BoundaryML/baml · error

c_for_init_stmt cannot accept empty input

Error message

c_for_init_stmt cannot accept empty input

What it means

parse_c_for_loop consumes an optional Rule::c_for_init_stmt in the for-loop header and .next().expect(...)s it is non-empty. A c_for_init_stmt pair with no children is treated as impossible, so it panics with 'c_for_init_stmt cannot accept empty input'.

Source

Thrown at engine/baml-lib/ast/src/parser/parse_expr.rs:199

            None
        }
    }
}

fn parse_c_for_loop(
    token: Pair<'_>,
    body: ExpressionBlock,
    span: Span,
    diagnostics: &mut Diagnostics,
) -> Option<CForLoopStmt> {
    assert_correct_parser(&token, &[Rule::c_for_loop], diagnostics);

    let mut header = token.into_inner();

    let init_stmt = consume_if_rule(&mut header, Rule::c_for_init_stmt).map(|rule| {
        rule.into_inner()
            .next()
            .expect("c_for_init_stmt cannot accept empty input")
    });
    let condition = consume_if_rule(&mut header, Rule::expression);
    let after_stmt = consume_if_rule(&mut header, Rule::c_for_after_stmt).map(|rule| {
        rule.into_inner()
            .next()
            .expect("c_for_after_stmt cannot accept empty input")
    });

    let init_stmt = parse_optional_rule(init_stmt, |rule| {
        let span = diagnostics.span(rule.as_span());
        parse_statement_inner_rule(rule, span, diagnostics)
    })?
    .map(Box::new);

    let condition = parse_optional_rule(condition, |rule| parse_expression(rule, diagnostics))?;

    let after_stmt = parse_optional_rule(after_stmt, |rule| {
        let span = diagnostics.span(rule.as_span());

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Ensure grammar (baml.bsl/pest) and parser code are from the same version.
2. Replace .expect with proper error handling (ok_or + anyhow!) if you maintain the crate.
3. Report the expression snippet upstream if reproducible on a release.

Example fix

// before
rule.into_inner().next().expect("c_for_init_stmt cannot accept empty input")
// after
rule.into_inner().next().ok_or_else(|| anyhow!("c_for_init_stmt cannot accept empty input"))?

let has_child = header
    .clone()
    .find(|p| p.as_rule() == Rule::c_for_init_stmt)
    .map(|p| p.into_inner().next().is_some())
    .unwrap_or(true);
if !has_child {
    eprintln!("empty c_for_init_stmt; aborting parse of this loop");
}

fn init_stmt_present(header: &Pairs<Rule>) -> bool {
    header.clone()
        .find(|p| p.as_rule() == Rule::c_for_init_stmt)
        .map(|p| p.into_inner().next().is_some())
        .unwrap_or(true)
}

let r = std::panic::catch_unwind(|| parse_c_for_loop(token, &mut diagnostics));
if r.is_err() {
    diagnostics.push(format!("unparseable for-loop at {}", token.as_str()));
}

When it happens

Trigger: Parsing a for-statement whose grammar-produced c_for_init_stmt token has zero inner pairs — a grammar/parse_expr desync (e.g. grammar made init optional internally) when parsing `for ... in ...` expressions with C-style headers.

Common situations: Custom ast crate builds where the expression grammar was modified; rarely on stock releases since the grammar guarantees a child.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• c_for_after_stmt cannot accept empty input
• Name should always be defined for attribute.
• Encountered impossible doc comment during parsing: {:?}, {:?
• block aware tail expression is not empty
• Unexpected prefix operator: {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/9b1b82a746a90a90. Report an issue: GitHub.