check-cfg
The tracking issue for this feature is: #82450.
This feature enables checking of conditional 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.
This specification has one form:
--check-cfg cfg(...)mark a configuration and it's expected values as expected.
No implicit expectation is added when using --cfg. Users are expected to
pass all expected names and values using the check cfg specification.
The cfg(...) form
The cfg(...) form enables checking the values within list-valued conditions. It has this
basic form:
rustc --check-cfg 'cfg(name, 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"))]
attribute and cfg!(name = "value") macro 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 check for the none value (ie #[cfg(foo)]) one can use the none() predicate inside
values(): values(none()). It can be followed or precessed by any number of "value".
To enable checking of values, but to provide an none/empty set of expected values
(ie. expect #[cfg(name)]), use these forms:
rustc --check-cfg 'cfg(name)'
rustc --check-cfg 'cfg(name, values(none()))'
To enable checking of name but not values, use one of these forms:
-
No expected values (will lint on every value):
rustc --check-cfg 'cfg(name, values())' -
Unknown expected values (will never lint):
rustc --check-cfg 'cfg(name, values(any()))'
To avoid repeating the same set of values, use this form:
rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'
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 (precedence is given to values(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 names and values checking is always enabled as long as at least one
--check-cfg argument is present.
As of 2024-02-15T, the list of known names is as follows:
clippydebug_assertionsdocdoctestmirioverflow_checkspanicproc_macrorelocation_modelsanitizesanitizer_cfi_generalize_pointerssanitizer_cfi_normalize_integerstarget_abitarget_archtarget_endiantarget_envtarget_familytarget_featuretarget_has_atomictarget_has_atomic_equal_alignmenttarget_has_atomic_load_storetarget_ostarget_pointer_widthtarget_thread_localtarget_vendortestunixwindows
Like with values(any()), well known names checking can be disabled by passing cfg(any())
as argument to --check-cfg.
Examples
Equivalence table
This table describe the equivalence of a --cfg argument to a --check-cfg argument.
--cfg | --check-cfg |
|---|---|
| nothing | nothing or --check-cfg=cfg() (to enable the checking) |
--cfg foo | --check-cfg=cfg(foo) or --check-cfg=cfg(foo, values(none())) |
--cfg foo="" | --check-cfg=cfg(foo, values("")) |
--cfg foo="bar" | --check-cfg=cfg(foo, values("bar")) |
--cfg foo="1" --cfg foo="2" | --check-cfg=cfg(foo, values("1", "2")) |
--cfg foo="1" --cfg bar="2" | --check-cfg=cfg(foo, values("1")) --check-cfg=cfg(bar, values("2")) |
--cfg foo --cfg foo="bar" | --check-cfg=cfg(foo, values(none(), "bar")) |
Example: Cargo-like feature example
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.
Given the --check-cfg arguments, exhaustive checking of names and
values are enabled.
example.rs:
#![allow(unused)] fn main() { #[cfg(feature = "lion")] // This condition is expected, as "lion" is an expected value of `feature` fn tame_lion(lion: Lion) {} #[cfg(feature = "zebra")] // This condition is expected, as "zebra" is an expected value of `feature` // but the condition will still evaluate to false // since only --cfg feature="lion" was passed fn ride_zebra(z: Zebra) {} #[cfg(feature = "platypus")] // This condition is UNEXPECTED, as "platypus" is NOT an expected value of // `feature` and will cause a compiler warning (by default). fn poke_platypus() {} #[cfg(feechure = "lion")] // This condition is UNEXPECTED, as 'feechure' is NOT a expected condition // name, no `cfg(feechure, ...)` was passed in `--check-cfg` fn tame_lion() {} #[cfg(windows = "unix")] // This condition is UNEXPECTED, as while 'windows' is a well known // condition name, it doens't expect any values fn tame_windows() {} }
Example: Multiple names and values
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 condition is expected, as 'is_embedded' was provided in --check-cfg fn do_embedded() {} // and doesn't take any value #[cfg(has_feathers)] // This condition is expected, as 'has_feathers' was provided in --check-cfg fn do_features() {} // and doesn't take any value #[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was NEVER provided // in any --check-cfg arguments fn do_mumble_frotz() {} #[cfg(feature = "lasers")] // This condition is expected, as "lasers" is an expected value of `feature` fn shoot_lasers() {} #[cfg(feature = "monkeys")] // This condition is UNEXPECTED, as "monkeys" is NOT an expected value of // `feature` fn write_shakespeare() {} }
Example: Condition names without 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 condition is expected, as 'is_embedded' was provided in --check-cfg // as condition name fn do_embedded() {} #[cfg(has_feathers)] // This condition is expected, as "has_feathers" was provided in --check-cfg // as condition name fn do_features() {} #[cfg(has_feathers = "zapping")] // This condition is expected, as "has_feathers" was provided in // and because *any* values is expected for 'has_feathers' no // warning is emitted for the value "zapping" fn do_zapping() {} #[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was not provided // in any --check-cfg arguments fn do_mumble_frotz() {} }