…or std/thread

Latter is simpler

This made too many assumptions about platforms, breaking some things.

Will need to be added later with a better way of gating on proper
alignment, without hardcoding cfg(target_arch)s.

Update the tracking issue number

Updated the documentation for `UnsafeCell`

Address review comments

Address more review comments + minor changes

test is already in scope, no need to use the full path

- Allow including defaults in `src/bootstrap/defaults` using `profile = "..."`
- Add default config files
- Combine config files using the merge dependency.
- Add comments to default config files
- Add a README asking to open an issue if the defaults are bad
- Give a loud error if trying to merge `.target`, since it's not
  currently supported
- Use an exhaustive match
- Use `<none>` in config.toml.example to avoid confusion
- Fix bugs in `Merge` derives

Previously, it would completely ignore the profile defaults if there
were any settings in `config.toml`. I sent an email to the `merge` maintainer
asking them to make the behavior in this commit the default.

This introduces a new dependency on `merge` that hasn't yet been vetted.

I want to improve the output when `include = "x"` isn't found:

```
thread 'main' panicked at 'fs::read_to_string(&file) failed with No such file or directory (os error 2) ("configuration file did not exist")', src/bootstrap/config.rs:522:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failed to run: /home/joshua/rustc/build/bootstrap/debug/bootstrap test tidy
Build completed unsuccessfully in 0:00:00
```

However that seems like it could be fixed in a follow-up.

…n-unsafe-fn, r=joshtriplett

Std/thread: deny unsafe op in unsafe fn

Partial fix of rust-lang#73904.

This encloses `unsafe` operations in `unsafe fn` in `libstd/thread`.
@rustbot modify labels: F-unsafe-block-in-unsafe-fn

…-Simulacrum

Add sample defaults for config.toml

- Allow including defaults in `src/bootstrap/defaults` using `profile = "..."`.
- Add default config files, with a README noting they're experimental and asking you to open an issue if you run into trouble. The config files have comments explaining why the defaults are set.
- Combine config files using the `merge` dependency.

This introduces a new dependency on `merge` that hasn't yet been vetted.

I want to improve the output when `include = "x"` isn't found:

```
thread 'main' panicked at 'fs::read_to_string(&file) failed with No such file or directory (os error 2) ("configuration file did not exist")', src/bootstrap/config.rs:522:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failed to run: /home/joshua/rustc/build/bootstrap/debug/bootstrap test tidy
Build completed unsuccessfully in 0:00:00
```

However that seems like it could be fixed in a follow-up.

Closes rust-lang#76619

…=jyn514

Use intra-doc links in core/src/iter when possible

Helps with rust-lang#75080.

I also updated lots of links to use `fn()` instead of `fn` when possible.

@rustbot modify labels: T-doc A-intra-doc-links

r? @jyn514

…jyn514

Finish moving to intra doc links for std::sync

Helps with rust-lang#75080.

@rustbot modify labels: T-doc A-intra-doc-links

r? @jyn514

…eddyb

Remove DeclareMethods

Most of the `DeclareMethods` API was only used internally by rustc_codegen_llvm. As such, it makes no sense to require other backends to implement them.

(`get_declared_value` and `declare_cfn` were used, in one place, specific to the `main` symbol, which I've replaced with a more specialized function to allow more flexibility in implementation - the intent is that `declare_c_main` can go away once we do something more clever, e.g. @eddyb has ideas around having a MIR shim or somesuch we can explore in a follow-up PR)

…mut, r=RalfJung

Add non-`unsafe` `.get_mut()` for `Unsafecell`

  - Tracking issue: rust-lang#76943

As discussed in: https://internals.rust-lang.org/t/add-non-unsafe-get-mut-for-unsafecell/12407

  - ### [Rendered documentation](https://modest-dubinsky-1f9f47.netlify.app/core/cell/struct.unsafecell)

This PR tries to move the sound `&mut UnsafeCell<T> -> &mut T` projection that all the "downstream" constructions were already relying on, up to the root abstraction, where it rightfully belongs, and officially blessing it.

  - this **helps reduce the amount of `unsafe` snippets out there** (_c.f._, the second commit of this PR: rust-lang@09503fd)

The fact that this getter is now expose for `UnsafeCell<T>` itself, will also help convey the idea that **`UnsafeCell` is not magical _w.r.t._ `&mut` accesses**, contrary to what some people incorrectly think.

  - Even the standard library itself at some point had such a confusion, _c.f._ this comment where there is a mention of multi-threaded (and thus _shared_) access despite dealing with exclusive references over unique ownership: https://github.com/rust-lang/rust/blob/59fb88d061544a035f3043b47594b34789204cee/library/core/src/cell.rs#L498-L499

r? @RalfJung

Replace manual as_nanos and as_secs_f64 reimplementations

Replace write_fmt with write!

Latter is simpler

Add test for issue rust-lang#34634

Closes rust-lang#34634

Use const_cstr macro in consts.rs

Remove unused static_assert macro

…mic-from-mut, r=kodrAus

Revert adding Atomic::from_mut.

This reverts rust-lang#74532, which made too many assumptions about platforms, breaking some things.

Will need to be added later with a better way of gating on proper alignment, without hardcoding cfg(target_arch)s.

---

To be merged if fixing from_mut (rust-lang#76965) takes too long.

r? @ghost

Add a regression test for copy propagation miscompilation

liballoc bench use imported path Bencher

test is already in scope, no need to use the full path

…mulacrum

BTreeMap: extra testing & fixed comments

r? @Mark-Simulacrum