Rollup merge of #144236 - yoshuawuyts:drop-guard, r=Mark-Simulacrum

Add `core::mem::DropGuard`

## 1.0 Summary

This PR introduces a new type `core::mem::DropGuard` which wraps a value and runs a closure when the value is dropped.

```rust
use core::mem::DropGuard;

// Create a new guard around a string that will
// print its value when dropped.
let s = String::from("Chashu likes tuna");
let mut s = DropGuard::new(s, |s| println!("{s}"));

// Modify the string contained in the guard.
s.push_str("!!!");

// The guard will be dropped here, printing:
// "Chashu likes tuna!!!"
```

## 2.0 Motivation

A number of programming languages include constructs like `try..finally` or `defer` to run code as the last piece of a particular sequence, regardless of whether an error occurred. This is typically used to clean up resources, like closing files, freeing memory, or unlocking resources. In Rust we use the `Drop` trait instead, allowing us to [never having to manually close sockets](https://blog.skylight.io/rust-means-never-having-to-close-a-socket/).

While `Drop` (and RAII in general) has been working incredibly well for Rust in general, sometimes it can be a little verbose to setup. In particular when upholding invariants are local to functions, having a quick inline way to setup an `impl Drop` can be incredibly convenient. We can see this in use in the Rust stdlib, which has a number of private `DropGuard` impls used internally:

- [library/alloc/src/vec/drain.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/vec/drain.rs#L177)
- [library/alloc/src/boxed/thin.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/boxed/thin.rs#L362)
- [library/alloc/src/slice.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/slice.rs#L413)
- [library/alloc/src/collections/linked_list.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/collections/linked_list.rs#L1135)
- [library/alloc/src/collections/binary_heap/mod.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/collections/binary_heap/mod.rs#L1816)
- [library/alloc/src/collections/btree/map.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/collections/btree/map.rs#L1715)
- [library/alloc/src/collections/vec_deque/drain.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/collections/vec_deque/drain.rs#L95)
- [library/alloc/src/vec/into_iter.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/alloc/src/vec/into_iter.rs#L488)
- [library/std/src/os/windows/process.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/library/std/src/os/windows/process.rs#L320)
- [tests/ui/process/win-proc-thread-attributes.rs](https://github.com/rust-lang/rust/blob/9982d6462bedf1e793f7b2dbd655a4e57cdf67d4/tests/ui/process/win-proc-thread-attributes.rs#L17)

## 3.0 Design

This PR implements what can be considered about the simplest possible design:

1. A single type `DropGuard` which takes both a generic type `T` and a closure `F`.
2. `Deref` + `DerefMut` impls to make it easy to work with the `T` in the guard.
3. An `impl Drop` on the guard which calls the closure `F` on drop.
4. An inherent `fn into_inner` which takes the type `T` out of the guard without calling the closure `F`.

Notably this design does not allow divergent behavior based on the type of drop that has occurred. The [`scopeguard` crate](https://docs.rs/scopeguard/latest/scopeguard/index.html) includes additional `on_success` and `on_onwind` variants which can be used to branch on unwind behavior instead. However [in a lot of cases](rust-lang/rust#143612 (comment)) this doesn’t seem necessary, and using the arm/disarm pattern seems to provide much the same functionality:

```rust
let guard = DropGuard::new((), |s| ...);  // 1. Arm the guard
other_function();                         // 2. Perform operations
guard.into_inner();                       // 3. Disarm the guard
```

`DropGuard` combined with this pattern seems like it should cover the vast majority of use cases for quick, inline destructors. It certainly seems like it should cover all existing uses in the stdlib, as well as all existing uses in crates like [hashbrown](https://github.com/search?q=repo%3Arust-lang%2Fhashbrown%20guard&type=code).

## 4.0 Acknowledgements

This implementation is based on the [mini-scopeguard crate](https://github.com/yoshuawuyts/mini-scopeguard) which in turn is based on the [scopeguard  crate](https://docs.rs/scopeguard). The implementations only differ superficially; because of the nature of the problem there is only really one obvious way to structure the solution. And the scopeguard crate got that right!

## 5.0 Conclusion

This PR adds a new type `core::mem::DropGuard` to the stdlib which adds a small convenience helper to create inline destructors with. This would bring the majority of the functionality of the `scopeguard` crate into the stdlib, which is the [49th most downloaded crate](https://crates.io/crates?sort=downloads) on crates.io (387 million downloads).

Given the actual implementation of `DropGuard` is only around 60 lines, it seems to hit that sweet spot of low-complexity / high-impact that makes for a particularly efficient stdlib addition. Which is why I’m putting this forward for consideration; thanks!

Author: Zalathar