`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 can take one of two forms:

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

These two options are independent. names checks only the namespace of condition names while values checks only the namespace of the values of 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 names(...) and values(...).

The `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 `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.

Examples

Consider this command line:

rustc --check-cfg 'names(feature)' \
      --check-cfg 'values(feature, "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. 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() {}
}

Note: The --check-cfg names(feature) option is necessary only to enable checking the condition name, as in the last example. feature is a well-known (always-expected) condition name, and so it is not necessary to specify it in a --check-cfg 'names(...)' option. That option can be shortened to > --check-cfg names() in order to enable checking well-known condition names.

Example: Checking condition names, but not values

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

#![allow(unused)]
fn main() {
#[cfg(is_embedded)]         // This is expected as "is_embedded" was provided in names()
fn do_embedded() {}

#[cfg(has_feathers)]        // This is expected as "has_feathers" was provided in names()
fn do_features() {}

#[cfg(has_feathers = "zapping")] // This is expected as "has_feathers" was provided in names()
                                 // 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 names()
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 'values(feature, "zapping", "lasers")' \
      --cfg 'feature="zapping"' -Z unstable-options

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

#[cfg(has_feathers)]        // Same as above, --check-cfg names(...) was never used so no name
                            // checking is performed
fn do_features() {}


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

#[cfg(feature = "monkeys")] // This is UNEXPECTED, because "monkeys" is not in the
                            // --check-cfg values(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 'names(is_embedded, has_feathers)' \
      --check-cfg 'values(feature, "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 names()
fn do_embedded() {}

#[cfg(has_feathers)]        // This is expected because "has_feathers" was provided in names()
fn do_features() {}

#[cfg(has_mumble_frotz)]    // This is UNEXPECTED, because has_mumble_frotz is not in the
                            // --check-cfg names(...) list
fn do_mumble_frotz() {}

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

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

The Rust Unstable Book

check-cfg

The names(...) form

The values(...) form