`unused_assignments` does not trigger on assignment through reference

Code

fn store32(output: &mut [u8; 16], i: usize, value: u32) {
    let output: &mut [u8; 4] = &mut output[i..][..4].try_into().unwrap();
    *output = u32::to_le_bytes(value);
}

fn main() {
    let mut r = [0u8; 16];
    store32(&mut r, 0, 0x12345678);
    println!("{r:?}")
}

Current output

None

Desired output

warning: value assigned to `output` is never read
note: `#[warn(unused_assignments)]` on by default

The fix is:

-    let output: &mut [u8; 4] = &mut output[i..][..4].try_into().unwrap();
+    let output: &mut [u8; 4] = (&mut output[i..][..4]).try_into().unwrap();

The compiler should have recognized that output in the buggy version is a reference to a temporary. The temporary is written (through output) but never read.

Rationale and extra context

The author of this code overlooked that .try_into().unwrap() binds tighter than &mut , so the store32 function is buggy: It doesn't write anything to output like it intends. It doesn't make sense to modify a temporary and then never read from it.

Other cases

Rust Version

% rustc --version --verbose
rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: aarch64-apple-darwin
release: 1.88.0
LLVM version: 20.1.5

Anything else?

See the thread https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/Accidental.20borrow.20of.20temporary.2Faccidental.20lifetime.20extension/with/528940661

Start by reproducing the unused_assignments behavior with the store32 example on Rust 1.88.0, then trace the compiler's handling of assignments through references to temporaries. Read the linked Zulip discussion for context. Done means the buggy form emits the requested warning while the corrected form remains valid.