Do not recover from `Trait()` if generic list is unterminated #151171

estebank · 2026-01-15T19:33:10Z

If we encounter fn foo<T: Trait(), the recovery logic would it as if Trait was intended to use the Fn-like trait syntax, but if we don't know for certain that we've parsed a full trait bound (fn foo<T: Trait()>), we bail from the recovery as more likely there could have been a missing closing > and the ( corresponds to the start of the fn parameter list.

Fix #141436.

If we encounter `fn foo<T: Trait()`, the recovery logic would it as if `Trait` was intended to use the Fn-like trait syntax, but if we don't know for certain that we've parsed a full trait bound (`fn foo<T: Trait()>`), we bail from the recovery as more likely there could have been a missing closing `>` and the `(` corresponds to the start of the fn parameter list.

rustbot · 2026-01-15T19:33:15Z

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estebank · 2026-01-15T19:40:55Z

compiler/rustc_parse/src/parser/ty.rs

+        let params = match self.parse_fn_params(&mode) {
+            Ok(params) => params,
+            Err(err) => {
+                if let Some(snapshot) = snapshot {
+                    self.restore_snapshot(snapshot);
+                    err.cancel();
+                    return Ok(());
+                } else {
+                    return Err(err);
+                }
+            }
+        };


These changes weren't strictly necessary for the test case, but we should always be restoring snapshots if we encounter parse errors in recovery logic.

JonathanBrouwer · 2026-01-17T10:52:54Z

@rustbot reroll
Looks right to me but don't feel comfortable enough with the parsing code to approve

petrochenkov · 2026-01-19T13:33:03Z

New global parser state and snapshots, there are always new rock bottoms for the parser to hit.
I'd rather revert #104531 entirely.
@rustbot reroll

davidtwco

While there clearly isn't broad consensus on how much recovery in the parser is too much, my disposition is that parser complexity is relatively low-risk for the impact changes like these have.

View changes since this review

davidtwco · 2026-01-19T16:05:43Z

@bors r+

rust-bors · 2026-01-19T16:05:46Z

📌 Commit c6c4372 has been approved by davidtwco

It is now in the queue for this repository.

@ghost

…uwer Rollup of 15 pull requests Successful merges: - #148623 (Ignore `#[doc(hidden)]` items when computing trimmed paths for printing) - #150550 (Miscellaneous cleanups to borrowck related code) - #150879 (Remove the diagnostic lints) - #150895 (rustc_errors: Add (heuristic) Syntax Highlighting for `rustc --explain`) - #150987 (remote-test-server: Fix header in batch mode) - #151004 (std: implement `sleep_until` on Apple platforms) - #151045 (Simplify some literal-value negations with `u128::wrapping_neg`) - #151119 (Support pointers in type reflection) - #151171 (Do not recover from `Trait()` if generic list is unterminated) - #151231 (HIR typeck cleanup: clarify and re-style `check_expr_unop`) - #151249 (Parse ident with allowing recovery when trying to diagnose) - #151295 (THIR patterns: Use `ty::Value` in more places throughout `const_to_pat`) - #151326 (Remove `DiagMessage::Translated` in favour of `DiagMessage::Str`) - #151361 (add test for issue 61463) - #151371 (Add `S-blocked` to `labels_blocking_approval`) r? @ghost

Rollup merge of #151171 - issue-141436, r=davidtwco Do not recover from `Trait()` if generic list is unterminated If we encounter `fn foo<T: Trait()`, the recovery logic would it as if `Trait` was intended to use the Fn-like trait syntax, but if we don't know for certain that we've parsed a full trait bound (`fn foo<T: Trait()>`), we bail from the recovery as more likely there could have been a missing closing `>` and the `(` corresponds to the start of the fn parameter list. Fix #141436.

@ghost

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#148623 (Ignore `#[doc(hidden)]` items when computing trimmed paths for printing) - rust-lang/rust#150550 (Miscellaneous cleanups to borrowck related code) - rust-lang/rust#150879 (Remove the diagnostic lints) - rust-lang/rust#150895 (rustc_errors: Add (heuristic) Syntax Highlighting for `rustc --explain`) - rust-lang/rust#150987 (remote-test-server: Fix header in batch mode) - rust-lang/rust#151004 (std: implement `sleep_until` on Apple platforms) - rust-lang/rust#151045 (Simplify some literal-value negations with `u128::wrapping_neg`) - rust-lang/rust#151119 (Support pointers in type reflection) - rust-lang/rust#151171 (Do not recover from `Trait()` if generic list is unterminated) - rust-lang/rust#151231 (HIR typeck cleanup: clarify and re-style `check_expr_unop`) - rust-lang/rust#151249 (Parse ident with allowing recovery when trying to diagnose) - rust-lang/rust#151295 (THIR patterns: Use `ty::Value` in more places throughout `const_to_pat`) - rust-lang/rust#151326 (Remove `DiagMessage::Translated` in favour of `DiagMessage::Str`) - rust-lang/rust#151361 (add test for issue 61463) - rust-lang/rust#151371 (Add `S-blocked` to `labels_blocking_approval`) r? @ghost

