Tracking issue for release notes of #159564: Allow elided ('static) lifetimes in thread_local! #163353

# Other
- [`'static` lifetimes in `thread_local!` are now elided like in ordinary `static` variables](https://github.com/rust-lang/rust/pull/159564)

# `'static` lifetimes in `thread_local!`

While the `thread_local!` macro is designed to mimic ordinary Rust syntax, it has one case that hasn't been updated until now! For example, consider the below `'static` variable:

```rust
static IMPORTANT_STRING: RefCell<Cow<'_, str>> = RefCell::new(Cow::Borrowed("hello world!"));
```

Because this is a `static` variable, it automatically elides `'static` lifetimes, meaning that the `'_` lifetime is automatically assumed to be `'static`. But for a thread-local:

```rust
thread_local! {
    static IMPORTANT_STRING: RefCell<Cow<'static, str>> = RefCell::new(Cow::Borrowed("hello world!"));
}
```

Until Rust 1.100, you had to explicitly specify the lifetime due to a limitation of the macro. Now, you can finally write:

```rust
thread_local! {
    static IMPORTANT_STRING: RefCell<Cow<'_, str>> = RefCell::new(Cow::Borrowed("hello world!"));
}
```

While code may want to always specify these lifetimes to ensure compatibility on old Rust versions, at least on the latest version of Rust, we've closed this gap between `thread_local!` variables and normal `static` variables.