chore: add regression test for #10219

compiler/noirc_frontend/src/elaborator/types.rs

@@ -1996,6 +1996,13 @@ impl Elaborator<'_> {
                     expr_location: location,
                 });
             }
+            Type::Error => {
+                self.push_err(TypeCheckError::ExpectingOtherError {
+                    message: "type_check_operator_method: encountered method_type of type 'error'"
+                        .to_string(),
+                    location,
+                });
+            }
             other => {
                 unreachable!(
                     "Expected operator method on {object_type} to have a function type, but found {other}"

compiler/noirc_frontend/src/node_interner/mod.rs

@@ -1236,7 +1236,7 @@ impl NodeInterner {
             method_ids,
             associated_types: vec![],
             associated_type_bounds: Default::default(),
-            name: Ident::new("Dummy".to_string(), Location::dummy()),
+            name: Ident::new("PopulateDummyOperatorTraitsTrait".to_string(), Location::dummy()),
             generics: vec![],
             location: Location::dummy(),
             visibility: ItemVisibility::Public,

compiler/noirc_frontend/src/tests/traits/trait_method_resolution.rs

@@ -715,3 +715,36 @@ fn regression_10537() {
     "#;
     check_errors_with_stdlib(src, [stdlib_src::EQ, stdlib_src::ORD]);
 }
+
+// Expecting missing `Eq` impl errors to resolve to a `NoMatchingImplFound` error
+// with a trait name of "PopulateDummyOperatorTraitsTrait" when the `Eq` trait
+// hasn't been explicitly defined, i.e. when it's only been defined by
+// `NodeInterner::populate_dummy_operator_traits`
+#[test]
+fn missing_eq_trait_error() {
+    let src = "
+    struct Foo {}
+    fn main() {
+        let x = Foo {};
+        let y = Foo {};
+        assert(x == y);
+               ^^^^^^ No matching impl found for `Foo: PopulateDummyOperatorTraitsTrait`
+               ~~~~~~ No impl for `Foo: PopulateDummyOperatorTraitsTrait`
+    }
+    ";
+    check_errors(src);
+}
+
+#[test]
+fn regression_10219() {
+    let src = "
+    fn main() {
+        let mut x: u32 = 0;
+        let mut y: u32 = 0;
+        assert(&mut x == &mut y);
+               ^^^^^^^^^^^^^^^^ No matching impl found for `&mut u32: Eq`
+               ~~~~~~~~~~~~~~~~ No impl for `&mut u32: Eq`
+    }
+    ";
+    check_errors_with_stdlib(src, [stdlib_src::EQ]);
+}