Forgetting a right angle bracket (`>`) gives: "error: `Fn` traits cannot take lifetime parameters"

[theemathas]

Code

trait Trait<'a> {}

fn foo<T: Trait<'static>() {}

Current output

error: `Fn` traits cannot take lifetime parameters
 --> src/lib.rs:3:16
  |
3 | fn foo<T: Trait<'static>() {}
  |                ^^^^^^^^^
  |
help: consider using a higher-ranked trait bound instead
  |
3 - fn foo<T: Trait<'static>() {}
3 + fn foo<T: for<'static> Trait() {}
  |

error: expected one of `+`, `,`, `->`, `=`, or `>`, found `{`
 --> src/lib.rs:3:28
  |
3 | fn foo<T: Trait<'static>() {}
  |                            ^ expected one of `+`, `,`, `->`, `=`, or `>`
  |
help: you might have meant to end the type parameters here
  |
3 | fn foo<T: Trait<'static>()> {}
  |                           +

error: could not compile `playground` (lib) due to 2 previous errors

Desired output

Rationale and extra context

The first error is confusing and unhelpful. Maybe only suggest it when the trait is named Fn/FnMut/FnOnce?

Also, the second error suggests adding the > in the wrong place.

Other cases

Rust Version

Reproducible on the playground with rustc 1.89.0-nightly (2025-05-22 2eef47813f25df637026)

Anything else?

No response

[fmease]

That diagnostic was suggested by me in #103490 but it sure doesn't make sense here. IIRC we do have some sort of stack of angle brackets in the parser, or at least some sort of flag indicating if we have unclosed angle brackets (in particular). If that stack is non-empty / that flag is set, we should simply suppress this diagnostic.

[fmease]

Maybe only suggest it when the trait is named Fn/FnMut/FnOnce?

Personally speaking not in favor. I dislike hard-coding lists of names like that. It would also need to include AsyncFn, AsyncFnMut, AsyncFnOnce, you get my point.

[xizheyin]

IIRC we do have some sort of stack of angle brackets in the parser, or at least some sort of flag indicating if we have unclosed angle brackets (in particular).

Maybe here

rust/compiler/rustc_parse/src/lexer/diagnostics.rs

Lines 11 to 26 in 3d86494

	#[derive(Default)]
	pub(super) struct TokenTreeDiagInfo {
	/// Stack of open delimiters and their spans. Used for error message.
	pub open_delimiters: Vec<(Delimiter, Span)>,
	pub unmatched_delims: Vec<UnmatchedDelim>,
	/// Used only for error recovery when arriving to EOF with mismatched braces.
	pub last_unclosed_found_span: Option<Span>,
	/// Collect empty block spans that might have been auto-inserted by editors.
	pub empty_block_spans: Vec<Span>,
	/// Collect the spans of braces (Open, Close). Used only
	/// for detecting if blocks are empty and only braces.
	pub matching_block_spans: Vec<(Span, Span)>,
	}