Merge #8875

8875: fix: false positive "Missing match arm" when an or-pattern has mismatched types r=flodiebold a=iDawer

![Screenshot_20210519_114510](https://user-images.githubusercontent.com/7803845/118768935-19e12c00-b86f-11eb-90c4-1eed3f2bf57f.jpg)
`InferenceResult` now records pattern type mismatches.

Co-authored-by: Dawer <[email protected]>

1 files changed, 19 insertions, 3 deletions

diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 47709c1e8..b9a69b79c 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -211,7 +211,7 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
 
         // FIXME: Due to shortcomings in the current type system implementation, only emit this
         // diagnostic if there are no type mismatches in the containing function.
-        if self.infer.type_mismatches.iter().next().is_some() {
+        if self.infer.expr_type_mismatches().next().is_some() {
             return;
         }
 
@@ -311,11 +311,12 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                 // necessary.
                 //
                 // FIXME we should use the type checker for this.
-                if pat_ty == match_expr_ty
+                if (pat_ty == match_expr_ty
                     || match_expr_ty
                         .as_reference()
                         .map(|(match_expr_ty, ..)| match_expr_ty == pat_ty)
-                        .unwrap_or(false)
+                        .unwrap_or(false))
+                    && types_of_subpatterns_do_match(pat, &cx.body, &infer)
                 {
                     // If we had a NotUsefulMatchArm diagnostic, we could
                     // check the usefulness of each pattern as we added it
@@ -496,6 +497,21 @@ pub fn record_pattern_missing_fields(
     Some((variant_def, missed_fields, exhaustive))
 }
 
+fn types_of_subpatterns_do_match(pat: PatId, body: &Body, infer: &InferenceResult) -> bool {
+    fn walk(pat: PatId, body: &Body, infer: &InferenceResult, has_type_mismatches: &mut bool) {
+        match infer.type_mismatch_for_pat(pat) {
+            Some(_) => *has_type_mismatches = true,
+            None => {
+                body[pat].walk_child_pats(|subpat| walk(subpat, body, infer, has_type_mismatches))
+            }
+        }
+    }
+
+    let mut has_type_mismatches = false;
+    walk(pat, body, infer, &mut has_type_mismatches);
+    !has_type_mismatches
+}
+
 #[cfg(test)]
 mod tests {
     use crate::diagnostics::tests::check_diagnostics;