Fix spurious error on non-ident-headed type arguments

[StreamDemon]

Summary

• Fixes Parser: spurious 'expected identifier' for non-ident-headed type args (Vec<&str>, tuples, arrays) #78: Vec<&str>, Vec<(i64, i64)>, Vec<[u8; 4]>, and Map<String, Vec<&str>> were rejected with a spurious "expected identifier" even though they are legal per §16 (type_arg = type | IDENT "=" type).
• Root cause: the associated-type-binding probe in type_args_after_open called ident() and backtracked by resetting self.pos — but ident() pushes its failure diagnostic into self.errors first, and the checkpoint never truncated the error list. The fallback type parse succeeded while the stale error survived, so parse_program returned Err.
• Fix: commit to the binding by peeking IDENT "=" before consuming anything — the same lookahead shape as turbofish and the send-statement head — so no failed probe ever runs. This also improves recovery for a genuinely bad binding value (Item = <garbage> now recovers to the next ,/> instead of re-parsing Item as a type argument).
• This was the parser's only probe-and-backtrack site (self.pos = checkpoint appears nowhere else), so no other production has this failure mode.

Related Issue

Closes #78. Found during the enhancement wave (PR #71) while designing the attribute-argument parser; deliberately split out because it is a behavior change.

Spec Sections Affected

None — the spec and docs/reference/grammar.md already say these shapes are legal; this makes the parser match them.

Checklist

• Code follows the Sploosh design principles (one way to do it, explicit over implicit, etc.)
• Documentation updated in relevant docs/ pages — N/A; crates/AGENTS.md's accepted-subset list already claimed these shapes, the fix makes reality match it
• Tests added or updated
• All build targets still compile (if applicable)
• Spec-only PR (skip Build Targets section if checked)

Build Targets Tested

• cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace all green locally (56 tests, 14 corpus fixtures).

Test Plan

• non_ident_headed_type_args_parse — the four probe cases from Parser: spurious 'expected identifier' for non-ident-headed type args (Vec<&str>, tuples, arrays) #78, plus a structural assertion that Vec<&str>'s argument survives as TypeArg::Type(Type::Reference { .. }) wrapping str.
• assoc_binding_with_non_ident_headed_value_parses — Iter<Item = Vec<&str>> keeps working through the new lookahead, asserted down to the TypeArg::Assoc shape.
• New tests/corpus/generic_args.sp fixture covers the shapes end-to-end via the auto-discovering corpus harness.
• Existing assoc-binding test (dyn_trait_type_args_with_assoc_binding) and the full suite pass unchanged.

---

Summary by cubic

Fixes a false "expected identifier" error when parsing generic type arguments that don’t start with an identifier. Uses IDENT '=' lookahead for associated-type bindings, so valid shapes like Vec<&str> now parse correctly (closes #78).

• Bug Fixes
  • Replaced probe-and-backtrack in type_args_after_open with IDENT '=' lookahead to avoid stale diagnostics and improve recovery to the next comma or '>' on bad bindings.
  • Added tests for non-ident-headed type args and assoc bindings with such values, plus a new corpus fixture tests/corpus/generic_args.sp.

^{Written for commit fcc29f0. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant Input as Token Stream
    participant Parser as type_args_after_open()
    participant Errors as Error List
    participant Output as Vec<TypeArg>

    Note over Parser: Loop over type arguments until '>' or EOF
    loop for each type arg
        Parser->>Input: peek current token
        Input-->>Parser: TokenKind
        Parser->>Input: peek next token (peek_kind_at(1))
        Input-->>Parser: TokenKind or None

        alt IDENT followed by '=' (NEW lookahead – no probe)
            Parser->>Input: consume IDENT
            Parser->>Input: consume '='
            Parser->>Input: parse type()
            Input-->>Parser: Ty
            Parser->>Parser: push TypeArg::Assoc { name, ty }
        else else – direct type parse
            Parser->>Input: parse type()
            alt type parsed successfully
                Input-->>Parser: Ty
                Parser->>Parser: push TypeArg::Type(ty)
            else type parse fails
                Parser->>Parser: recover_until(Comma, Gt)
                Note over Parser: No stale error from abandoned ident() probe
            end
        end

        Parser->>Input: try consume comma
        alt comma consumed
            Parser->>Parser: continue loop
        else no comma
            Parser->>Parser: break
        end
    end

    Parser-->>Output: list of TypeArg

Loading

<sub>Auto-approved: Fix #78: replace probe-and-backtrack with lookahead in type_args_after_open to avoid stale errors on non-ident-headed type args like Vec<&str>. Adds tests.

Re-trigger cubic</sub>