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Stabilized APIs and Mismatched lifetime syntaxes lint
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‎content/Rust-1.89.0.md

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@@ -45,7 +45,81 @@ pub const MY_BITSET: [bool; _] = make_bitset::<10>();
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### Mismatched lifetimes syntax lint
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TC/LANG to write this
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### Mismatched lifetime syntaxes lint
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[Lifetime elision][elision] in function signatures is an ergonomic aspect of the Rust language, but it can also be a stumbling point for newcomers and experts alike. This is especially true when lifetimes are inferred in types where it isn't syntactically obvious that a lifetime is even present:
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```rust
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// The returned type `std::slice::Iter` has a lifetime,
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// but there's no visual indication of that.
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//
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// Lifetime elision infers the lifetime of the return
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// value to be the same as the argument `scores`.
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fn items(scores: &[u8]) -> std::slice::Iter<u8> {
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scores.iter()
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}
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```
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Code like this will now produce a warning by default:
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```text
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warning: hiding a lifetime that's elided elsewhere is confusing
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--> src/lib.rs:1:18
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|
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1 | fn items(scores: &[u8]) -> std::slice::Iter<u8> {
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| ^^^^^ -------------------- the same lifetime is hidden here
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| |
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| the lifetime is elided here
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|
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= help: the same lifetime is referred to in inconsistent ways, making the signature confusing
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= note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
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help: use `'_` for type paths
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|
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1 | fn items(scores: &[u8]) -> std::slice::Iter<'_, u8> {
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| +++
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```
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We [first attempted][elided_lifetime_in_path] to improve this situation back in 2018 as part of the [`rust_2018_idioms`][2018-by-default] lint group, but [strong feedback][bevy] about the `elided_lifetimes_in_paths` lint showed that it was too blunt of a hammer as it warns about lifetimes which don't matter to understand the function:
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```rust
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use std::fmt;
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struct Greeting;
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impl fmt::Display for Greeting {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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// -----^^^^^^^^^ expected lifetime parameter
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// Knowing that `Formatter` has a lifetime does not help the programmer
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"howdy".fmt(f)
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}
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}
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```
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We then realized that the confusion we want to eliminate occurs when both
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1. lifetime elision inference rules *connect* an input lifetime to an output lifetime
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2. it's not syntactically obvious that a lifetime exists
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There are two pieces of Rust syntax that indicate that a lifetime exists: `&` and `'`, with `'` being subdivided into the inferred lifetime `'_` and named lifetimes `'a`. When a type uses a named lifetime, lifetime elision will not infer a lifetime for that type. Using these criteria, we can construct three groups:
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| Self-evident it has a lifetime | Allow lifetime elision to infer a lifetime | Examples |
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|--------------------------------|--------------------------------------------|---------------------------------------|
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| No | Yes | `ContainsLifetime` |
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| Yes | Yes | `&T`, `&'_ T`, `ContainsLifetime<'_>` |
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| Yes | No | `&'a T`, `ContainsLifetime<'a>` |
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The `mismatched_lifetime_syntaxes` lint checks that the inputs and outputs of a function belong to the same group. For the initial motivating example above, `&[u8]` falls into the second group while `std::slice::Iter<u8>` falls into the first group. We say that the lifetimes in the first group are *hidden*.
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Because the input and output lifetimes belong to different groups, the lint will warn about this function, reducing confusion about when a value has a meaningful lifetime that isn't visually obvious.
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The `mismatched_lifetime_syntaxes` lint supersedes the `elided_named_lifetimes` lint, which did something similar for named lifetimes specifically.
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Future work on the `elided_lifetimes_in_paths` lint intends to split it into more focused sub-lints with an eye to warning about a subset of them eventually.
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[elision]: https://doc.rust-lang.org/1.89/book/ch10-03-lifetime-syntax.html#lifetime-elision
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[elided_lifetime_in_path]: https://github.com/rust-lang/rust/pull/46254
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[2018-by-default]: https://github.com/rust-lang/rust/issues/54910
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[bevy]: https://github.com/rust-lang/rust/issues/131725
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### More x86 target features
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@@ -67,7 +141,31 @@ Refer to Rust’s [platform support page][platform_support_page] for more inform
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### Stabilized APIs
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TODO
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- [`NonZero<char>`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html)
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- Many intrinsics for x86, not enumerated here
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- [AVX512 intrinsics](https://github.com/rust-lang/rust/issues/111137)
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- [`SHA512`, `SM3` and `SM4` intrinsics](https://github.com/rust-lang/rust/issues/126624)
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- [`File::lock`](https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.lock)
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- [`File::lock_shared`](https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.lock_shared)
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- [`File::try_lock`](https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.try_lock)
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- [`File::try_lock_shared`](https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.try_lock_shared)
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- [`File::unlock`](https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.unlock)
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- [`NonNull::from_ref`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.from_ref)
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- [`NonNull::from_mut`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.from_mut)
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- [`NonNull::without_provenance`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.without_provenance)
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- [`NonNull::with_exposed_provenance`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.with_exposed_provenance)
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- [`NonNull::expose_provenance`](https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.expose_provenance)
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- [`OsString::leak`](https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.leak)
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- [`PathBuf::leak`](https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.leak)
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- [`Result::flatten`](https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.flatten)
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- [`std::os::linux::net::TcpStreamExt::quickack`](https://doc.rust-lang.org/stable/std/os/linux/net/trait.TcpStreamExt.html#tymethod.quickack)
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- [`std::os::linux::net::TcpStreamExt::set_quickack`](https://doc.rust-lang.org/stable/std/os/linux/net/trait.TcpStreamExt.html#tymethod.set_quickack)
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These previously stable APIs are now stable in const contexts:
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- [`<[T; N]>::as_mut_slice`](https://doc.rust-lang.org/stable/std/primitive.array.html#method.as_mut_slice)
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- [`<[u8]>::eq_ignore_ascii_case`](https://doc.rust-lang.org/stable/std/primitive.slice.html#impl-%5Bu8%5D/method.eq_ignore_ascii_case)
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- [`str::eq_ignore_ascii_case`](https://doc.rust-lang.org/stable/std/primitive.str.html#impl-str/method.eq_ignore_ascii_case)
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### Other changes
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