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Avoid unbounded O(n^2) when parsing nested type args
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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.
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estebank committed Oct 24, 2023
1 parent 855444e commit 54dffa1
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Showing 4 changed files with 51 additions and 6 deletions.
6 changes: 4 additions & 2 deletions compiler/rustc_parse/src/parser/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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 {
Expand Down
25 changes: 21 additions & 4 deletions compiler/rustc_parse/src/parser/path.rs
Original file line number Diff line number Diff line change
Expand Up @@ -487,10 +487,24 @@ impl<'a> Parser<'a> {
// Take a snapshot before attempting to parse - we can restore this later.
let snapshot = is_first_invocation.then(|| self.clone());

self.angle_bracket_nesting += 1;
debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
match self.parse_angle_args(ty_generics) {
Ok(args) => Ok(args),
Ok(args) => {
self.angle_bracket_nesting -= 1;
Ok(args)
}
Err(mut e) if self.angle_bracket_nesting > 10 => {
self.angle_bracket_nesting -= 1;
// When encountering severly malformed code where there are several levels of
// nested unclosed angle args (`f::<f::<f::<f::<...`), we avoid severe O(n^2)
// behavior by bailing out earlier (#117080).
e.emit();
rustc_errors::FatalError.raise();
}
Err(e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
self.angle_bracket_nesting -= 1;

// Swap `self` with our backup of the parser state before attempting to parse
// generic arguments.
let snapshot = mem::replace(self, snapshot.unwrap());
Expand Down Expand Up @@ -520,8 +534,8 @@ impl<'a> Parser<'a> {
// 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));
let span = lo
.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count.into()));
self.sess.emit_err(errors::UnmatchedAngle {
span,
plural: snapshot.unmatched_angle_bracket_count > 1,
Expand All @@ -531,7 +545,10 @@ impl<'a> Parser<'a> {
self.parse_angle_args(ty_generics)
}
}
Err(e) => Err(e),
Err(e) => {
self.angle_bracket_nesting -= 1;
Err(e)
}
}
}

Expand Down
13 changes: 13 additions & 0 deletions tests/ui/parser/deep-unmatched-angle-brackets.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
trait Mul<T> {
type Output;
}
trait Matrix: Mul<<Self as Matrix>::Row, Output = ()> {
type Row;
type Transpose: Matrix<Row = Self::Row>;
}
fn is_mul<S, T: Mul<S, Output = ()>>() {}
fn f<T: Matrix>() {
is_mul::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<>();

Check failure on line 10 in tests/ui/parser/deep-unmatched-angle-brackets.rs

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GitHub Actions / PR - mingw-check-tidy

line longer than 100 chars
//~^ ERROR expected one of `!`, `+`, `,`, `::`, or `>`, found `(`
}
fn main() {}
13 changes: 13 additions & 0 deletions tests/ui/parser/deep-unmatched-angle-brackets.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
error: expected one of `!`, `+`, `,`, `::`, or `>`, found `(`
--> $DIR/deep-unmatched-angle-brackets.rs:10:415
|
LL | ...::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<>();
| ^ expected one of `!`, `+`, `,`, `::`, or `>`
|
help: you might have meant to end the type parameters here
|
LL | is_mul::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<f::<>>();
| +

error: aborting due to previous error

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