Correctly suggest adding bounds to impl Trait argument

src/librustc_typeck/check/method/suggest.rs

@@ -743,8 +743,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         // We do this to avoid suggesting code that ends up as `T: FooBar`,
                         // instead we suggest `T: Foo + Bar` in that case.
                         let mut has_bounds = false;
+                        let mut impl_trait = false;
                         if let Node::GenericParam(ref param) = hir.get(id) {
-                            has_bounds = !param.bounds.is_empty();
+                            match param.kind {
+                                hir::GenericParamKind::Type { synthetic: Some(_), .. } => {
+                                    impl_trait = true;  // #63706
+                                    has_bounds = param.bounds.len() > 1;
+                                }
+                                _ => {
+                                    has_bounds = !param.bounds.is_empty();
+                                }
+                            }
                         }
                         let sp = hir.span(id);
                         // `sp` only covers `T`, change it so that it covers
@@ -765,8 +774,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             sp,
                             &msg[..],
                             candidates.iter().map(|t| format!(
-                                "{}: {}{}",
+                                "{}{} {}{}",
                                 param,
+                                if impl_trait { " +" } else { ":" },
                                 self.tcx.def_path_str(t.def_id),
                                 if has_bounds { " +"} else { "" },
                             )),

src/test/ui/suggestions/impl-trait-with-missing-trait-bounds-in-arg.fixed

@@ -0,0 +1,20 @@
+// run-rustfix
+
+trait Foo {}
+
+trait Bar {
+    fn hello(&self) {}
+}
+
+struct S;
+
+impl Foo for S {}
+impl Bar for S {}
+
+fn test(foo: impl Foo + Bar) {
+    foo.hello(); //~ ERROR E0599
+}
+
+fn main() {
+    test(S);
+}

src/test/ui/suggestions/impl-trait-with-missing-trait-bounds-in-arg.rs

@@ -0,0 +1,20 @@
+// run-rustfix
+
+trait Foo {}
+
+trait Bar {
+    fn hello(&self) {}
+}
+
+struct S;
+
+impl Foo for S {}
+impl Bar for S {}
+
+fn test(foo: impl Foo) {
+    foo.hello(); //~ ERROR E0599
+}
+
+fn main() {
+    test(S);
+}

src/test/ui/suggestions/impl-trait-with-missing-trait-bounds-in-arg.stderr

@@ -0,0 +1,15 @@
+error[E0599]: no method named `hello` found for type `impl Foo` in the current scope
+  --> $DIR/impl-trait-with-missing-trait-bounds-in-arg.rs:15:9
+   |
+LL |     foo.hello();
+   |         ^^^^^
+   |
+   = help: items from traits can only be used if the type parameter is bounded by the trait
+help: the following trait defines an item `hello`, perhaps you need to restrict type parameter `impl Foo` with it:
+   |
+LL | fn test(foo: impl Foo + Bar) {
+   |              ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0599`.