{"record":{"id":"488ee14c94618560","repo":"BoundaryML/baml","slug":"c-for-after-stmt-cannot-accept-empty-input","errorCode":null,"errorMessage":"c_for_after_stmt cannot accept empty input","messagePattern":"c_for_after_stmt cannot accept empty input","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"error","filePath":"engine/baml-lib/ast/src/parser/parse_expr.rs","lineNumber":205,"sourceCode":"    token: Pair<'_>,\n    body: ExpressionBlock,\n    span: Span,\n    diagnostics: &mut Diagnostics,\n) -> Option<CForLoopStmt> {\n    assert_correct_parser(&token, &[Rule::c_for_loop], diagnostics);\n\n    let mut header = token.into_inner();\n\n    let init_stmt = consume_if_rule(&mut header, Rule::c_for_init_stmt).map(|rule| {\n        rule.into_inner()\n            .next()\n            .expect(\"c_for_init_stmt cannot accept empty input\")\n    });\n    let condition = consume_if_rule(&mut header, Rule::expression);\n    let after_stmt = consume_if_rule(&mut header, Rule::c_for_after_stmt).map(|rule| {\n        rule.into_inner()\n            .next()\n            .expect(\"c_for_after_stmt cannot accept empty input\")\n    });\n\n    let init_stmt = parse_optional_rule(init_stmt, |rule| {\n        let span = diagnostics.span(rule.as_span());\n        parse_statement_inner_rule(rule, span, diagnostics)\n    })?\n    .map(Box::new);\n\n    let condition = parse_optional_rule(condition, |rule| parse_expression(rule, diagnostics))?;\n\n    let after_stmt = parse_optional_rule(after_stmt, |rule| {\n        let span = diagnostics.span(rule.as_span());\n\n        match rule.as_rule() {\n            Rule::block_aware_assign_stmt => {\n                let mut tokens = rule.into_inner();\n\n                let left = parse_expression(tokens.next()?, diagnostics)?;","sourceCodeStart":187,"sourceCodeEnd":223,"githubUrl":"https://github.com/BoundaryML/baml/blob/bd85ce9dee1463ff04d27efd20531013a4ff46c1/engine/baml-lib/ast/src/parser/parse_expr.rs#L187-L223","documentation":"Same invariant as the init statement, applied to the after-statement slot of a C-style for loop: a Rule::c_for_after_stmt pair with no inner tokens panics with 'c_for_after_stmt cannot accept empty input' inside parse_c_for_loop.","triggerScenarios":"Parsing an expression whose c_for_after_stmt grammar token contains no children — only expected when grammar and hand-written parser code are out of sync, or on malformed token streams.","commonSituations":"Modified/custom grammar builds of the baml ast crate; fuzz inputs producing degenerate parse trees.","solutions":["Rebuild the ast crate so grammar and parser come from one version.","Swap .expect for ok_or_else(anyhow!) error propagation if maintaining the code.","Isolate and report the triggering expression snippet upstream."],"exampleFix":"// before\nrule.into_inner().next().expect(\"c_for_after_stmt cannot accept empty input\")\n// after\nrule.into_inner().next().ok_or_else(|| anyhow!(\"c_for_after_stmt cannot accept empty input\"))?","handlingStrategy":"type-guard","validationCode":"let has_child = header\n    .clone()\n    .find(|p| p.as_rule() == Rule::c_for_after_stmt)\n    .map(|p| p.into_inner().next().is_some())\n    .unwrap_or(true);\nif !has_child {\n    eprintln!(\"empty c_for_after_stmt; aborting parse of this loop\");\n}","typeGuard":"fn after_stmt_present(header: &Pairs<Rule>) -> bool {\n    header.clone()\n        .find(|p| p.as_rule() == Rule::c_for_after_stmt)\n        .map(|p| p.into_inner().next().is_some())\n        .unwrap_or(true)\n}","tryCatchPattern":"let r = std::panic::catch_unwind(|| parse_c_for_loop(token, &mut diagnostics));\nif r.is_err() {\n    return Err(anyhow!(\"for-loop header is malformed\"));\n}","preventionTips":["Keep grammar and parser code versioned together.","Replace expect with error propagation when touching this code.","Add regression tests for for-loop headers with/without after-statements."],"tags":["rust","parser","panic","for-loop","invariant"],"backgroundTag":"internal-invariant-violation","analyzedSha":"bd85ce9dee1463ff04d27efd20531013a4ff46c1","analyzedAt":"2026-09-12T03:38:25.718Z","contentChangedAt":"2026-09-12T03:38:25.718Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}