Rollup merge of #119710 - Nilstrieb:let-_-=-oops, r=TaKO8Ki

Improve `let_underscore_lock`

- lint if the lock was in a nested pattern
- lint if the lock is inside a `Result<Lock, _>`

addresses https://github.com/rust-lang/rust/pull/119704#discussion_r1444044745

tests/ui/lint/let_underscore/let_underscore_drop.rs

@@ -11,4 +11,6 @@ impl Drop for NontrivialDrop {
 
 fn main() {
     let _ = NontrivialDrop; //~WARNING non-binding let on a type that implements `Drop`
+
+    let (_, _) = (NontrivialDrop, NontrivialDrop); // This should be ignored.
 }

tests/ui/lint/let_underscore/let_underscore_lock.rs

@@ -1,7 +1,22 @@
 // check-fail
 use std::sync::{Arc, Mutex};
 
+struct Struct<T> {
+    a: T,
+}
+
 fn main() {
     let data = Arc::new(Mutex::new(0));
     let _ = data.lock().unwrap(); //~ERROR non-binding let on a synchronization lock
+
+    let _ = data.lock(); //~ERROR non-binding let on a synchronization lock
+
+    let (_, _) = (data.lock(), 1); //~ERROR non-binding let on a synchronization lock
+
+    let (_a, Struct { a: _ }) = (0, Struct { a: data.lock() }); //~ERROR non-binding let on a synchronization lock
+
+    (_ , _) = (data.lock(), 1); //~ERROR non-binding let on a synchronization lock
+
+    let _b;
+    (_b, Struct { a: _ }) = (0, Struct { a: data.lock() }); //~ERROR non-binding let on a synchronization lock
 }

tests/ui/lint/let_underscore/let_underscore_lock.stderr

@@ -1,10 +1,8 @@
 error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:6:9
+  --> $DIR/let_underscore_lock.rs:10:9
    |
 LL |     let _ = data.lock().unwrap();
-   |         ^   ^^^^^^^^^^^^^^^^^^^^ this binding will immediately drop the value assigned to it
-   |         |
-   |         this lock is not assigned to a binding and is immediately dropped
+   |         ^ this lock is not assigned to a binding and is immediately dropped
    |
    = note: `#[deny(let_underscore_lock)]` on by default
 help: consider binding to an unused variable to avoid immediately dropping the value
@@ -16,5 +14,70 @@ help: consider immediately dropping the value
 LL |     drop(data.lock().unwrap());
    |     ~~~~~                    +
 
-error: aborting due to 1 previous error
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:12:9
+   |
+LL |     let _ = data.lock();
+   |         ^ this lock is not assigned to a binding and is immediately dropped
+   |
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let _unused = data.lock();
+   |         ~~~~~~~
+help: consider immediately dropping the value
+   |
+LL |     drop(data.lock());
+   |     ~~~~~           +
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:14:10
+   |
+LL |     let (_, _) = (data.lock(), 1);
+   |          ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let (_unused, _) = (data.lock(), 1);
+   |          ~~~~~~~
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:16:26
+   |
+LL |     let (_a, Struct { a: _ }) = (0, Struct { a: data.lock() });
+   |                          ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let (_a, Struct { a: _unused }) = (0, Struct { a: data.lock() });
+   |                          ~~~~~~~
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:18:6
+   |
+LL |     (_ , _) = (data.lock(), 1);
+   |      ^ this lock is not assigned to a binding and is immediately dropped
+   |
+help: consider binding to an unused variable to avoid immediately dropping the value
+  --> $DIR/let_underscore_lock.rs:18:6
+   |
+LL |     (_ , _) = (data.lock(), 1);
+   |      ^
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:21:22
+   |
+LL |     (_b, Struct { a: _ }) = (0, Struct { a: data.lock() });
+   |                      ^ this lock is not assigned to a binding and is immediately dropped
+   |
+help: consider binding to an unused variable to avoid immediately dropping the value
+  --> $DIR/let_underscore_lock.rs:21:22
+   |
+LL |     (_b, Struct { a: _ }) = (0, Struct { a: data.lock() });
+   |                      ^
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+
+error: aborting due to 6 previous errors