever the real `WithHeader<H>` type is needed.

Fixes rust-lang#93117
Fixes rust-lang#96258

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

It makes the code a little nicer to read.

The `&[ast::Variant]` field isn't used.

The `Vec<Ident>` field is only used for its length, but that's always
the same as the length of the `&[Ident]` and so isn't necessary.

It's unused.

They are always `false`.

It's unused.

This was accidentally left behind in a previous commit.

Improve a comment, and panic on an impossible code path.

The existing derive code allows for various possibilities that aren't
needed in practice, which complicates the code. There are only a few
auto-derived traits and new ones are unlikely, so this commit simplifies
things.

- `PtrTy` has been eliminated. The `Raw` variant was never used, and the
  lifetime for the `Borrowed` variant was always `None`. That left just
  the mutability field, which has been inlined as necessary.
- `MethodDef::explicit_self` was a confusing `Option<Option<PtrTy>>`.
  Indicating either `&self` or nothing. It's now a `bool`.
- `borrowed_self` is renamed as `self_ref`.
- `Ty::Ptr` is renamed to `Ty::Ref`.

It's unused.

Because a `Literal` is a type of expression, and is simply the wrong
name for this.

Because that's all that is needed in practice.

…, r=compiler-errors

Suggest blanket impl to the local traits

This PR will add additional suggestion regarding the blanket implementation when it is possible, by generation a new help message + suggestion.

Closes rust-lang#96076

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Make `ThinBox<T>` covariant in `T`

Just like `Box<T>`, we want `ThinBox<T>` to be covariant in `T`, but the
projection in `WithHeader<<T as Pointee>::Metadata>` was making it
invariant. This is now hidden as `WithOpaqueHeader`, which we type-cast
whenever the real `WithHeader<H>` type is needed.

Fixes the problem noted in <rust-lang#92791 (comment)>.

…_inc_clos_cap, r=lcnr

fix ICE with -Wrust-2021-incompatible-closure-captures

Fixes rust-lang#93117
Fixes rust-lang#96258

…lan-DPC

fix grammar in useless doc comment lint

…rk-Simulacrum

Many small deriving cleanups

These commits remove lots of little unnecessary things, and clarifies other things.

r? `@Mark-Simulacrum`

…on, r=Dylan-DPC

Use const instead of function and make it private