Properly infer tuple struct patterns when encountering ellipsis

crates/hir_ty/src/infer/pat.rs

@@ -23,6 +23,7 @@ impl<'a> InferenceContext<'a> {
         expected: &Ty,
         default_bm: BindingMode,
         id: PatId,
+        ellipsis: Option<usize>,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
         let var_data = def.map(|it| variant_data(self.db.upcast(), it));
@@ -34,8 +35,15 @@ impl<'a> InferenceContext<'a> {
         let substs = ty.substs().unwrap_or_else(Substs::empty);
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
+        let (pre, post) = match ellipsis {
+            Some(idx) => subpats.split_at(idx),
+            None => (&subpats[..], &[][..]),
+        };
+        let post_idx_offset = field_tys.iter().count() - post.len();
 
-        for (i, &subpat) in subpats.iter().enumerate() {
+        let pre_iter = pre.iter().enumerate();
+        let post_iter = (post_idx_offset..).zip(post.iter());
+        for (i, &subpat) in pre_iter.chain(post_iter) {
             let expected_ty = var_data
                 .as_ref()
                 .and_then(|d| d.field(&Name::new_tuple_field(i)))
@@ -111,23 +119,23 @@ impl<'a> InferenceContext<'a> {
         let expected = expected;
 
         let ty = match &body[pat] {
-            Pat::Tuple { ref args, ellipsis } => {
+            &Pat::Tuple { ref args, ellipsis } => {
                 let expectations = match expected.as_tuple() {
                     Some(parameters) => &*parameters.0,
                     _ => &[],
                 };
 
                 let (pre, post) = match ellipsis {
-                    &Some(idx) => args.split_at(idx),
+                    Some(idx) => args.split_at(idx),
                     None => (&args[..], &[][..]),
                 };
-                let uncovered_range = pre.len()..expectations.len().saturating_sub(post.len());
+                let n_uncovered_patterns = expectations.len().saturating_sub(args.len());
                 let mut expectations_iter = expectations.iter().chain(repeat(&Ty::Unknown));
                 let mut infer_pat = |(&pat, ty)| self.infer_pat(pat, ty, default_bm);
 
-                let mut inner_tys = Vec::with_capacity(expectations.len());
+                let mut inner_tys = Vec::with_capacity(n_uncovered_patterns + args.len());
                 inner_tys.extend(pre.iter().zip(expectations_iter.by_ref()).map(&mut infer_pat));
-                inner_tys.extend(expectations_iter.by_ref().take(uncovered_range.len()).cloned());
+                inner_tys.extend(expectations_iter.by_ref().take(n_uncovered_patterns).cloned());
                 inner_tys.extend(post.iter().zip(expectations_iter).map(infer_pat));
 
                 Ty::apply(
@@ -159,9 +167,14 @@ impl<'a> InferenceContext<'a> {
                 let subty = self.infer_pat(*pat, expectation, default_bm);
                 Ty::apply_one(TypeCtor::Ref(*mutability), subty)
             }
-            Pat::TupleStruct { path: p, args: subpats, .. } => {
-                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm, pat)
-            }
+            Pat::TupleStruct { path: p, args: subpats, ellipsis } => self.infer_tuple_struct_pat(
+                p.as_ref(),
+                subpats,
+                expected,
+                default_bm,
+                pat,
+                *ellipsis,
+            ),
             Pat::Record { path: p, args: fields, ellipsis: _ } => {
                 self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
             }

crates/hir_ty/src/tests/patterns.rs

@@ -726,3 +726,51 @@ fn foo(tuple: (u8, i16, f32)) {
         "#]],
     );
 }
+
+#[test]
+fn tuple_struct_ellipsis_pattern() {
+    check_infer(
+        r#"
+struct Tuple(u8, i16, f32);
+fn foo(tuple: Tuple) {
+    match tuple {
+        Tuple(.., b, c) => {},
+        Tuple(a, .., c) => {},
+        Tuple(a, b, ..) => {},
+        Tuple(a, b) => {/*too short*/}
+        Tuple(a, b, c, d) => {/*too long*/}
+        _ => {}
+    }
+}"#,
+        expect![[r#"
+            35..40 'tuple': Tuple
+            49..268 '{     ...   } }': ()
+            55..266 'match ...     }': ()
+            61..66 'tuple': Tuple
+            77..92 'Tuple(.., b, c)': Tuple
+            87..88 'b': i16
+            90..91 'c': f32
+            96..98 '{}': ()
+            108..123 'Tuple(a, .., c)': Tuple
+            114..115 'a': u8
+            121..122 'c': f32
+            127..129 '{}': ()
+            139..154 'Tuple(a, b, ..)': Tuple
+            145..146 'a': u8
+            148..149 'b': i16
+            158..160 '{}': ()
+            170..181 'Tuple(a, b)': Tuple
+            176..177 'a': u8
+            179..180 'b': i16
+            185..200 '{/*too short*/}': ()
+            209..226 'Tuple(... c, d)': Tuple
+            215..216 'a': u8
+            218..219 'b': i16
+            221..222 'c': f32
+            224..225 'd': {unknown}
+            230..244 '{/*too long*/}': ()
+            253..254 '_': Tuple
+            258..260 '{}': ()
+        "#]],
+    );
+}