parser: syntactically allow `self` in all `fn` contexts
Part of https://github.com/rust-lang/rust/pull/68728.
`self` parameters are now *syntactically* allowed as the first parameter irrespective of item context (and in function pointers). Instead, semantic validation (`ast_validation`) is used.
r? @petrochenkov
This commit reduces the size of `Nonterminal` from a whopping 240 bytes
to 72 bytes (on x86-64), which gets it below the `memcpy` threshold.
It also removes some impedance mismatches with `Annotatable`, which
already uses `P` for these variants.
Implement `?const` opt-out for trait bounds
For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](https://github.com/rust-lang/rfcs/pull/2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment.
Prior to this PR, the constness of a trait bound/impl was stored in `TraitRef`. Now, the constness of an `impl` is stored in `ast::ItemKind::Impl` and the constness of a bound in `ast::TraitBoundModifer`. Additionally, constness of trait bounds is now stored in an additional field of `ty::Predicate::Trait`, and the combination of the constness of the item along with any `TraitBoundModifier` determines the constness of the bound in accordance with the RFC. Encoding the constness of impls at the `ty` level is left for a later PR.
After a discussion in \#wg-grammar on Discord, it was decided that the grammar should not encode the mutual exclusivity of trait bound modifiers. The grammar for trait bound modifiers remains `[?const] [?]`. To encode this, I add a dummy variant to `ast::TraitBoundModifier` that is used when the syntax `?const ?` appears. This variant causes an error in AST validation and disappears during HIR lowering.
cc #67794
r? @oli-obk
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#67995. Follow up to #67597.
- 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#67995.
