Rustc diagnostics don’t realize that assignments from a match guard can be used in the following match arms.

[steffahn]

fn main() {
    let mut x = 0;
    match () {
        () if {
            x = 42;
            false
        } => {}
        _ => {
            println!("{}", x); // prints 42
        }
    }
}

(Playground)

Output:

42

Errors:

   Compiling playground v0.0.1 (/playground)
warning: value assigned to `x` is never read
 --> src/main.rs:5:13
  |
5 |             x = 42;
  |             ^
  |
  = note: `#[warn(unused_assignments)]` on by default
  = help: maybe it is overwritten before being read?

warning: 1 warning emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.74s
     Running `target/debug/playground`

@rustbot modify labels: T-compiler, A-diagnostics, A-lint, D-incorrect, C-bug

[jyn514]

@steffahn you're correct, but also ... I hope you feel properly ashamed of yourself for writing this code 😆

[steffahn]

@jyn514 Always a question of context, here's mine. This is not supposed to be proper Rust code but was me trying to figure out how bad guards can become in order to evaluate my intuition on how deref patterns could potentially be translated. TL;DR, it's intentionally bad rust code 😜 (and yet, I do still feel a bit ashamed to have come up with this, nonetheless 😅).

[tmiasko]

The current implementation of liveness analysis doesn't model control flow between guards and following arms. Additional example showcasing related but somewhat different issue:

fn main() {
    let mut n = 0;
    match 0 {
        | _ | _ | _ | _ | _ | _ | _ | _ | _ | _ if
        { println!("{}", n); n += 1; false } => {}
        _ => {}
    }
}

warning: value assigned to `n` is never read
 --> a.rs:5:30
  |
5 |         { println!("{}", n); n += 1; false } => {}
  |                              ^
  |
  = note: `#[warn(unused_assignments)]` on by default
  = help: maybe it is overwritten before being read?

[dianne]

This was fixed by #142390. Neither of the examples produce warnings under current nightly rustc.