In Rust for Linux we are using these to make `alloc` a bit
more modular.

A `run-make-fulldeps` test is added for each of them, so that
enabling each of them independently is kept in a compilable state.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

The prose talked about doing this, so might as well add a simple code example of it too.

State up-front and center what shape the returned extension will have, without
making the user read through the description and examples.

Rationale: Various frameworks and libraries for different platforms have their
different conventions as to whether an "extension" is ".ext" or just "ext" and
anyone that's had to deal with this ambiguity in the past is always double- or
triple-checking to make sure the function call returns an extension that matches
the expected semantics. Offer the answer to this important question right off
the bat instead of making them dig to find it.

It's now much more like the `-Zhir-stats` output.
- Each line is preceded with `meta-stats`, which makes the provenance
  clearer and allows filtering of the output.
- Sections are now sorted in reverse order of size.
- Column headings avoid the need to repeat the word "bytes" on every line.
- Long numbers now have `_` separators for easier reading.
- Consistent use of '-' within section labels, rather than a mix of '-',
  '_', and ' '.

The code itself is shorter and easier to read thanks to:
- the `stat` macro, which encapsulates each section's encoding, avoids
  some boilerplate, and removes the need for some low-value comments;
- the `stats` vector, which replaces dozens of local variables.

`alloc`: add unstable cfg features `no_rc` and `no_sync`

In Rust for Linux we are using these to make `alloc` a bit more modular.

See rust-lang#86048 and rust-lang#84266 for similar requests.

Of course, the particular names are not important.

…ark-Simulacrum

Add another example for `uN::carrying_mul`

The prose talks about doing this, so might as well add a simple code example of it too.

…race, r=tmandry

Adding ignore fuchsia tests for Backtrace, ErrorKind cases

Tests where Backtrace parses are required (invalid since Fuchsia backtraces are not symbolized), and test where ErrorKind is not properly translated from a Fuchsia-style to Unix-style error code

cc. ```@djkoloski```

r? ```@tmandry```

…bjorn3

Improve `-Zmeta-stats` some more

A follow-up to rust-lang#97384.

r? ```@bjorn3```

…, r=oli-obk

fix ConstProp handling of written_only_inside_own_block_locals

Fixes a regression introduced by rust-lang#100239, which adds an early return and thus skips some code in `visit_terminator` that must be run for soundness.

Fixes rust-lang#101973

Clarify Path::extension() semantics in docs abstract

State up-front and center what shape the returned extension will have, without making the user read through the description and examples.

This is a doc-only change. There are no changes to the API contract and the clarification is in line with what was already stated/promised in the existing doc text - just clarified, summarized, and served bright and early.

Rationale: Various frameworks and libraries for different platforms have their different conventions as to whether an "extension" is ".ext" or just "ext" and anyone that's had to deal with this ambiguity in the past is always double- or triple-checking to make sure the function call returns an extension that matches the expected semantics. Offer the answer to this important question right off the bat instead of making them dig to find it.

```@rustbot``` label +A-docs

…ckh726

Use rebind instead of dummy binder in `SameTypeModuloInfer` relation

Lazy binder usage (sorry, my fault) + assertion in `Binder::dummy`

Fixes rust-lang#101984