What causes "config key {} not in sections or top_level_keys" in rust-lang/rust?

List valid section names from the config.toml template: look for [section] headers.

rust-lang/rust · error · RuntimeError

config key {} not in sections or top_level_keys

Error message

config key {} not in sections or top_level_keys

What it means

Raised by configure_file() in configure.py at line 738. This is a structural validation: when processing parsed config sections, each section_key must be either a known section (in the sections dict, populated from [section] headers in the template) or a known top-level key (in top_level_keys). If a section_key matches neither, the configuration is structurally invalid and cannot be written.

Source

Thrown at src/bootstrap/configure.py:738

                raise RuntimeError("failed to find config line for {}".format(key))


def configure_top_level_key(lines, top_level_key, value):
    for i, line in enumerate(lines):
        if line.startswith("#" + top_level_key + " = ") or line.startswith(
            top_level_key + " = "
        ):
            lines[i] = "{} = {}".format(top_level_key, to_toml(value))
            return

    raise RuntimeError("failed to find config line for {}".format(top_level_key))


# Modify `sections` to reflect the parsed arguments and example configs.
def configure_file(sections, top_level_keys, targets, config):
    for section_key, section_config in config.items():
        if section_key not in sections and section_key not in top_level_keys:
            raise RuntimeError(
                "config key {} not in sections or top_level_keys".format(section_key)
            )
        if section_key in top_level_keys:
            configure_top_level_key(sections[None], section_key, section_config)

        elif section_key == "target":
            for target in section_config:
                configure_section(targets[target], section_config[target])
        else:
            configure_section(sections[section_key], section_config)


def write_uncommented(target, f):
    """Writes each block in 'target' that is not composed entirely of comments to 'f'.

    A block is a sequence of non-empty lines separated by empty lines.
    """
    block = []

View on GitHub (pinned to 7088e4b63a)

Solutions

List valid section names from the config.toml template: look for [section] headers.
Check top_level_keys for recognized non-section options.
Correct the section/key name in the configure invocation.
If the key is new and valid, add the corresponding [section] header to the template.

Defensive patterns

Strategy: validation

Validate before calling

# Before calling configure_file, validate all section keys
def validate_config_keys(config, sections, top_level_keys):
    for section_key in config:
        if section_key not in sections and section_key not in top_level_keys:
            print(f'Unknown config key: {section_key}')
            print(f'Valid sections: {list(sections.keys())}')
            print(f'Valid top-level keys: {list(top_level_keys)}')
            return False
    return True

Try / catch

try:
    configure_file(sections, top_level_keys, targets, config)
except RuntimeError as e:
    if 'not in sections or top_level_keys' in str(e):
        key = str(e).split('key ')[1].split(' not')[0]
        print(f'Section key "{key}" is not a recognized section or top-level key.')
        print(f'Valid sections: {list(sections.keys())}')
    raise

Prevention

Cross-check section names against the [section] headers in the config template before configuring.
Validate top-level keys against the known set.
Use consistent key naming — typos are the most common cause.

When it happens

Trigger: configure_file(sections, top_level_keys, targets, config) at line 736-740. For each section_key in config.items(): if section_key not in sections AND section_key not in top_level_keys, raise. This fires when the user provides a configuration dict whose top-level key name is neither a recognized TOML section header nor a recognized top-level scalar key.

Common situations: Typo in a section name in the configure arguments (e.g. '--set build.cargo-vs '--set bild.cargo'); using a section name from a different Rust version; passing arbitrary/unknown configuration keys that don't map to any section or top-level key in the current template.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/d93588106bf8a7c8. Report an issue: GitHub.