Closed
Description
This issue tracks the release notes text for #122792.
- Issue is nominated for the responsible team (and T-release nomination is removed).
- Proposed text is drafted by team responsible for underlying change.
- Issue is nominated for release team review of clarity for wider audience.
- Release team includes text in release notes/blog posts.
Release notes text:
The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.
# Language
- [Patterns matching empty types can now be omitted in common cases](https://github.com/rust-lang/rust/pull/122792)Release blog section (if any, leave blank if no section is expected):
Empty patterns can now be omitted in common cases:
```rust
pub fn safe_unwrap<T>(x: Result<T, !>) -> T {
let Ok(x) = x; // the `Err` case does not need to appear
x
}
// TODO: other examples, maybe std's PoisonError?
```
This is particularly useful in combination with the never type `!`, but also works with other empty types such a variantless enum (`enum Void {}`).
For reasons related to uninitialized values and unsafe code, this behavior is not allowed if the empty type is accessed through a reference, pointer, or union field:
```rust
pub fn safe_unwrap_ref<T>(x: &Result<T, !>) -> &T {
match x {
Ok(x) => x,
Err(never) => match *never {}, // this branch cannot be omitted because of the reference
}
}
```
To avoid interfering with crates that wish to support several rust versions, these branches are not yet warned as "unreachable", despite the fact that they can be removed.