Add parentheses for closure when suggesting calling closure

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -820,16 +820,20 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArg { .. })
             && obligation.cause.span.can_be_used_for_suggestions()
         {
+            let (span, sugg) = if let Some(snippet) =
+                self.tcx.sess.source_map().span_to_snippet(obligation.cause.span).ok()
+                && snippet.starts_with("|")
+            {
+                (obligation.cause.span, format!("({snippet})({args})"))
+            } else {
+                (obligation.cause.span.shrink_to_hi(), format!("({args})"))
+            };
+
             // When the obligation error has been ensured to have been caused by
             // an argument, the `obligation.cause.span` points at the expression
             // of the argument, so we can provide a suggestion. Otherwise, we give
             // a more general note.
-            err.span_suggestion_verbose(
-                obligation.cause.span.shrink_to_hi(),
-                msg,
-                format!("({args})"),
-                Applicability::HasPlaceholders,
-            );
+            err.span_suggestion_verbose(span, msg, sugg, Applicability::HasPlaceholders);
         } else if let DefIdOrName::DefId(def_id) = def_id_or_name {
             let name = match self.tcx.hir_get_if_local(def_id) {
                 Some(hir::Node::Expr(hir::Expr {

tests/ui/const-generics/adt_const_params/const_param_ty_bad.stderr

@@ -32,8 +32,9 @@ LL | fn check(_: impl std::marker::UnsizedConstParamTy) {}
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `check`
 help: use parentheses to call this closure
    |
-LL |     check(|| {}());
-   |                ++
+LL -     check(|| {});
+LL +     check((|| {})());
+   |
 
 error[E0277]: `fn()` can't be used as a const parameter type
   --> $DIR/const_param_ty_bad.rs:9:11

tests/ui/suggestions/use-parentheses-to-call-closure-issue-145404.fixed

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+use std::fmt::Display;
+
+struct S;
+
+impl S {
+    fn call(&self, _: impl Display) {}
+}
+
+fn main() {
+    S.call((|| "hello")()); //~ ERROR [E0277]
+}

tests/ui/suggestions/use-parentheses-to-call-closure-issue-145404.rs

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+use std::fmt::Display;
+
+struct S;
+
+impl S {
+    fn call(&self, _: impl Display) {}
+}
+
+fn main() {
+    S.call(|| "hello"); //~ ERROR [E0277]
+}

tests/ui/suggestions/use-parentheses-to-call-closure-issue-145404.stderr

@@ -0,0 +1,23 @@
+error[E0277]: `{closure@$DIR/use-parentheses-to-call-closure-issue-145404.rs:12:12: 12:14}` doesn't implement `std::fmt::Display`
+  --> $DIR/use-parentheses-to-call-closure-issue-145404.rs:12:12
+   |
+LL |     S.call(|| "hello");
+   |       ---- ^^^^^^^^^^ unsatisfied trait bound
+   |       |
+   |       required by a bound introduced by this call
+   |
+   = help: the trait `std::fmt::Display` is not implemented for closure `{closure@$DIR/use-parentheses-to-call-closure-issue-145404.rs:12:12: 12:14}`
+note: required by a bound in `S::call`
+  --> $DIR/use-parentheses-to-call-closure-issue-145404.rs:8:28
+   |
+LL |     fn call(&self, _: impl Display) {}
+   |                            ^^^^^^^ required by this bound in `S::call`
+help: use parentheses to call this closure
+   |
+LL -     S.call(|| "hello");
+LL +     S.call((|| "hello")());
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.