Since rustc doesn't do the assembly parsing itself, it is unable
to detect when inline assembly ends with an instruction prefix,
which doesn't make sense since it would apply to instructions from
the compiler. This fixes rust-lang#82314 by mentioning that x86 instruction
prefixes must not be used in inline assembly.

Inspired by rust-lang#90521 , and by the
various `must_use` PRs; in all of those cases, the submitter of the PR
could know that `relnotes` applied, but couldn't apply it themselves.

People should be able to help triage by observing that a change needs an
FCP before we can apply it.

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

… r=jackh726

Suggest `extern crate alloc` when using undeclared module `alloc`

closes rust-lang#90136

…r=notriddle

Simplify js tester a bit

r? `````@notriddle`````

Add note about x86 instruction prefixes in asm! to unstable book

Since rustc doesn't do the assembly parsing itself, it is unable to detect when inline assembly ends with an instruction prefix, which doesn't make sense since it would apply to instructions from the compiler. This fixes rust-lang#82314 by mentioning that x86 instruction prefixes must not be used in inline assembly. AFAICT x86 is the only instruction set with instruction prefixes.

…ure, r=Amanieu

Update aarch64 `target_feature` list for LLVM 12.

Many of these feature are now available on all valid LLVM versions.

I've also added a few new ones to the list.

r? `@Amanieu`

…henkov

Demote metadata load warning to "info".

There is a warn log message for whenever the crate loader fails to load metadata from a candidate file. I think this warning is too aggressive, as there are several situations where metadata information might not be found in a candidate file, which is normal. Also, this warning is somewhat confusing, and non-actionable in most cases for a user (most users will not know what it means).

If the crate loader ultimately does not find a valid crate, then an error will be reported (and hopefully rust-lang#88368 will improve that error message).

If a rustc developer wants to debug a loader problem, they can still use `RUSTC_LOG=rustc_metadata=debug` and get the details.

There is more discussion of this particular warning at rust-lang#89795 (comment).

Fixes rust-lang#90525

…oshtriplett

Clean up some `-Z unstable-options` in tests.

Several of these tests were for features that have been stabilized, or otherwise don't need `-Z unstable-options`.

…riplett

Add more text and examples to `carrying_{add|mul}`

`feature(bigint_helper_methods)` tracking issue rust-lang#85532

cc `````@clarfonthey`````

…r=Mark-Simulacrum

rustbot allow labels

`relnotes` was inspired by rust-lang#90521 , and by the various `must_use` PRs; in all of those cases, the submitter of the PR could know that `relnotes` applied, but couldn't apply it themselves.

For `needs-fcp`, I think people should be able to help triage by observing that a change needs an FCP before we can apply it.

…r-sidebar, r=jsha

Fix missing bottom border for headings in sidebar

Fixes rust-lang#90568.

r? ```@jsha```