`check-cfg`

The tracking issue for this feature is: #82450.

This feature allows you to enable complete or partial checking of configuration.

rustc accepts the --check-cfg option, which specifies whether to check conditions and how to check them. The --check-cfg option takes a value, called the check cfg specification. The check cfg specification is parsed using the Rust metadata syntax, just as the --cfg option is.

--check-cfg option take one form:

--check-cfg cfg(...) enables checking the values within list-valued conditions.

NOTE: No implicit expectation is added when using --cfg for both forms. Users are expected to pass all expected names and values using cfg(...).

The `cfg(...)` form

The cfg(...) form enables checking the values within list-valued conditions. It has this basic form:

rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'

where name is a bare identifier (has no quotes) and each "value" term is a quoted literal string. name specifies the name of the condition, such as feature or my_cfg.

When the cfg(...) option is specified, rustc will check every #[cfg(name = "value")] attribute, #[cfg_attr(name = "value")] attribute, #[link(name = "a", cfg(name = "value"))] and cfg!(name = "value") call. It will check that the "value" specified is present in the list of expected values. If "value" is not in it, then rustc will report an unexpected_cfgs lint diagnostic. The default diagnostic level for this lint is Warn.

The command line --cfg arguments are currently NOT checked but may very well be checked in the future.

To enable checking of values, but to provide an empty set of expected values, use these forms:

rustc --check-cfg 'cfg(name1, ..., nameN)'
rustc --check-cfg 'cfg(name1, ..., nameN, values())'

To enable checking of name but not values (i.e. unknown expected values), use this form:

rustc --check-cfg 'cfg(name1, ..., nameN, values(any()))'

The --check-cfg cfg(...) option can be repeated, both for the same condition name and for different names. If it is repeated for the same condition name, then the sets of values for that condition are merged together (presedence is given to any()).

Well known names and values

rustc has a internal list of well known names and their corresponding values. Those well known names and values follows the same stability as what they refer to.

Well known values checking is always enabled as long as a --check-cfg argument is present.

Well known names checking is always enable as long as a --check-cfg argument is present unless any cfg(any()) argument is passed.

To disable checking of well known names, use this form:

rustc --check-cfg 'cfg(any())'

NOTE: If one want to enable values and names checking without having any cfg to declare, one can use an empty cfg() argument.

Examples

Consider this command line:

rustc --check-cfg 'cfg(feature, values("lion", "zebra"))' \
      --cfg 'feature="lion"' -Z unstable-options \
      example.rs

This command line indicates that this crate has two features: lion and zebra. The lion feature is enabled, while the zebra feature is disabled. Exhaustive checking of names and values are enabled by default. Consider compiling this code:

#![allow(unused)]
fn main() {
// This is expected, and tame_lion() will be compiled
#[cfg(feature = "lion")]
fn tame_lion(lion: Lion) {}

// This is expected, and ride_zebra() will NOT be compiled.
#[cfg(feature = "zebra")]
fn ride_zebra(zebra: Zebra) {}

// This is UNEXPECTED, and will cause a compiler warning (by default).
#[cfg(feature = "platypus")]
fn poke_platypus() {}

// This is UNEXPECTED, because 'feechure' is not a known condition name,
// and will cause a compiler warning (by default).
#[cfg(feechure = "lion")]
fn tame_lion() {}

// This is UNEXPECTED, because 'windows' is a well known condition name,
// and because 'windows' doens't take any values,
// and will cause a compiler warning (by default).
#[cfg(windows = "unix")]
fn tame_windows() {}
}

Example: Checking condition names, but not values

# This turns on checking for condition names, but not values, such as 'feature' values.
rustc --check-cfg 'cfg(is_embedded, has_feathers, values(any()))' \
      --cfg has_feathers -Z unstable-options

#![allow(unused)]
fn main() {
#[cfg(is_embedded)]         // This is expected as "is_embedded" was provided in cfg()
fn do_embedded() {}         // and because names exhaustiveness was not disabled

#[cfg(has_feathers)]        // This is expected as "has_feathers" was provided in cfg()
fn do_features() {}         // and because names exhaustiveness was not disabled

#[cfg(has_feathers = "zapping")] // This is expected as "has_feathers" was provided in cfg()
                                 // and because no value checking was enable for "has_feathers"
                                 // no warning is emitted for the value "zapping"
fn do_zapping() {}

#[cfg(has_mumble_frotz)]    // This is UNEXPECTED because names checking is enable and
                            // "has_mumble_frotz" was not provided in cfg()
fn do_mumble_frotz() {}
}

