dedup for placeholder typs

compiler/rustc_hir_analysis/src/collect.rs

@@ -176,7 +176,7 @@ pub(crate) fn placeholder_type_error<'tcx>(
 pub(crate) fn placeholder_type_error_diag<'tcx>(
     tcx: TyCtxt<'tcx>,
     generics: Option<&hir::Generics<'_>>,
-    placeholder_types: Vec<Span>,
+    mut placeholder_types: Vec<Span>,
     additional_spans: Vec<Span>,
     suggest: bool,
     hir_ty: Option<&hir::Ty<'_>>,
@@ -188,6 +188,8 @@ pub(crate) fn placeholder_type_error_diag<'tcx>(
 
     let params = generics.map(|g| g.params).unwrap_or_default();
     let type_name = params.next_type_param_name(None);
+
+    placeholder_types.dedup();
     let mut sugg: Vec<_> =
         placeholder_types.iter().map(|sp| (*sp, (*type_name).to_string())).collect();
 

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 +1,18 @@
+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
+   |
+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`.