Set let_underscore_lock to Deny by default.

Clippy sets this lint to Deny by default, and it having the lint be Deny
is useful for when we test the lint against a Crater run.

compiler/rustc_lint/src/let_underscore.rs

@@ -55,7 +55,7 @@ declare_lint! {
     /// of at end of scope, which is typically incorrect.
     ///
     /// ### Example
-    /// ```rust
+    /// ```compile_fail
     /// use std::sync::{Arc, Mutex};
     /// use std::thread;
     /// let data = Arc::new(Mutex::new(0));
@@ -83,7 +83,7 @@ declare_lint! {
     /// calling `std::mem::drop` on the expression is clearer and helps convey
     /// intent.
     pub LET_UNDERSCORE_LOCK,
-    Warn,
+    Deny,
     "non-binding let on a synchronization lock"
 }
 

src/test/ui/let_underscore/let_underscore_lock.rs

@@ -1,8 +1,6 @@
-// run-pass
-
 use std::sync::{Arc, Mutex};
 
 fn main() {
     let data = Arc::new(Mutex::new(0));
-    let _ = data.lock().unwrap(); //~WARNING non-binding let on a synchronization lock
+    let _ = data.lock().unwrap(); //~ERROR non-binding let on a synchronization lock
 }

src/test/ui/let_underscore/let_underscore_lock.stderr

@@ -1,10 +1,10 @@
-warning: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:7:5
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:5:5
    |
 LL |     let _ = data.lock().unwrap();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#[warn(let_underscore_lock)]` on by default
+   = note: `#[deny(let_underscore_lock)]` on by default
 help: consider binding to an unused variable
    |
 LL |     let _unused = data.lock().unwrap();
@@ -14,5 +14,5 @@ help: consider explicitly droping with `std::mem::drop`
 LL |     let _ = drop(...);
    |             ~~~~~~~~~
 
-warning: 1 warning emitted
+error: aborting due to previous error