Deeply normalize type trace in type error reporting

Author: compiler-errors

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -1256,8 +1256,11 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
         let (expected_found, exp_found, is_simple_error, values) = match values {
             None => (None, Mismatch::Fixed("type"), false, None),
-            Some((values, _param_env)) => {
-                let values = self.resolve_vars_if_possible(values);
+            Some((values, param_env)) => {
+                let mut values = self.resolve_vars_if_possible(values);
+                if self.next_trait_solver() {
+                    values = deeply_normalize_for_diagnostics(self, param_env, values);
+                }
                 let (is_simple_error, exp_found) = match values {
                     ValuePairs::Terms(ExpectedFound { expected, found }) => {
                         match (expected.unpack(), found.unpack()) {

tests/ui/traits/next-solver/diagnostics/deeply-normalize-type-expectation.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -Znext-solver
+
+// Make sure we try to mention a deeply normalized type in a type mismatch error.
+
+trait Mirror {
+    type Assoc;
+}
+impl<T> Mirror for T {
+    type Assoc = T;
+}
+
+fn needs<T>(_: <T as Mirror>::Assoc) {}
+
+fn main() {
+    needs::<i32>(());
+    //~^ ERROR mismatched types
+}

tests/ui/traits/next-solver/diagnostics/deeply-normalize-type-expectation.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/deeply-normalize-type-expectation.rs:15:18
+   |
+LL |     needs::<i32>(());
+   |     ------------ ^^ expected `i32`, found `()`
+   |     |
+   |     arguments to this function are incorrect
+   |
+note: function defined here
+  --> $DIR/deeply-normalize-type-expectation.rs:12:4
+   |
+LL | fn needs<T>(_: <T as Mirror>::Assoc) {}
+   |    ^^^^^    -----------------------
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.