Throw error when encountering ... instead of .. while destructing a pattern

Added tests and stderr output

Author: rakshith-ravi

src/librustc_parse/parser/pat.rs

@@ -295,6 +295,8 @@ impl<'a> Parser<'a> {
             // A rest pattern `..`.
             self.bump(); // `..`
             PatKind::Rest
+        } else if self.check(&token::DotDotDot) && !self.is_pat_range_end_start(1) {
+            self.recover_dotdotdot_rest_pat(lo)
         } else if let Some(form) = self.parse_range_end() {
             self.parse_pat_range_to(form)? // `..=X`, `...X`, or `..X`.
         } else if self.eat_keyword(kw::Underscore) {
@@ -362,6 +364,25 @@ impl<'a> Parser<'a> {
         Ok(pat)
     }
 
+    /// Recover from a typoed `...` pattern that was encountered
+    /// Ref: Issue #70388
+    fn recover_dotdotdot_rest_pat(&mut self, lo: Span) -> PatKind {
+        // A typoed rest pattern `...`.
+        self.bump(); // `...`
+
+        // The user probably mistook `...` for a rest pattern `..`.
+        self.struct_span_err(lo, "unexpected `...`")
+            .span_label(lo, "not a valid pattern")
+            .span_suggestion_short(
+                lo,
+                "for a rest pattern, use `..` instead of `...`",
+                "..".to_owned(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        PatKind::Rest
+    }
+
     /// Try to recover the more general form `intersect ::= $pat_lhs @ $pat_rhs`.
     ///
     /// Allowed binding patterns generated by `binding ::= ref? mut? $ident @ $pat_rhs`

src/test/ui/parser/issue-70388-recover-dotdotdot-rest-pat.rs

@@ -0,0 +1,7 @@
+struct Foo(i32);
+
+fn main() {
+    let Foo(...) = Foo(0); //~ ERROR unexpected `...`
+    let [_, ..., _] = [0, 1]; //~ ERROR unexpected `...`
+    let _recovery_witness: () = 0; //~ ERROR mismatched types
+}

src/test/ui/parser/issue-70388-recover-dotdotdot-rest-pat.stderr

@@ -0,0 +1,29 @@
+error: unexpected `...`
+  --> $DIR/issue-70388-recover-dotdotdot-rest-pat.rs:4:13
+   |
+LL |     let Foo(...) = Foo(0);
+   |             ^^^
+   |             |
+   |             not a valid pattern
+   |             help: for a rest pattern, use `..` instead of `...`
+
+error: unexpected `...`
+  --> $DIR/issue-70388-recover-dotdotdot-rest-pat.rs:5:13
+   |
+LL |     let [_, ..., _] = [0, 1];
+   |             ^^^
+   |             |
+   |             not a valid pattern
+   |             help: for a rest pattern, use `..` instead of `...`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-70388-recover-dotdotdot-rest-pat.rs:6:33
+   |
+LL |     let _recovery_witness: () = 0;
+   |                            --   ^ expected `()`, found integer
+   |                            |
+   |                            expected due to this
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.