Rollup merge of rust-lang#91098 - compiler-errors:issue-91058, r=estebank

Don't suggest certain fixups (`.field`, `.await`, etc) when reporting errors while matching on arrays

When we have a type mismatch with a `cause.code` that is an `ObligationCauseCode::Pattern`, skip suggesting fixes like adding `.await` or accessing a struct's `.field` if the pattern's `root_ty` differs from the `expected` ty. This occurs in situations like this:

```rust
struct S(());

fn main() {
    let array = [S(())];

    match array {
        [()] => {}
        _ => {}
    }
}
```

I think what's happening here is a layer of `[_; N]` is peeled off of both types and we end up seeing the mismatch between just `S` and `()`, but when we suggest a fixup, that applies to the expression with type `root_ty`.

---

Questions:
1. Should this check live here, above all of the suggestions, or should I push this down into every suggestion when we match `ObligationCauseCode`?
2. Any other `ObligationCauseCode`s to check here?
3. Am I overlooking an easier way to get to this same conclusion without pattern matching on `ObligationCauseCode` and comparing `root_ty`?

Fixes rust-lang#91058

Author: matthiaskrgr

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

@@ -1695,11 +1695,23 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             _ => exp_found,
         };
-        debug!("exp_found {:?} terr {:?}", exp_found, terr);
+        debug!("exp_found {:?} terr {:?} cause.code {:?}", exp_found, terr, cause.code);
         if let Some(exp_found) = exp_found {
-            self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
-            self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
-            self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
+            let should_suggest_fixes = if let ObligationCauseCode::Pattern { root_ty, .. } =
+                &cause.code
+            {
+                // Skip if the root_ty of the pattern is not the same as the expected_ty.
+                // If these types aren't equal then we've probably peeled off a layer of arrays.
+                same_type_modulo_infer(self.resolve_vars_if_possible(*root_ty), exp_found.expected)
+            } else {
+                true
+            };
+
+            if should_suggest_fixes {
+                self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
+                self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
+                self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
+            }
         }
 
         // In some (most?) cases cause.body_id points to actual body, but in some cases
@@ -1879,7 +1891,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 .iter()
                 .filter(|field| field.vis.is_accessible_from(field.did, self.tcx))
                 .map(|field| (field.ident.name, field.ty(self.tcx, expected_substs)))
-                .find(|(_, ty)| ty::TyS::same_type(ty, exp_found.found))
+                .find(|(_, ty)| same_type_modulo_infer(ty, exp_found.found))
             {
                 if let ObligationCauseCode::Pattern { span: Some(span), .. } = cause.code {
                     if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
@@ -1944,7 +1956,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                         | (_, ty::Infer(_))
                                         | (ty::Param(_), _)
                                         | (ty::Infer(_), _) => {}
-                                        _ if ty::TyS::same_type(exp_ty, found_ty) => {}
+                                        _ if same_type_modulo_infer(exp_ty, found_ty) => {}
                                         _ => show_suggestion = false,
                                     };
                                 }

src/test/ui/issues/issue-5358-1.stderr

@@ -8,6 +8,10 @@ LL |         Either::Right(_) => {}
    |
    = note: expected struct `S`
                 found enum `Either<_, _>`
+help: you might have meant to use field `0` whose type is `Either<usize, usize>`
+   |
+LL |     match S(Either::Left(5)).0 {
+   |           ~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 

src/test/ui/match/issue-91058.rs

@@ -0,0 +1,11 @@
+struct S(());
+
+fn main() {
+    let array = [S(())];
+
+    match array {
+        [()] => {}
+        //~^ ERROR mismatched types [E0308]
+        _ => {}
+    }
+}

src/test/ui/match/issue-91058.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-91058.rs:7:10
+   |
+LL |     match array {
+   |           ----- this expression has type `[S; 1]`
+LL |         [()] => {}
+   |          ^^ expected struct `S`, found `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.