Stops unavoidable `missing_docs` warning/error on proc-macro crates.
Resolves rust-lang#42008.

impl_trait_in_bindings is not yet complete and can lead to compiler crashes.

Fixes rust-lang#60764.

…=pnkfelix

Explain error when yielding a reference to a local variable

Closes rust-lang#56508

coretest: Downgrade deny to warn

The `deny` causes a build failure in https://github.com/RalfJung/miri-test-libstd. Since we use `-D warnings` for rustc builds, `warn` should be enough to lead to compile errors here, without impeding external builds.

…s, r=alexcrichton

Add #[doc(hidden)] attribute on compiler generated module.

Resolves unavoidable `missing_docs` warning/error on proc-macro crates.
Resolves rust-lang#42008.

Changes not yet tested locally, however I wanted to submit first since `rustc` takes forever to compile.

… r=matthewjasper

Use `delay_span_bug` for error cases when checking `AnonConst` parent

Fixes rust-lang#60704.
Fixes rust-lang#60650.

add impl_trait_in_bindings to INCOMPLETE_FEATURES

impl_trait_in_bindings is not yet complete and can lead to compiler crashes.

Fixes rust-lang#60764.