fix panic on ellipsis in pattern

crates/ra_hir_def/src/body/lower.rs

@@ -650,6 +650,7 @@ impl ExprCollector<'_> {
             ast::Pat::SlicePat(p) => {
                 let SlicePatComponents { prefix, slice, suffix } = p.components();
 
+                // FIXME properly handle `DotDotPat`
                 Pat::Slice {
                     prefix: prefix.into_iter().map(|p| self.collect_pat(p)).collect(),
                     slice: slice.map(|p| self.collect_pat(p)),
@@ -666,9 +667,15 @@ impl ExprCollector<'_> {
                     Pat::Missing
                 }
             }
-            ast::Pat::DotDotPat(_) => unreachable!(
-                "`DotDotPat` requires special handling and should not be mapped to a Pat."
-            ),
+            ast::Pat::DotDotPat(_) => {
+                // `DotDotPat` requires special handling and should not be mapped
+                // to a Pat. Here we are using `Pat::Missing` as a fallback for
+                // when `DotDotPat` is mapped to `Pat`, which can easily happen
+                // when the source code being analyzed has a malformed pattern
+                // which includes `..` in a place where it isn't valid.
+
+                Pat::Missing
+            }
             // FIXME: implement
             ast::Pat::BoxPat(_) | ast::Pat::RangePat(_) | ast::Pat::MacroPat(_) => Pat::Missing,
         };

crates/ra_hir_ty/src/tests/regression.rs

@@ -484,3 +484,52 @@ fn main() {
 
     assert_eq!("()", super::type_at_pos(&db, pos));
 }
+
+#[test]
+fn issue_3999_slice() {
+    assert_snapshot!(
+        infer(r#"
+fn foo(params: &[usize]) {
+    match params {
+        [ps @ .., _] => {}
+    }
+}
+"#),
+        @r###"
+    [8; 14) 'params': &[usize]
+    [26; 81) '{     ...   } }': ()
+    [32; 79) 'match ...     }': ()
+    [38; 44) 'params': &[usize]
+    [55; 67) '[ps @ .., _]': [usize]
+    [65; 66) '_': usize
+    [71; 73) '{}': ()
+    "###
+    );
+}
+
+#[test]
+fn issue_3999_struct() {
+    // rust-analyzer should not panic on seeing this malformed
+    // record pattern.
+    assert_snapshot!(
+        infer(r#"
+struct Bar {
+    a: bool,
+}
+fn foo(b: Bar) {
+    match b {
+        Bar { a: .. } => {},
+    }
+}
+"#),
+        @r###"
+    [36; 37) 'b': Bar
+    [44; 96) '{     ...   } }': ()
+    [50; 94) 'match ...     }': ()
+    [56; 57) 'b': Bar
+    [68; 81) 'Bar { a: .. }': Bar
+    [77; 79) '..': bool
+    [85; 87) '{}': ()
+    "###
+    );
+}