Fix handling of unmatched angle brackets in parser

compiler/rustc_parse/src/parser/path.rs

@@ -352,29 +352,38 @@ impl<'a> Parser<'a> {
         debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
         match self.parse_angle_args() {
             Ok(args) => Ok(args),
-            Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
+            Err(mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
+                // Swap `self` with our backup of the parser state before attempting to parse
+                // generic arguments.
+                let snapshot = mem::replace(self, snapshot.unwrap());
+
+                // Eat the unmatched angle brackets.
+                let all_angle_brackets = (0..snapshot.unmatched_angle_bracket_count)
+                    .fold(true, |a, _| a && self.eat_lt());
+
+                if !all_angle_brackets {
+                    // If there are other tokens in between the extraneous `<`s, we cannot simply
+                    // suggest to remove them. This check also prevents us from accidentally ending
+                    // up in the middle of a multibyte character (issue #84104).
+                    let _ = mem::replace(self, snapshot);
+                    Err(e)
+                } else {
                     // Cancel error from being unable to find `>`. We know the error
                     // must have been this due to a non-zero unmatched angle bracket
                     // count.
                     e.cancel();
 
-                // Swap `self` with our backup of the parser state before attempting to parse
-                // generic arguments.
-                let snapshot = mem::replace(self, snapshot.unwrap());
-
                     debug!(
                         "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
                          snapshot.count={:?}",
                         snapshot.unmatched_angle_bracket_count,
                     );
 
-                // Eat the unmatched angle brackets.
-                for _ in 0..snapshot.unmatched_angle_bracket_count {
-                    self.eat_lt();
-                }
-
                     // Make a span over ${unmatched angle bracket count} characters.
-                let span = lo.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count));
+                    // This is safe because `all_angle_brackets` ensures that there are only `<`s,
+                    // i.e. no multibyte characters, in this range.
+                    let span =
+                        lo.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count));
                     self.struct_span_err(
                         span,
                         &format!(
@@ -396,6 +405,7 @@ impl<'a> Parser<'a> {
                     // Try again without unmatched angle bracket characters.
                     self.parse_angle_args()
                 }
+            }
             Err(e) => Err(e),
         }
     }

src/test/ui/parser/issue-84104.rs

@@ -0,0 +1,3 @@
+// error-pattern: this file contains an unclosed delimiter
+// error-pattern: expected one of
+#[i=i::<ښܖ<

src/test/ui/parser/issue-84104.stderr

@@ -0,0 +1,16 @@
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-84104.rs:3:13
+   |
+LL | #[i=i::<ښܖ<
+   |  -          ^
+   |  |
+   |  unclosed delimiter
+
+error: expected one of `>`, a const expression, lifetime, or type, found `]`
+  --> $DIR/issue-84104.rs:3:13
+   |
+LL | #[i=i::<ښܖ<
+   |             ^ expected one of `>`, a const expression, lifetime, or type
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/unmatched-langle-1.rs

@@ -0,0 +1,9 @@
+// Check that a suggestion is issued if there are too many `<`s in a
+// generic argument list, and that the parser recovers properly.
+
+fn main() {
+    foo::<<<<Ty<i32>>();
+    //~^ ERROR: unmatched angle brackets
+    //~| ERROR: cannot find function `foo` in this scope [E0425]
+    //~| ERROR: cannot find type `Ty` in this scope [E0412]
+}

src/test/ui/parser/unmatched-langle-1.stderr

@@ -0,0 +1,22 @@
+error: unmatched angle brackets
+  --> $DIR/unmatched-langle-1.rs:5:10
+   |
+LL |     foo::<<<<Ty<i32>>();
+   |          ^^^ help: remove extra angle brackets
+
+error[E0425]: cannot find function `foo` in this scope
+  --> $DIR/unmatched-langle-1.rs:5:5
+   |
+LL |     foo::<<<<Ty<i32>>();
+   |     ^^^ not found in this scope
+
+error[E0412]: cannot find type `Ty` in this scope
+  --> $DIR/unmatched-langle-1.rs:5:14
+   |
+LL |     foo::<<<<Ty<i32>>();
+   |              ^^ not found in this scope
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0412, E0425.
+For more information about an error, try `rustc --explain E0412`.

src/test/ui/parser/unmatched-langle-2.rs

@@ -0,0 +1,15 @@
+// When there are too many opening `<`s, the compiler would previously
+// suggest nonsense if the `<`s were interspersed with other tokens:
+//
+//   error: unmatched angle brackets
+//    --> unmatched-langle.rs:2:10
+//     |
+//   2 |     foo::<Ty<<<i32>();
+//     |          ^^^ help: remove extra angle brackets
+//
+// This test makes sure that this is no longer happening.
+
+fn main() {
+    foo::<Ty<<<i32>();
+    //~^ ERROR: expected `::`, found `(`
+}

src/test/ui/parser/unmatched-langle-2.stderr

@@ -0,0 +1,8 @@
+error: expected `::`, found `(`
+  --> $DIR/unmatched-langle-2.rs:13:20
+   |
+LL |     foo::<Ty<<<i32>();
+   |                    ^ expected `::`
+
+error: aborting due to previous error
+