From 35dd62e915905b6a93e2069a6d155cdf00cd254a Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Tue, 24 Nov 2020 13:41:21 +0100
Subject: Properly infer tuple patterns when encountering ellipsis

---
 crates/hir_ty/src/infer/pat.rs      | 25 +++++++++++++-------
 crates/hir_ty/src/tests/patterns.rs | 47 +++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 8 deletions(-)

diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
index cde2ab82b..b3fd74eab 100644
--- a/crates/hir_ty/src/infer/pat.rs
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -111,20 +111,29 @@ impl<'a> InferenceContext<'a> {
         let expected = expected;
 
         let ty = match &body[pat] {
-            Pat::Tuple { ref args, .. } => {
+            Pat::Tuple { ref args, ellipsis } => {
                 let expectations = match expected.as_tuple() {
                     Some(parameters) => &*parameters.0,
                     _ => &[],
                 };
-                let expectations_iter = expectations.iter().chain(repeat(&Ty::Unknown));
 
-                let inner_tys = args
-                    .iter()
-                    .zip(expectations_iter)
-                    .map(|(&pat, ty)| self.infer_pat(pat, ty, default_bm))
-                    .collect();
+                let (pre, post) = match ellipsis {
+                    &Some(idx) => args.split_at(idx),
+                    None => (&args[..], &[][..]),
+                };
+                let uncovered_range = pre.len()..expectations.len().saturating_sub(post.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());
+                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(post.iter().zip(expectations_iter).map(infer_pat));
 
-                Ty::apply(TypeCtor::Tuple { cardinality: args.len() as u16 }, Substs(inner_tys))
+                Ty::apply(
+                    TypeCtor::Tuple { cardinality: inner_tys.len() as u16 },
+                    Substs(inner_tys.into()),
+                )
             }
             Pat::Or(ref pats) => {
                 if let Some((first_pat, rest)) = pats.split_first() {
diff --git a/crates/hir_ty/src/tests/patterns.rs b/crates/hir_ty/src/tests/patterns.rs
index 6a965ac4f..5776de280 100644
--- a/crates/hir_ty/src/tests/patterns.rs
+++ b/crates/hir_ty/src/tests/patterns.rs
@@ -679,3 +679,50 @@ fn box_pattern() {
         "#]],
     );
 }
+
+#[test]
+fn tuple_ellipsis_pattern() {
+    check_infer(
+        r#"
+fn foo(tuple: (u8, i16, f32)) {
+    match tuple {
+        (.., b, c) => {},
+        (a, .., c) => {},
+        (a, b, ..) => {},
+        (a, b) => {/*too short*/}
+        (a, b, c, d) => {/*too long*/}
+        _ => {}
+    }
+}"#,
+        expect![[r#"
+            7..12 'tuple': (u8, i16, f32)
+            30..224 '{     ...   } }': ()
+            36..222 'match ...     }': ()
+            42..47 'tuple': (u8, i16, f32)
+            58..68 '(.., b, c)': (u8, i16, f32)
+            63..64 'b': i16
+            66..67 'c': f32
+            72..74 '{}': ()
+            84..94 '(a, .., c)': (u8, i16, f32)
+            85..86 'a': u8
+            92..93 'c': f32
+            98..100 '{}': ()
+            110..120 '(a, b, ..)': (u8, i16, f32)
+            111..112 'a': u8
+            114..115 'b': i16
+            124..126 '{}': ()
+            136..142 '(a, b)': (u8, i16, f32)
+            137..138 'a': u8
+            140..141 'b': i16
+            146..161 '{/*too short*/}': ()
+            170..182 '(a, b, c, d)': (u8, i16, f32, {unknown})
+            171..172 'a': u8
+            174..175 'b': i16
+            177..178 'c': f32
+            180..181 'd': {unknown}
+            186..200 '{/*too long*/}': ()
+            209..210 '_': (u8, i16, f32)
+            214..216 '{}': ()
+        "#]],
+    );
+}
-- 
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