Description
We're planning to stabilize #![feature(precise_capturing)] ahead of Rust 2024 (in all editions). This feature allows one to write:
fn captures<'a: 'a, 'b: 'b, T>() -> impl Sized + use<'b, T> {}
// ~~~~~~~~~~~~~~~~~~~~~~~
// This opaque type does not capture `'a`.
fn outlives<'o, T: 'o>(_: T) {}
fn caller<'o, 'a, 'b: 'o, T: 'o>() {
// ~~
// ^ Note that we don't need `'a: 'o`.
outlives::<'o>(captures::<'a, 'b, T>());
}That is, it allows for the precise capturing of generic parameters in RPIT-like impl Trait opaque types (the initial stabilization will require all in scope type and const parameters to be included in the use<..> bound; see the stabilization report for other restrictions). We need this to stabilize so that we can stabilize the Lifetime Capture Rules 2024 (RFC 3498) in Rust 2024. These are lang team priority items for the Rust 2024 edition.
Currently r-a gives errors about this syntax:
fn captures<'a: 'a, 'b: 'b, T>() -> impl Sized + use<'b, T> {}
// ~ ~~ ~ ~ ^ Syntax Error: expected an item
// | | | ^ Syntax Error: expected BANG
// | | | ^ Syntax Error: expected `{`, `[`, `(`
// | | ^ Syntax Error: expected an item
// | ^ Syntax Error: expected SEMICOLON
// ^ Syntax Error: expected a blockThe earliest this syntax could conceivably stabilize is with Rust 1.82, which will branch on 2024-08-30 and will be released on 2024-10-17.
Tracking:
Activity