`unused_unit` false positive when removing `-> ()` leads to type inference failure

[narpfel]

Summary

When returning the result of a function fn f<T>(...) -> T from a lambda || -> () { ... }, removing the lambda’s return type annotation leads to a type inference failure. The unused_unit lint should not trigger in that case because the unit is not actually unused.

Lint Name

unused_unit

Reproducer

I tried this code:

trait Convert<T> {
    fn from(value: T) -> Self;
}

impl Convert<u64> for () {
    fn from(_value: u64) -> Self {}
}

fn handle<T>(value: u64) -> T where T: Convert<u64> {
    eprintln!("error: {value}");
    Convert::from(value)
}

pub fn f() -> Option<bool> {
    let result: Result<bool, u64> = Err(42);
    result.map_err(|err| -> () { handle(err) }).ok()
}

pub fn main() {
    dbg!(f());
}

I saw this happen:

warning: unneeded unit return type
  --> src/main.rs:16:25
   |
16 |     result.map_err(|err| -> () { handle(err) }).ok()
   |                         ^^^^^^ help: remove the `-> ()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
   = note: `#[warn(clippy::unused_unit)]` on by default

warning: `t` (bin "t") generated 1 warning (run `cargo clippy --fix --bin "t"` to apply 1 suggestion)

I expected to see this happen: No warning should be generated.

Godbolt link: https://godbolt.org/z/ojv53fWe8

Version

rustc 1.89.0-nightly (d13a431a6 2025-06-09)
binary: rustc
commit-hash: d13a431a6cc69cd65efe7c3eb7808251d6fd7a46
commit-date: 2025-06-09
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Additional Labels

@rustbot label +I-suggestion-causes-error