Auto merge of #112380 - jieyouxu:useless-bindings-lint, r=WaffleLapkin

Add allow-by-default lint for unit bindings

### Example

```rust
#![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.

compiler/rustc_lint/src/lib.rs

@@ -85,6 +85,7 @@ mod redundant_semicolon;
 mod reference_casting;
 mod traits;
 mod types;
+mod unit_bindings;
 mod unused;
 
 pub use array_into_iter::ARRAY_INTO_ITER;
@@ -123,6 +124,7 @@ use redundant_semicolon::*;
 use reference_casting::*;
 use traits::*;
 use types::*;
+use unit_bindings::*;
 use unused::*;
 
 /// Useful for other parts of the compiler / Clippy.
@@ -202,6 +204,7 @@ late_lint_methods!(
             InvalidReferenceCasting: InvalidReferenceCasting,
             // Depends on referenced function signatures in expressions
             UnusedResults: UnusedResults,
+            UnitBindings: UnitBindings,
             NonUpperCaseGlobals: NonUpperCaseGlobals,
             NonShorthandFieldPatterns: NonShorthandFieldPatterns,
             UnusedAllocation: UnusedAllocation,