Use multipart_suggestion to create an applicable suggestion.

The "consider explicitly droping" can now suggest a machine applicable
suggestion now.

compiler/rustc_lint/src/let_underscore.rs

@@ -154,11 +154,13 @@ fn build_and_emit_lint(
             "_unused",
             Applicability::MachineApplicable,
         )
-        .span_suggestion_verbose(
+        .multipart_suggestion(
-            init_span,
             "consider explicitly droping with `std::mem::drop`",
-            "drop(...)",
+            vec![
-            Applicability::HasPlaceholders,
+                (init_span.shrink_to_lo(), "drop(".to_string()),
+                (init_span.shrink_to_hi(), ")".to_string()),
+            ],
+            Applicability::MachineApplicable,
         )
         .emit();
 }

src/test/ui/lint/let_underscore/let_underscore_drop.stderr

@@ -11,8 +11,8 @@ LL |     let _unused = NontrivialDrop;
    |         ~~~~~~~
 help: consider explicitly droping with `std::mem::drop`
    |
-LL |     let _ = drop(...);
+LL |     let _ = drop(NontrivialDrop);
-   |             ~~~~~~~~~
+   |             +++++              +
 
 warning: 1 warning emitted
 

src/test/ui/lint/let_underscore/let_underscore_lock.stderr

@@ -11,8 +11,8 @@ LL |     let _unused = data.lock().unwrap();
    |         ~~~~~~~
 help: consider explicitly droping with `std::mem::drop`
    |
-LL |     let _ = drop(...);
+LL |     let _ = drop(data.lock().unwrap());
-   |             ~~~~~~~~~
+   |             +++++                    +
 
 error: aborting due to previous error