…teger addresses

…PlaceTy::dangling`) so you can't get the address back out.

Co-Authored-By: Ralf Jung <[email protected]>

 - Account for `impl Trait<_>`.
 - Provide a reasonable `Span` for empty `Generics` in `impl`s.
 - Account for `fn foo<_>(_: _) {}` to suggest `fn foo<T>(_: T) {}`.
 - Fix rust-lang#67995.

This is used for both the `?const` syntax in bounds as well as the `impl
const Trait` syntax. I also considered handling these separately by
adding a variant of `TraitBoundModifier` and a field to
`ItemKind::Impl`, but this approach was less intrusive.

The grammar also handles `?const ?Trait` even though this is
semantically redundant.

This means the new syntax will always fail to compile, even when the
feature gate is enabled. These checks will be removed in a later PR
once the implementation is done.

…e-types, r=Centril

Point at opaque and closure type definitions in type errors

Fixes rust-lang#57266, fixes rust-lang#67117.

…Jung

Reduce special treatment for zsts

addresses rust-lang#67467 (comment)

cc @RalfJung

Parse the syntax described in RFC 2632

This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out.

I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled?

@oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking?

cc rust-lang#67792 rust-lang#67794

r? @Centril

…enkov

rustc_ast_lowering: misc cleanup & rustc dep reductions

- The first two commits do some code simplification.
- The next three do some file splitting (getting `lib.rs` below the 3kloc tidy lint).
- The remaining commits reduce the number of `rustc::` imports. This works towards making lowering independent of the `rustc` crate.

r? @oli-obk cc @Zoxc

…xcrichton

Allow specifying LLVM args in target specifications

Extend support of `_` in type parameters

 - Account for `impl Trait<_>`.
 - Provide a reasonable `Span` for empty `Generics` in `impl`s.
 - Account for `fn foo<_>(_: _) {}` to suggest `fn foo<T>(_: T) {}`.
 - Fix rust-lang#67995. Follow up to rust-lang#67597.

expect `fn` after `const unsafe` / `const extern`

Fixes rust-lang#68062

r? @estebank @petrochenkov
cc @Aaron1011