Fix clippy lint suggestion incorrectly treated as span_help

Author: estebank

src/tools/clippy/clippy_lints/src/unnecessary_wraps.rs

@@ -144,7 +144,9 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryWraps {
                 (
                     "this function's return value is unnecessary".to_string(),
                     "remove the return type...".to_string(),
-                    snippet(cx, fn_decl.output.span(), "..").to_string(),
+                    // FIXME: we should instead get the span including the `->` and suggest an
+                    // empty string for this case.
+                    "()".to_string(),
                     "...and then remove returned values",
                 )
             } else {

src/tools/clippy/tests/ui/unnecessary_wraps.stderr

@@ -116,6 +116,10 @@ LL | |     }
 LL | | }
    | |_^
    |
+help: remove the return type...
+   |
+LL | fn issue_6640_1(a: bool, b: bool) -> () {
+   |                                      ~~
 help: ...and then remove returned values
    |
 LL ~         return ;
@@ -139,6 +143,10 @@ LL | |     }
 LL | | }
    | |_^
    |
+help: remove the return type...
+   |
+LL | fn issue_6640_2(a: bool, b: bool) -> () {
+   |                                      ~~
 help: ...and then remove returned values
    |
 LL ~         return ;