Emit an error when RTN is used in an incorrect place

We miss one place: associated type bindings aka. `impl Trait<Type(..): Send>`, but we also miss it for Fn-style parenthesizes error so I left it out for now.

crates/ide-diagnostics/src/handlers/bad_rtn.rs

@@ -0,0 +1,52 @@
+use ide_db::Severity;
+
+use crate::{Diagnostic, DiagnosticCode, DiagnosticsContext};
+
+// Diagnostic: bad-rtn
+//
+// This diagnostic is shown when a RTN (Return Type Notation, `Type::method(..): Send`) is written in an improper place.
+pub(crate) fn bad_rtn(ctx: &DiagnosticsContext<'_>, d: &hir::BadRtn) -> Diagnostic {
+    Diagnostic::new_with_syntax_node_ptr(
+        ctx,
+        DiagnosticCode::Ra("bad-rtn", Severity::Error),
+        "return type notation not allowed in this position yet",
+        d.rtn.map(Into::into),
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_diagnostics;
+
+    #[test]
+    fn fn_traits_also_emit() {
+        check_diagnostics(
+            r#"
+//- minicore: fn
+fn foo<
+    A: Fn(..),
+      // ^^^^ error: return type notation not allowed in this position yet
+>() {}
+        "#,
+        );
+    }
+
+    #[test]
+    fn bad_rtn() {
+        check_diagnostics(
+            r#"
+mod module {
+    pub struct Type;
+}
+trait Trait {}
+
+fn foo()
+where
+    module(..)::Type: Trait
+       // ^^^^ error: return type notation not allowed in this position yet
+{
+}
+        "#,
+        );
+    }
+}

crates/ide-diagnostics/src/lib.rs

@@ -25,6 +25,7 @@
 
 mod handlers {
     pub(crate) mod await_outside_of_async;
+    pub(crate) mod bad_rtn;
     pub(crate) mod break_outside_of_loop;
     pub(crate) mod expected_function;
     pub(crate) mod generic_args_prohibited;
@@ -493,6 +494,7 @@ pub fn semantic_diagnostics(
             AnyDiagnostic::ParenthesizedGenericArgsWithoutFnTrait(d) => {
                 handlers::parenthesized_generic_args_without_fn_trait::parenthesized_generic_args_without_fn_trait(&ctx, &d)
             }
+            AnyDiagnostic::BadRtn(d) => handlers::bad_rtn::bad_rtn(&ctx, &d),
         };
         res.push(d)
     }