Avoid unbounded O(n^2) when parsing nested type args

When encountering code like `f::<f::<f::<f::<f::<f::<f::<f::<...` with
unmatched closing angle brackets, add a linear check that avoids the
exponential behavior of the parse recovery mechanism.

Fix #117080.

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 597303cae73..41b19ecb63a 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -159,8 +159,9 @@ pub struct Parser<'a> {
     /// appropriately.
     ///
     /// See the comments in the `parse_path_segment` function for more details.
-    unmatched_angle_bracket_count: u32,
-    max_angle_bracket_count: u32,
+    unmatched_angle_bracket_count: u16,
+    max_angle_bracket_count: u16,
+    angle_bracket_nesting: u16,
 
     last_unexpected_token_span: Option<Span>,
     /// If present, this `Parser` is not parsing Rust code but rather a macro call.
@@ -394,6 +395,7 @@ impl<'a> Parser<'a> {
             break_last_token: false,
             unmatched_angle_bracket_count: 0,
             max_angle_bracket_count: 0,
+            angle_bracket_nesting: 0,
             last_unexpected_token_span: None,
             subparser_name,
             capture_state: CaptureState {