Rollup merge of #97739 - a2aaron:let_underscore, r=estebank

Uplift the `let_underscore` lints from clippy into rustc.

This PR resolves #97241.

This PR adds three lints from clippy--`let_underscore_drop`, `let_underscore_lock`, and `let_underscore_must_use`, which are meant to capture likely-incorrect uses of `let _ = ...` bindings (in particular, doing this on a type with a non-trivial `Drop` causes the `Drop` to occur immediately, instead of at the end of the scope. For a type like `MutexGuard`, this effectively releases the lock immediately, which is almost certainly the wrong behavior)

In porting the lints from clippy I had to copy over a bunch of utility functions from `clippy_util` that these lints also relied upon. Is that the right approach?

Note that I've set the `must_use` and `drop` lints to Allow by default and set `lock` to Deny by default (this matches the same settings that clippy has). In talking with `@estebank` he informed me to do a Crater run (I am not sure what type of Crater run to request here--I think it's just "check only"?)

On the linked issue, there's some discussion about using `must_use` and `Drop` together as a heuristic for when to warn--I did not implement this yet.

r? `@estebank`

4 files changed, 63 insertions, 0 deletions

diff --git a/src/test/ui/lint/let_underscore/let_underscore_drop.rs b/src/test/ui/lint/let_underscore/let_underscore_drop.rs
new file mode 100644
index 00000000000..f298871f122
--- /dev/null
+++ b/src/test/ui/lint/let_underscore/let_underscore_drop.rs
@@ -0,0 +1,14 @@
+// check-pass
+#![warn(let_underscore_drop)]
+
+struct NontrivialDrop;
+
+impl Drop for NontrivialDrop {
+    fn drop(&mut self) {
+        println!("Dropping!");
+    }
+}
+
+fn main() {
+    let _ = NontrivialDrop; //~WARNING non-binding let on a type that implements `Drop`
+}
diff --git a/src/test/ui/lint/let_underscore/let_underscore_drop.stderr b/src/test/ui/lint/let_underscore/let_underscore_drop.stderr
new file mode 100644
index 00000000000..7b7de202e46
--- /dev/null
+++ b/src/test/ui/lint/let_underscore/let_underscore_drop.stderr
@@ -0,0 +1,22 @@
+warning: non-binding let on a type that implements `Drop`
+  --> $DIR/let_underscore_drop.rs:13:5
+   |
+LL |     let _ = NontrivialDrop;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/let_underscore_drop.rs:2:9
+   |
+LL | #![warn(let_underscore_drop)]
+   |         ^^^^^^^^^^^^^^^^^^^
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let _unused = NontrivialDrop;
+   |         ~~~~~~~
+help: consider immediately dropping the value
+   |
+LL |     drop(NontrivialDrop);
+   |     ~~~~~              +
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/lint/let_underscore/let_underscore_lock.rs b/src/test/ui/lint/let_underscore/let_underscore_lock.rs
new file mode 100644
index 00000000000..7423862cdf0
--- /dev/null
+++ b/src/test/ui/lint/let_underscore/let_underscore_lock.rs
@@ -0,0 +1,7 @@
+// check-fail
+use std::sync::{Arc, Mutex};
+
+fn main() {
+    let data = Arc::new(Mutex::new(0));
+    let _ = data.lock().unwrap(); //~ERROR non-binding let on a synchronization lock
+}
diff --git a/src/test/ui/lint/let_underscore/let_underscore_lock.stderr b/src/test/ui/lint/let_underscore/let_underscore_lock.stderr
new file mode 100644
index 00000000000..fb58af0a42f
--- /dev/null
+++ b/src/test/ui/lint/let_underscore/let_underscore_lock.stderr
@@ -0,0 +1,20 @@
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:6:9
+   |
+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
+   |
+   = note: `#[deny(let_underscore_lock)]` on by default
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let _unused = data.lock().unwrap();
+   |         ~~~~~~~
+help: consider immediately dropping the value
+   |
+LL |     drop(data.lock().unwrap());
+   |     ~~~~~                    +
+
+error: aborting due to previous error
+