Improve test coverage for unit_bindings lint

tests/ui/lint/unit_bindings.deny_level.stderr

@@ -0,0 +1,40 @@
+error: binding has unit type `()`
+  --> $DIR/unit_bindings.rs:50:5
+   |
+LL |     let _ = expr;
+   |     ^^^^-^^^^^^^^
+   |         |
+   |         this pattern is inferred to be the unit type `()`
+   |
+note: the lint level is defined here
+  --> $DIR/unit_bindings.rs:22:30
+   |
+LL | #![cfg_attr(deny_level, deny(unit_bindings))]
+   |                              ^^^^^^^^^^^^^
+
+error: binding has unit type `()`
+  --> $DIR/unit_bindings.rs:51:5
+   |
+LL |     let pat = expr;
+   |     ^^^^---^^^^^^^^
+   |         |
+   |         this pattern is inferred to be the unit type `()`
+
+error: binding has unit type `()`
+  --> $DIR/unit_bindings.rs:52:5
+   |
+LL |     let _pat = expr;
+   |     ^^^^----^^^^^^^^
+   |         |
+   |         this pattern is inferred to be the unit type `()`
+
+error: binding has unit type `()`
+  --> $DIR/unit_bindings.rs:55:5
+   |
+LL |     let list = v.sort();
+   |     ^^^^----^^^^^^^^^^^^
+   |         |
+   |         this pattern is inferred to be the unit type `()`
+
+error: aborting due to 4 previous errors
+

tests/ui/lint/unit_bindings.rs

@@ -0,0 +1,60 @@
+//! Basic checks for `unit_bindings` lint.
+//!
+//! The `unit_bindings` lint tries to detect cases like `let list = list.sort()`. The lint will
+//! trigger on bindings that have the unit `()` type **except** if:
+//!
+//! - The user wrote `()` on either side, i.e.
+//!     - `let () = <expr>;` or `let <expr> = ();`
+//!     - `let _ = ();`
+//! - The binding occurs within macro expansions, e.g. `foo!();`.
+//! - The user explicitly provided type annotations, e.g. `let x: () = <expr>`.
+//!
+//! Examples where the lint *should* fire on include:
+//!
+//! - `let _ = <expr>;`
+//! - `let pat = <expr>;`
+//! - `let _pat = <expr>;`
+
+//@ revisions: default_level deny_level
+//@[default_level] check-pass (`unit_bindings` is currently allow-by-default)
+
+#![allow(unused)]
+#![cfg_attr(deny_level, deny(unit_bindings))]
+
+// The `list` binding below should trigger the lint if it's not contained in a macro expansion.
+macro_rules! expands_to_sus {
+    () => {
+        let mut v = [1, 2, 3];
+        let list = v.sort();
+    }
+}
+
+// No warning for `y` and `z` because it is provided as type parameter.
+fn ty_param_check<T: Copy>(x: T) {
+    let y = x;
+    let z: T = x;
+}
+
+fn main() {
+    // No warning if user explicitly wrote `()` on either side.
+    let expr = ();
+    let () = expr;
+    let _ = ();
+    // No warning if user explicitly annotates the unit type on the binding.
+    let pat: () = expr;
+    // No warning for let bindings with unit type in macro expansions.
+    expands_to_sus!();
+    // No warning for unit bindings in generic fns.
+    ty_param_check(());
+
+    let _ = expr; //[deny_level]~ ERROR binding has unit type
+    let pat = expr; //[deny_level]~ ERROR binding has unit type
+    let _pat = expr; //[deny_level]~ ERROR binding has unit type
+
+    let mut v = [1, 2, 3];
+    let list = v.sort(); //[deny_level]~ ERROR binding has unit type
+
+    // Limitation: the lint currently does not fire on nested unit LHS bindings, i.e.
+    // this will not currently trigger the lint.
+    let (nested, _) = (expr, 0i32);
+}