…ex, r=dtolnay

Static mutex is static

StaticMutex is only ever used with as a static (as the name already suggests). So it doesn't have to be generic over a lifetime, but can simply assume 'static.

This 'static lifetime guarantees the object is never moved, so this is no longer a manually checked requirement for unsafe calls to lock().

@rustbot modify labels: +T-libs +A-concurrency +C-cleanup

…ex, r=dtolnay

Static mutex is static

StaticMutex is only ever used with as a static (as the name already suggests). So it doesn't have to be generic over a lifetime, but can simply assume 'static.

This 'static lifetime guarantees the object is never moved, so this is no longer a manually checked requirement for unsafe calls to lock().

@rustbot modify labels: +T-libs +A-concurrency +C-cleanup

Rollup of 14 pull requests

Successful merges:

 - rust-lang#75023 (ensure arguments are included in count mismatch span)
 - rust-lang#75265 (Add `str::{Split,RSplit,SplitN,RSplitN,SplitTerminator,RSplitTerminator,SplitInclusive}::as_str` methods)
 - rust-lang#75675 (mangling: mangle impl params w/ v0 scheme)
 - rust-lang#76084 (Refactor io/buffered.rs into submodules)
 - rust-lang#76119 (Stabilize move_ref_pattern)
 - rust-lang#77493 (ICEs should always print the top of the query stack)
 - rust-lang#77619 (Use futex-based thread-parker for Wasm32.)
 - rust-lang#77646 (For backtrace, use StaticMutex instead of a raw sys Mutex.)
 - rust-lang#77648 (Static mutex is static)
 - rust-lang#77657 (Cleanup cloudabi mutexes and condvars)
 - rust-lang#77672 (Simplify doc-cfg rendering based on the current context)
 - rust-lang#77780 (rustc_parse: fix spans on cast and range exprs with attrs)
 - rust-lang#77935 (BTreeMap: make PartialCmp/PartialEq explicit and tested)
 - rust-lang#77980 (Fix intra doc link for needs_drop)

Failed merges:

r? `@ghost`

…=Mark-Simulacrum

Enforce no-move rule of ReentrantMutex using Pin and fix UB in stdio

A `sys_common::ReentrantMutex` may not be moved after initializing it with `.init()`. This was not enforced, but only stated as a requirement in the comments on the unsafe functions. This change enforces this no-moving rule using `Pin`, by changing `&self` to a `Pin` in the `init()` and `lock()` functions.

This uncovered a bug I introduced in rust-lang#77154: stdio.rs (the only user of ReentrantMutex) called `init()` on its ReentrantMutexes while constructing them in the intializer of `SyncOnceCell::get_or_init`, which would move them afterwards. Interestingly, the ReentrantMutex unit tests already had the same bug, so this invalid usage has been tested on all (CI-tested) platforms for a long time. Apparently this doesn't break badly on any of the major platforms, but it does break the rules.\*

To be able to keep using SyncOnceCell, this adds a `SyncOnceCell::get_or_init_pin` function, which makes it possible to work with pinned values inside a (pinned) SyncOnceCell. Whether this function should be public or not and what its exact behaviour and interface should be if it would be public is something I'd like to leave for a separate issue or PR. In this PR, this function is internal-only and marked with `pub(crate)`.

\* Note: That bug is now included in 1.48, while this patch can only make it to ~~1.49~~ 1.50. We should consider the implications of 1.48 shipping with a wrong usage of `pthread_mutex_t` / `CRITICAL_SECTION` / .. which technically invokes UB according to their specification. The risk is very low, considering the objects are not 'used' (locked) before the move, and the ReentrantMutex unit tests have verified this works fine in practice.

Edit: This has been backported and included in 1.48. And soon 1.49 too.

---

In future changes, I want to push this usage of Pin further inside `sys` instead of only `sys_common`, and apply it to all 'unmovable' objects there (`Mutex`, `Condvar`, `RwLock`). Also, while `sys_common`'s mutexes and condvars are already taken care of by rust-lang#77147 and rust-lang#77648, its `RwLock` should still be made movable or get pinned.