dedup for duplicate suggestions

Author: bvanjoi

compiler/rustc_errors/src/diagnostic.rs

@@ -622,10 +622,12 @@ impl Diagnostic {
     pub fn multipart_suggestion_with_style(
         &mut self,
         msg: impl Into<SubdiagnosticMessage>,
-        suggestion: Vec<(Span, String)>,
+        mut suggestion: Vec<(Span, String)>,
         applicability: Applicability,
         style: SuggestionStyle,
     ) -> &mut Self {
+        suggestion.dedup();
+
         let mut parts = suggestion
             .into_iter()
             .map(|(span, snippet)| SubstitutionPart { snippet, span })

tests/ui/macros/issue-118048.rs

@@ -0,0 +1,10 @@
+macro_rules! foo {
+    ($ty:ty) => {
+        fn foo(_: $ty, _: $ty) {}
+    }
+}
+
+foo!(_);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
+
+fn main() {}

tests/ui/macros/issue-118048.stderr

@@ -0,0 +1,21 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
+  --> $DIR/issue-118048.rs:7:6
+   |
+LL | foo!(_);
+   |      ^
+   |      |
+   |      not allowed in type signatures
+   |      not allowed in type signatures
+   |
+help: use type parameters instead
+   |
+LL ~         fn foo<T>(_: $ty, _: $ty) {}
+LL |     }
+LL | }
+LL | 
+LL ~ foo!(T);
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.