I believe rustdoc should not be conflating private items (visibility
lower than `pub`) and hidden items (attribute `doc(hidden)`). This
matters now that Cargo is passing --document-private-items by default
for bin crates. In bin crates that rely on macros, intentionally hidden
implementation details of the macros can overwhelm the actual useful
internal API that one would want to document.

This PR restores the strip-hidden pass when documenting private items,
and introduces a separate unstable --document-hidden-items option to
skip the strip-hidden pass. The two options are orthogonal to one
another.

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.

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

Distinguish between private items and hidden items in rustdoc

I believe rustdoc should not be conflating private items (visibility lower than `pub`) and hidden items (attribute `doc(hidden)`). This matters now that Cargo is passing --document-private-items by default for bin crates. In bin crates that rely on macros, intentionally hidden implementation details of the macros can overwhelm the actual useful internal API that one would want to document.

This PR restores the strip-hidden pass when documenting private items, and introduces a separate unstable --document-hidden-items option to skip the strip-hidden pass. The two options are orthogonal to one another.

Fixes rust-lang#67851. Closes rust-lang#60884.

…gisa

Split MIR building into its own crate

This moves `rustc_mir::{build, hair, lints}` to `rustc_mir_build`.
The new crate only has a `provide` function as it's public API.

Based on rust-lang#67898

cc @Centril @rust-lang/compiler
r? @oli-obk

Updates for VxWorks

r? @alexcrichton