Rollup of 16 pull requests

Successful merges:

 - #53311 (Window Mutex: Document that we properly initialize the SRWLock)
 - #53503 (Discourage overuse of mem::forget)
 - #53545 (Fix #50865: ICE on impl-trait returning functions reaching private items)
 - #53559 (add macro check for lint)
 - #53562 (Lament the invincibility of the Turbofish)
 - #53563 (use String::new() instead of String::from(""), "".to_string(), "".to_owned() or "".into())
 - #53592 (docs: minor stylistic changes to str/string docs)
 - #53594 (Update RELEASES.md to include clippy-preview)
 - #53600 (Fix a grammatical mistake in "expected generic arguments" errors)
 - #53614 (update nomicon and book)
 - #53617 (tidy: Stop requiring a license header)
 - #53618 (Add missing fmt examples)
 - #53636 (Prefer `.nth(n)` over `.skip(n).next()`.)
 - #53644 (Use SmallVec for SmallCStr)
 - #53664 (Remove unnecessary closure in rustc_mir/build/mod.rs)
 - #53666 (Added rustc_codegen_llvm to compiler documentation.)

Added rustc_codegen_llvm to compiler documentation.

Fixes #51737.

Fix #50865: ICE on impl-trait returning functions reaching private items

Adds a test case as suggested in #50865, and implements @petrochenkov's suggestion. Fixes #50865.

Impl-trait-returning functions are marked under a new (low) access level, which they propagate rather than `AccessLevels::Reachable`. `AccessLevels::is_reachable` returns false for such items (leaving stability analysis unaffected), these items may still be visible to the lints phase however.

Stabilize 'attr_literals' feature.

RFC Issue: rust-lang/rfcs#1559
Tracking Issue: rust-lang/rust#34981

Reference PR: https://github.com/rust-lang-nursery/reference/pull/388.

Remove unnecessary closure in rustc_mir/build/mod.rs

Fixes #53608

Use SmallVec for SmallCStr

This reuses the awesome optimizations from Servo's `SmallVec` to speed up `SmallCStr`.

Prefer `.nth(n)` over `.skip(n).next()`.

Found by clippy.

Add missing fmt examples

r? @QuietMisdreavus

tidy: Stop requiring a license header

Previously approved in rust-lang/rust#43498 ; update tidy to match.

update nomicon and book

Fix a grammatical mistake in "expected generic arguments" errors

r? @EddyB

Update RELEASES.md to include clippy-preview

[Rendered](https://github.com/rust-lang/rust/blob/Aaronepower-patch-1/RELEASES.md#misc)

r? @Mark-Simulacrum

docs: minor stylistic changes to str/string docs

std::string::String.repeat(): slightly rephrase to be more in-line with other descriptions.

add ticks around a few keywords in other descriptions.

Address two regressions

Remove assert checking that expansion data is immutable until I have time to investigate why it's firing, cc https://github.com/rust-lang/rust/issues/52363
Turn the error for module-relative access to macro-expanded `macro_export` macros into a deprecation lint, closes https://github.com/rust-lang/rust/issues/53495

use String::new() instead of String::from(""), "".to_string(), "".to_owned() or "".into()

Lament the invincibility of the Turbofish

Here a test case is added to ensure that any others attempting to drive the Turbofish to extinction have second thoughts. Previously the [entire test suite would succeed](https://github.com/rust-lang/rust/pull/53511) if generic arguments were accepted without disambiguation, making for [confusing and heartbreaking circumstances](https://github.com/rust-lang/rfcs/pull/2527).

add macro check for lint

Fixes #53537

Discourage overuse of mem::forget

Some uses of `mem::forget` have been replaced by better methods of `Box`, so I've removed obsoleted use-cases from these docs.

I've removed emphasis on leaking, because it's not obvious `mem::forget` does not guarantee leaking of memory: memory of stack-allocated objects and values partially moved out of `Box` will still be freed. That's a potential error when used to pass objects to FFI, so it's better to direct users to `Box::into_raw` instead.

Window Mutex: Document that we properly initialize the SRWLock

See https://github.com/rust-lang/rust/issues/35836

Turn the error for module-relative access to macro-expanded `macro_export` macros into a deprecation lint

This reverts commit b15785b6.

Update clippy

r? @oli-obk @Manishearth

Auto merge of #53588 - tristanburgess:52985_diagnostics_no_concrete_type_behind_existential_type, r=oli-obk

52985 diagnostics no concrete type behind existential type

@oli-obk FYI. See below for new cycle error generated.

```rust
error[E0391]: cycle detected when processing `Foo`
 --> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:3:1
  |
3 | existential type Foo: Copy;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: ...which requires processing `bar`...
 --> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:6:23
  |
6 |   fn bar(x: Foo) -> Foo {
  |  _______________________^
7 | |     x
8 | | }
  | |_^
  = note: ...which again requires processing `Foo`, completing the cycle

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.
```

Fix unsoundness for VecDeque

 See individual commit for more details.

r? @RalfJung.

Fixes https://github.com/rust-lang/rust/issues/53566, fixes https://github.com/rust-lang/rust/issues/53529

Use optimized SmallVec implementation

This PR replaces current SmallVec implementation with the one from the Servo project.

Closes https://github.com/rust-lang/rust/issues/51640

r? @Mark-Simulacrum