@ghost

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#148623 (Ignore `#[doc(hidden)]` items when computing trimmed paths for printing) - rust-lang/rust#150550 (Miscellaneous cleanups to borrowck related code) - rust-lang/rust#150879 (Remove the diagnostic lints) - rust-lang/rust#150895 (rustc_errors: Add (heuristic) Syntax Highlighting for `rustc --explain`) - rust-lang/rust#150987 (remote-test-server: Fix header in batch mode) - rust-lang/rust#151004 (std: implement `sleep_until` on Apple platforms) - rust-lang/rust#151045 (Simplify some literal-value negations with `u128::wrapping_neg`) - rust-lang/rust#151119 (Support pointers in type reflection) - rust-lang/rust#151171 (Do not recover from `Trait()` if generic list is unterminated) - rust-lang/rust#151231 (HIR typeck cleanup: clarify and re-style `check_expr_unop`) - rust-lang/rust#151249 (Parse ident with allowing recovery when trying to diagnose) - rust-lang/rust#151295 (THIR patterns: Use `ty::Value` in more places throughout `const_to_pat`) - rust-lang/rust#151326 (Remove `DiagMessage::Translated` in favour of `DiagMessage::Str`) - rust-lang/rust#151361 (add test for issue 61463) - rust-lang/rust#151371 (Add `S-blocked` to `labels_blocking_approval`) r? @ghost

@ghost

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#148623 (Ignore `#[doc(hidden)]` items when computing trimmed paths for printing) - rust-lang/rust#150550 (Miscellaneous cleanups to borrowck related code) - rust-lang/rust#150879 (Remove the diagnostic lints) - rust-lang/rust#150895 (rustc_errors: Add (heuristic) Syntax Highlighting for `rustc --explain`) - rust-lang/rust#150987 (remote-test-server: Fix header in batch mode) - rust-lang/rust#151004 (std: implement `sleep_until` on Apple platforms) - rust-lang/rust#151045 (Simplify some literal-value negations with `u128::wrapping_neg`) - rust-lang/rust#151119 (Support pointers in type reflection) - rust-lang/rust#151171 (Do not recover from `Trait()` if generic list is unterminated) - rust-lang/rust#151231 (HIR typeck cleanup: clarify and re-style `check_expr_unop`) - rust-lang/rust#151249 (Parse ident with allowing recovery when trying to diagnose) - rust-lang/rust#151295 (THIR patterns: Use `ty::Value` in more places throughout `const_to_pat`) - rust-lang/rust#151326 (Remove `DiagMessage::Translated` in favour of `DiagMessage::Str`) - rust-lang/rust#151361 (add test for issue 61463) - rust-lang/rust#151371 (Add `S-blocked` to `labels_blocking_approval`) r? @ghost

@ghost

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#148623 (Ignore `#[doc(hidden)]` items when computing trimmed paths for printing) - rust-lang/rust#150550 (Miscellaneous cleanups to borrowck related code) - rust-lang/rust#150879 (Remove the diagnostic lints) - rust-lang/rust#150895 (rustc_errors: Add (heuristic) Syntax Highlighting for `rustc --explain`) - rust-lang/rust#150987 (remote-test-server: Fix header in batch mode) - rust-lang/rust#151004 (std: implement `sleep_until` on Apple platforms) - rust-lang/rust#151045 (Simplify some literal-value negations with `u128::wrapping_neg`) - rust-lang/rust#151119 (Support pointers in type reflection) - rust-lang/rust#151171 (Do not recover from `Trait()` if generic list is unterminated) - rust-lang/rust#151231 (HIR typeck cleanup: clarify and re-style `check_expr_unop`) - rust-lang/rust#151249 (Parse ident with allowing recovery when trying to diagnose) - rust-lang/rust#151295 (THIR patterns: Use `ty::Value` in more places throughout `const_to_pat`) - rust-lang/rust#151326 (Remove `DiagMessage::Translated` in favour of `DiagMessage::Str`) - rust-lang/rust#151361 (add test for issue 61463) - rust-lang/rust#151371 (Add `S-blocked` to `labels_blocking_approval`) r? @ghost

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 15, 2026

rustbot assigned JonathanBrouwer Jan 15, 2026

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jan 15, 2026

estebank commented Jan 15, 2026

View reviewed changes

Use default field values in Parser

c6c4372

rustbot assigned petrochenkov and unassigned JonathanBrouwer Jan 17, 2026

rustbot assigned davidtwco and unassigned petrochenkov Jan 19, 2026

davidtwco approved these changes Jan 19, 2026

View reviewed changes

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 19, 2026

JonathanBrouwer mentioned this pull request Jan 19, 2026

Rollup of 15 pull requests #151381

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rust-bors bot merged commit 1968df8 into rust-lang:main Jan 20, 2026
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Do not recover from `Trait()` if generic list is unterminated #151171

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