This avoids adding archives multiple times, which results in duplicate
objects in the resulting rlib, leading to symbol collision and link
failures. This could happen when crate contains multiple link attributes
that all reference the same archive.

RFC 2457 declares: "A `non_ascii_idents` lint is added to the
compiler. This lint is allow by default."

They always end up interned anyway

…=Manishearth

`non_ascii_idents` lint (part of RFC 2457)

RFC 2457 [declares](https://github.com/rust-lang/rfcs/blob/121bbeff500c3274cea22c7e0ca176274d592646/text/2457-non-ascii-idents.md): "A `non_ascii_idents` lint is added to the compiler. This lint is allow by default."

(Part of rust-lang#55467.)

r? @Manishearth

Support stability and deprecation checking for all macros

RELNOTES: Deprecation attributes on macros now have effect.

Fixes rust-lang#34079
Fixes rust-lang#49912
Unblocks rust-lang#62086
Unblocks rust-lang#61000

…laumeGomez

rustdoc: set cfg(doctest) when collecting doctests

Note: This PR builds on top of rust-lang#61199; only the last commit is specific to this PR.

As discussed in rust-lang#61199, we want the ability to isolate items to only when rustdoc is collecting doctests, but we can't use `cfg(test)` because of libcore's `#![cfg(not(test))]`. This PR proposes a new cfg flag, `cfg(doctest)`, specific to this situation, rather than reusing an existing flag. I've isolated it behind a feature gate so that we can contain the effects to nightly only. (A stable workaround that can be used in lieu of `#[cfg(doctest)]` is `#[cfg(rustdoc)] #[doc(hidden)]`, at least once rust-lang#61351 lands.)

Tracking issue: rust-lang#62210

…es, r=cramertj

Check if the archive has already been added to avoid duplicates

This avoids adding archives multiple times, which results in duplicate
objects in the resulting rlib, leading to symbol collision and link
failures. This could happen when crate contains multiple link attributes
that all reference the same archive.