Function arguments in nontrivial pattern can be reassigned with a different lifetime.

I tried this code:

#![allow(unused)]

fn works<'a, 'b>(mut x @ _: &'a i32, y: &'b i32) {
    x = y;
}

fn fails<'a, 'b>(mut x: &'a i32, y: &'b i32) {
    x = y;
}

I expected the two functions to either both compile or both error. Instead, only the first function compiles:

error: lifetime may not live long enough
 --> src/lib.rs:8:5
  |
7 | fn fails<'a, 'b>(mut x: &'a i32, y: &'b i32) {
  |          --  -- lifetime `'b` defined here
  |          |
  |          lifetime `'a` defined here
8 |     x = y;
  |     ^^^^^ assignment requires that `'b` must outlive `'a`
  |
  = help: consider adding the following bound: `'b: 'a`

See also #128225, where a variable can be assigned a value with a different lifetime.

Related to https://github.com/rust-lang/rust/issues/148389, where a similar inconsistency occurs with async fn.

Meta

Reproducible on the playground with version 1.93.0-nightly (2025-11-02 b15a874aafe7eab9ea3a)

Start by running the two functions in src/lib.rs on the cited Rust nightly playground version and compare the assignment diagnostics. Read the related issues #128225 and #148389 for the surrounding lifetime and async behavior. Done means the compiler's behavior for these equivalent argument patterns is made consistent, with appropriate regression coverage identified.