Add allow-by-default lint for unit bindings by jieyouxu · Pull Request #112380 ·...

Example

#![warn(unit_bindings)]

macro_rules! owo {
    () => {
        let whats_this = ();
    }
}

fn main() {
    // No warning if user explicitly wrote `()` on either side.
    let expr = ();
    let () = expr;
    let _ = ();

    let _ = expr; //~ WARN binding has unit type
    let pat = expr; //~ WARN binding has unit type
    let _pat = expr; //~ WARN binding has unit type

    // No warning for let bindings with unit type in macro expansions.
    owo!();

    // No warning if user explicitly annotates the unit type on the binding.
    let pat: () = expr;
}

outputs

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:17:5
   |
LL |     let _ = expr;
   |     ^^^^-^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`
   |
note: the lint level is defined here
  --> $DIR/unit-bindings.rs:3:9
   |
LL | #![warn(unit_bindings)]
   |         ^^^^^^^^^^^^^

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:18:5
   |
LL |     let pat = expr;
   |     ^^^^---^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:19:5
   |
LL |     let _pat = expr;
   |     ^^^^----^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`

warning: 3 warnings emitted

This lint is not triggered if any of the following conditions are met:

• The user explicitly annotates the binding with the () type.
• The binding is from a macro expansion.
• The user explicitly wrote let () = init;
• The user explicitly wrote let pat = ();. This is allowed for local lifetimes.

Known Issue

It is known that this lint can trigger on some proc-macro generated code whose span returns false for Span::from_expansion because e.g. the proc-macro simply forwards user code spans, and otherwise don't have distinguishing syntax context compared to non-macro-generated code. For those kind of proc-macros, I believe the correct way to fix them is to instead emit identifers with span like Span::mixed_site().located_at(user_span).

Closes #71432.