Highlight clarifying information in "expected/found" error

When the expected and found types have the same textual representation, we add clarifying in parentheses. We now visually highlight it in the output.

Detect a corner case where the clarifying information would be the same for both types and skip it, as it doesn't add anything useful.

compiler/rustc_errors/src/diagnostic.rs

@@ -641,7 +641,14 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         found_label: &dyn fmt::Display,
         found: DiagStyledString,
     ) -> &mut Self {
-        self.note_expected_found_extra(expected_label, expected, found_label, found, &"", &"")
+        self.note_expected_found_extra(
+            expected_label,
+            expected,
+            found_label,
+            found,
+            DiagStyledString::normal(""),
+            DiagStyledString::normal(""),
+        )
     }
 
     #[rustc_lint_diagnostics]
@@ -651,8 +658,8 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         expected: DiagStyledString,
         found_label: &dyn fmt::Display,
         found: DiagStyledString,
-        expected_extra: &dyn fmt::Display,
-        found_extra: &dyn fmt::Display,
+        expected_extra: DiagStyledString,
+        found_extra: DiagStyledString,
     ) -> &mut Self {
         let expected_label = expected_label.to_string();
         let expected_label = if expected_label.is_empty() {
@@ -677,10 +684,13 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
             expected_label
         ))];
         msg.extend(expected.0);
-        msg.push(StringPart::normal(format!("`{expected_extra}\n")));
+        msg.push(StringPart::normal(format!("`")));
+        msg.extend(expected_extra.0);
+        msg.push(StringPart::normal(format!("\n")));
         msg.push(StringPart::normal(format!("{}{} `", " ".repeat(found_padding), found_label)));
         msg.extend(found.0);
-        msg.push(StringPart::normal(format!("`{found_extra}")));
+        msg.push(StringPart::normal(format!("`")));
+        msg.extend(found_extra.0);
 
         // For now, just attach these as notes.
         self.highlighted_note(msg);

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

@@ -1725,32 +1725,42 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     }
                 }
                 TypeError::Sorts(values) => {
-                    let extra = expected == found;
+                    let extra = expected == found
+                        // Ensure that we don't ever say something like
+                        // expected `impl Trait` (opaque type `impl Trait`)
+                        //    found `impl Trait` (opaque type `impl Trait`)
+                        && values.expected.sort_string(self.tcx)
+                            != values.found.sort_string(self.tcx);
                     let sort_string = |ty: Ty<'tcx>| match (extra, ty.kind()) {
                         (true, ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. })) => {
                             let sm = self.tcx.sess.source_map();
                             let pos = sm.lookup_char_pos(self.tcx.def_span(*def_id).lo());
-                            format!(
+                            DiagStyledString::normal(format!(
                                 " (opaque type at <{}:{}:{}>)",
                                 sm.filename_for_diagnostics(&pos.file.name),
                                 pos.line,
                                 pos.col.to_usize() + 1,
-                            )
+                            ))
                         }
                         (true, ty::Alias(ty::Projection, proj))
                             if self.tcx.is_impl_trait_in_trait(proj.def_id) =>
                         {
                             let sm = self.tcx.sess.source_map();
                             let pos = sm.lookup_char_pos(self.tcx.def_span(proj.def_id).lo());
-                            format!(
+                            DiagStyledString::normal(format!(
                                 " (trait associated opaque type at <{}:{}:{}>)",
                                 sm.filename_for_diagnostics(&pos.file.name),
                                 pos.line,
                                 pos.col.to_usize() + 1,
-                            )
+                            ))
                         }
-                        (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
-                        (false, _) => "".to_string(),
+                        (true, _) => {
+                            let mut s = DiagStyledString::normal(" (");
+                            s.push_highlighted(ty.sort_string(self.tcx));
+                            s.push_normal(")");
+                            s
+                        }
+                        (false, _) => DiagStyledString::normal(""),
                     };
                     if !(values.expected.is_simple_text() && values.found.is_simple_text())
                         || (exp_found.is_some_and(|ef| {
@@ -1767,23 +1777,23 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             }
                         }))
                     {
-                        if let Some(ExpectedFound { found: found_ty, .. }) = exp_found {
+                        if let Some(ExpectedFound { found: found_ty, .. }) = exp_found
+                            && !self.tcx.ty_is_opaque_future(found_ty)
+                        {
                             // `Future` is a special opaque type that the compiler
                             // will try to hide in some case such as `async fn`, so
                             // to make an error more use friendly we will
                             // avoid to suggest a mismatch type with a
                             // type that the user usually are not using
                             // directly such as `impl Future<Output = u8>`.
-                            if !self.tcx.ty_is_opaque_future(found_ty) {
-                                diag.note_expected_found_extra(
-                                    &expected_label,
-                                    expected,
-                                    &found_label,
-                                    found,
-                                    &sort_string(values.expected),
-                                    &sort_string(values.found),
-                                );
-                            }
+                            diag.note_expected_found_extra(
+                                &expected_label,
+                                expected,
+                                &found_label,
+                                found,
+                                sort_string(values.expected),
+                                sort_string(values.found),
+                            );
                         }
                     }
                 }

tests/ui/error-emitter/E0308-clarification.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -Zunstable-options --error-format=human-unicode --color=always
+//@ only-linux
+// Ensure that when we have a type error where both types have the same textual representation, the
+// diagnostic machinery highlights the clarifying comment that comes after in parentheses.
+trait Foo: Copy + ToString {}
+
+impl<T: Copy + ToString> Foo for T {}
+
+fn hide<T: Foo>(x: T) -> impl Foo {
+    x
+}
+
+fn main() {
+    let mut x = (hide(0_u32), hide(0_i32));
+    x = (x.1, x.0);
+}

tests/ui/impl-trait/impl-trait-in-macro.stderr

@@ -12,8 +12,8 @@ LL |     let mut a = x;
 LL |     a = y;
    |         ^ expected type parameter `impl Debug`, found a different type parameter `impl Debug`
    |
-   = note: expected type parameter `impl Debug` (type parameter `impl Debug`)
-              found type parameter `impl Debug` (type parameter `impl Debug`)
+   = note: expected type parameter `impl Debug`
+              found type parameter `impl Debug`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 

tests/ui/impl-trait/universal-two-impl-traits.stderr

@@ -10,8 +10,8 @@ LL |     let mut a = x;
 LL |     a = y;
    |         ^ expected type parameter `impl Debug`, found a different type parameter `impl Debug`
    |
-   = note: expected type parameter `impl Debug` (type parameter `impl Debug`)
-              found type parameter `impl Debug` (type parameter `impl Debug`)
+   = note: expected type parameter `impl Debug`
+              found type parameter `impl Debug`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters