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
Allocate HIR on an arena 4/4
This is the fourth and last PR in the series started by #66931, #66936 and #66942.
The last commits should compile on their own.
The difference with the previous PR is given by https://github.com/cjgillot/rust/compare/hirene-ty...hirene
A few more cleanups may be necessary, please tell me.
r? @eddyb like the other
cc @Zoxc
expand: Simplify expansion of derives
And make it more uniform with other macros.
This is done by merging placeholders for future derives' outputs into the derive container's output fragment early (addressing FIXMEs from https://github.com/rust-lang/rust/pull/63667).
Also, macros with names starting with `_` are no longer reported as unused, in accordance with the usual behavior of `unused` lints.
r? @matthewjasper or @mark-i-m