Example: Checking feature values, but not condition names

# This turns on checking for feature values, but not for condition names.
rustc --check-cfg 'cfg(feature, values("zapping", "lasers"))' \
      --check-cfg 'cfg(any())' \
      --cfg 'feature="zapping"' -Z unstable-options

#![allow(unused)]
fn main() {
#[cfg(is_embedded)]         // This is doesn't raise a warning, because names checking was
                            // disabled by 'cfg(any())'
fn do_embedded() {}

#[cfg(has_feathers)]        // Same as above, 'cfg(any())' was provided so no name
                            // checking is performed
fn do_features() {}

#[cfg(feature = "lasers")]  // This is expected, "lasers" is in the cfg(feature) list
fn shoot_lasers() {}

#[cfg(feature = "monkeys")] // This is UNEXPECTED, because "monkeys" is not in the
                            // cfg(feature) list
fn write_shakespeare() {}
}

Example: Checking both condition names and feature values

# This turns on checking for feature values and for condition names.
rustc --check-cfg 'cfg(is_embedded, has_feathers)' \
      --check-cfg 'cfg(feature, values("zapping", "lasers"))' \
      --cfg has_feathers --cfg 'feature="zapping"' -Z unstable-options

#![allow(unused)]
fn main() {
#[cfg(is_embedded)]         // This is expected because "is_embedded" was provided in cfg()
fn do_embedded() {}         // and doesn't take any value

#[cfg(has_feathers)]        // This is expected because "has_feathers" was provided in cfg()
fn do_features() {}         // and deosn't take any value

#[cfg(has_mumble_frotz)]    // This is UNEXPECTED, because "has_mumble_frotz" was never provided
fn do_mumble_frotz() {}

#[cfg(feature = "lasers")]  // This is expected, "lasers" is in the cfg(feature) list
fn shoot_lasers() {}

#[cfg(feature = "monkeys")] // This is UNEXPECTED, because "monkeys" is not in
                            // the cfg(feature) list
fn write_shakespeare() {}
}

The deprecated `names(...)` form

The names(...) form enables checking the names. This form uses a named list:

rustc --check-cfg 'names(name1, name2, ... nameN)'

where each name is a bare identifier (has no quotes). The order of the names is not significant.

If --check-cfg names(...) is specified at least once, then rustc will check all references to condition names. rustc will check every #[cfg] attribute, #[cfg_attr] attribute, cfg clause inside #[link] attribute and cfg!(...) call against the provided list of expected condition names. If a name is not present in this list, then rustc will report an unexpected_cfgs lint diagnostic. The default diagnostic level for this lint is Warn.

If --check-cfg names(...) is not specified, then rustc will not check references to condition names.

--check-cfg names(...) may be specified more than once. The result is that the list of valid condition names is merged across all options. It is legal for a condition name to be specified more than once; redundantly specifying a condition name has no effect.

To enable checking condition names with an empty set of valid condition names, use the following form. The parentheses are required.

rustc --check-cfg 'names()'

Note that --check-cfg 'names()' is not equivalent to omitting the option entirely. The first form enables checking condition names, while specifying that there are no valid condition names (outside of the set of well-known names defined by rustc). Omitting the --check-cfg 'names(...)' option does not enable checking condition names.

The deprecated `values(...)` form

The values(...) form enables checking the values within list-valued conditions. It has this form:

rustc --check-cfg `values(name, "value1", "value2", ... "valueN")'

where name is a bare identifier (has no quotes) and each "value" term is a quoted literal string. name specifies the name of the condition, such as feature or target_os.

When the values(...) option is specified, rustc will check every #[cfg(name = "value")] attribute, #[cfg_attr(name = "value")] attribute, #[link(name = "a", cfg(name = "value"))] and cfg!(name = "value") call. It will check that the "value" specified is present in the list of expected values. If "value" is not in it, then rustc will report an unexpected_cfgs lint diagnostic. The default diagnostic level for this lint is Warn.

To enable checking of values, but to provide an empty set of valid values, use this form:

rustc --check-cfg `values(name)`

The --check-cfg values(...) option can be repeated, both for the same condition name and for different names. If it is repeated for the same condition name, then the sets of values for that condition are merged together.

If values() is specified, then rustc will enable the checking of well-known values defined by itself. Note that it's necessary to specify the values() form to enable the checking of well known values, specifying the other forms doesn't implicitly enable it.

The Rust Unstable Book