`unused_assignment` triggered by closure even though value is used

[y86-dev]

Code

fn main() {
    let mut a: Option<u32> = None;
    let mut cl = move || {
        a = Some(42);
        a.take().unwrap()
    };
    println!("{}", cl())
}

Current output

warning: value captured by `a` is never read
 --> src/main.rs:4:9
  |
4 |         a = Some(42);
  |         ^
  |
  = help: did you mean to capture by reference instead?
  = note: `#[warn(unused_assignments)]` on by default

Desired output

warning: value assigned to `a` is never read
 --> src/main.rs:2:13
  |
2 |     let mut a: Option<u32> = None;
  |             ^
  |
  = help: maybe it is overwritten before being read?
  = note: `#[warn(unused_assignments)]` on by default

Rationale and extra context

No response

Other cases

If the initial value is used, then the warning goes away, suggesting that it should warn on that instead:

fn main() {
    let mut a: Option<u32> = None;
    let mut cl = move || {
        let _ = a;
        a = Some(42);
        a.take().unwrap()
    };
    println!("{}", cl())
}

Rust Version

rustc 1.89.0-nightly (d97326eab 2025-05-15)
binary: rustc
commit-hash: d97326eabfc3b2c33abcb08d6bc117aefa697cb7
commit-date: 2025-05-15
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.4

Anything else?

No response

[hanna-kruppe]

The diagnostic has a point, it just points at a misleading place in the code. The initial value of a, the one that’s assigned outside the closure and then moved into the closure, is never read. You could achieve the same effect with:

let cl = || {
    let mut a = Some(42);
    a.take().unwrap()
};

[fmease]

I would agree with that but consider this case (for which this lint also fires):

fn main() {
    let a: Option<u32>;
    let mut cl = move || {
        a = Some(42); //~ WARN value captured by `a` is never read
        a.take().unwrap()
    };
    println!("{}", cl())
}

where we only have a single assignment and there's no "initial value of a".

[fmease]

Lol, what I wrote doesn't even compile 🤦 I didn't read the rest of the playground's output on mobile.