Allow let_underscore_drop and let_underscore_must_use by default.

These lints are very noisy and are allow-by-default in clippy anyways.
Hence, setting them to allow-by-default here makes more sense than
warning constantly on these cases.

compiler/rustc_lint/src/let_underscore.rs

@@ -41,7 +41,7 @@
     /// calling `std::mem::drop` on the expression is clearer and helps convey
     /// intent.
     pub LET_UNDERSCORE_DROP,
-    Warn,
+    Allow,
     "non-binding let on a type that implements `Drop`"
 }
 
@@ -104,7 +104,7 @@
     /// expression to immediately drop. Usually, it's better to explicitly handle
     /// the `must_use` expression.
     pub LET_UNDERSCORE_MUST_USE,
-    Warn,
+    Allow,
     "non-binding let on a expression marked `must_use`"
 }
 

src/test/ui/let_underscore/let_underscore_drop.rs

@@ -1,4 +1,5 @@
 // run-pass
+// compile-flags: -W let_underscore_drop
 
 struct NontrivialDrop;
 

src/test/ui/let_underscore/let_underscore_drop.stderr

@@ -1,10 +1,10 @@
 warning: non-binding let on a type that implements `Drop`
-  --> $DIR/let_underscore_drop.rs:12:5
+  --> $DIR/let_underscore_drop.rs:13:5
    |
 LL |     let _ = NontrivialDrop;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#[warn(let_underscore_drop)]` on by default
+   = note: requested on the command line with `-W let-underscore-drop`
    = help: consider binding to an unused variable
    = help: consider explicitly droping with `std::mem::drop`
 

src/test/ui/let_underscore/let_underscore_must_use.rs

@@ -1,4 +1,5 @@
 // run-pass
+// compile-flags: -W let_underscore_must_use
 
 #[must_use]
 struct MustUseType;

src/test/ui/let_underscore/let_underscore_must_use.stderr

@@ -1,15 +1,15 @@
 warning: non-binding let on a expression marked `must_use`
-  --> $DIR/let_underscore_must_use.rs:10:5
+  --> $DIR/let_underscore_must_use.rs:11:5
    |
 LL |     let _ = MustUseType;
    |     ^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#[warn(let_underscore_must_use)]` on by default
+   = note: requested on the command line with `-W let-underscore-must-use`
    = help: consider binding to an unused variable
    = help: consider explicitly droping with `std::mem::drop`
 
 warning: non-binding let on a expression marked `must_use`
-  --> $DIR/let_underscore_must_use.rs:11:5
+  --> $DIR/let_underscore_must_use.rs:12:5
    |
 LL |     let _ = must_use_function();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^