…arnings

`--extern proc_macro` is used to add the proc_macro crate to the extern
prelude for all procmacros. In general pathless `--extern` only references
sysroot/standard libraries and so should be exempt from
unused-crate-dependencies warnings.

split select into submodules

https://github.com/rust-lang/rust/blob/a0f06d11ae7fe29c8e263e2a5d8cd41b372f0283/src/librustc_trait_selection/traits/select.rs#L1

I extracted two submodules:

- confirmation: apart from `pub(super) fn confirm_candidate`, everything else is private
- candidate_assembly: exports `pub(super) fn candidate_from_obligation` and `pub(super) fn assemble_candidates`

I tried to change as little as possible while doing this and hopefully split this into well reviewable commits.

…nkov

Don't count pathless --extern for unused-crate-dependencies warnings

`--extern proc_macro` is used to add the proc_macro crate to the extern
prelude for all procmacros. In general pathless `--extern` only references
sysroot/standard libraries and so should be exempt from
unused-crate-dependencies warnings.

r? @petrochenkov

…=shepmaster

Stabilize `std::io::Buf{Reader, Writer}::capacity`

Closes rust-lang#68833
FCP is done here: rust-lang#68833 (comment)

…an-DPC

clean up E0641 explanation

r? @Dylan-DPC

…acrum

Updated documentation for Control Flow Guard

Update user-facing documentation for the Control Flow Guard (CFG) exploit mitigation in the unstable book, as requested in rust-lang#68793.