Lint for unused empty function value

[Oakchris1955]

What it does

Suggests turning let _unused = returns_nothing(); to returns_nothing();

Advantage

Increases code hygiene

Drawbacks

None

Example

let _unused: () = returns_nothing();

Could be written as:

returns_nothing();

Comparison with existing lints

No similar lints were found

Additional Context

A quick Github search shows around 116 thousand instances of the aforementioned example

[samueltardieu]

This lint exists and is called let_unit_value, however it doesn't trigger when the type is explicitly present. It should be fixed.

@rustbot label -A-lint C-bug I-false-negative "good first issue"

[samueltardieu]

Note that there is a small subtlety here: the lint should not trigger if the explicit type is needed to force the type of the initialization expression, for example when generics are involved.

[matanshavit]

@rustbot claim

[matanshavit]

Is this what was intended? #15975

[Oakchris1955]

From what I see yes.

[Jarcho]

Not linting here is intentional. People use let _: () = ... as an assertion that the expression actually evaluates to a unit type.

[samueltardieu]

In this case they can use let () = … which is more compact while being as explicit. Should we suggest that in a note?