Rollup merge of #86010 - FabianWolff:ICE-parser, r=varkor

Fix two ICEs in the parser

This pull request fixes #84104 and fixes #84148. The latter is caused by an invalid `assert_ne!()` in the parser, which I have simply removed because the error is then caught in another part of the parser.

#84104 is somewhat more subtle and has to do with a suggestion to remove extraneous `<` characters; for instance:
```rust
fn main() {
    foo::<Ty<<<i32>();
}
```
currently leads to
```
error: unmatched angle brackets
 --> unmatched-langle.rs:2:10
  |
2 |     foo::<Ty<<<i32>();
  |          ^^^ help: remove extra angle brackets
```
which is obviously wrong and stems from the fact that the code for issuing the above suggestion does not consider the possibility that there might be other tokens in between the opening angle brackets. In #84104, this has led to a span being generated that ends in the middle of a multi-byte character (because the code issuing the suggestion thought that it was only skipping over `<`, which are single-byte), causing an ICE.

2 files changed, 47 insertions, 38 deletions

diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 9cc600d9ede..953c6915068 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -352,49 +352,59 @@ 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 => {
-                // 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();
-
+            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());
 
-                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));
-                self.struct_span_err(
-                    span,
-                    &format!(
-                        "unmatched angle bracket{}",
-                        pluralize!(snapshot.unmatched_angle_bracket_count)
-                    ),
-                )
-                .span_suggestion(
-                    span,
-                    &format!(
-                        "remove extra angle bracket{}",
-                        pluralize!(snapshot.unmatched_angle_bracket_count)
-                    ),
-                    String::new(),
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+                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();
+
+                    debug!(
+                        "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
+                         snapshot.count={:?}",
+                        snapshot.unmatched_angle_bracket_count,
+                    );
+
+                    // Make a span over ${unmatched angle bracket count} characters.
+                    // 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!(
+                            "unmatched angle bracket{}",
+                            pluralize!(snapshot.unmatched_angle_bracket_count)
+                        ),
+                    )
+                    .span_suggestion(
+                        span,
+                        &format!(
+                            "remove extra angle bracket{}",
+                            pluralize!(snapshot.unmatched_angle_bracket_count)
+                        ),
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
 
-                // Try again without unmatched angle bracket characters.
-                self.parse_angle_args()
+                    // Try again without unmatched angle bracket characters.
+                    self.parse_angle_args()
+                }
             }
             Err(e) => Err(e),
         }
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 89cf2d7876e..de5a5632600 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -334,7 +334,6 @@ impl<'a> Parser<'a> {
         mut bounds: GenericBounds,
         plus: bool,
     ) -> PResult<'a, TyKind> {
-        assert_ne!(self.token, token::Question);
         if plus {
             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);