Conditional compilation
Syntax
ConfigurationPredicate :
ConfigurationOption
| ConfigurationAll
| ConfigurationAny
| ConfigurationNotConfigurationOption :
IDENTIFIER (=
(STRING_LITERAL | RAW_STRING_LITERAL))?ConfigurationAll
all
(
ConfigurationPredicateList?)
ConfigurationAny
any
(
ConfigurationPredicateList?)
ConfigurationNot
not
(
ConfigurationPredicate)
ConfigurationPredicateList
ConfigurationPredicate (,
ConfigurationPredicate)*,
?
Conditionally compiled source code is source code that may or may not be
considered a part of the source code depending on certain conditions. Source code can be conditionally compiled
using the attributes cfg
and cfg_attr
and the built-in cfg
macro.
These conditions are based on the target architecture of the compiled crate,
arbitrary values passed to the compiler, and a few other miscellaneous things
further described below in detail.
Each form of conditional compilation takes a configuration predicate that evaluates to true or false. The predicate is one of the following:
- A configuration option. It is true if the option is set and false if it is unset.
all()
with a comma separated list of configuration predicates. It is false if at least one predicate is false. If there are no predicates, it is true.any()
with a comma separated list of configuration predicates. It is true if at least one predicate is true. If there are no predicates, it is false.not()
with a configuration predicate. It is true if its predicate is false and false if its predicate is true.
Configuration options are names and key-value pairs that are either set or
unset. Names are written as a single identifier such as, for example, unix
.
Key-value pairs are written as an identifier, =
, and then a string. For
example, target_arch = "x86_64"
is a configuration option.
Note: Whitespace around the
=
is ignored.foo="bar"
andfoo = "bar"
are equivalent configuration options.
Keys are not unique in the set of key-value configuration options. For example,
both feature = "std"
and feature = "serde"
can be set at the same time.
Set Configuration Options
Which configuration options are set is determined statically during the compilation of the crate. Certain options are compiler-set based on data about the compilation. Other options are arbitrarily-set, set based on input passed to the compiler outside of the code. It is not possible to set a configuration option from within the source code of the crate being compiled.
Note: For
rustc
, arbitrary-set configuration options are set using the--cfg
flag.
Note: Configuration options with the key
feature
are a convention used by Cargo for specifying compile-time options and optional dependencies.
Warning: It is possible for arbitrarily-set configuration options to have the
same value as compiler-set configuration options. For example, it is possible
to do rustc --cfg "unix" program.rs
while compiling to a Windows target, and
have both unix
and windows
configuration options set at the same time. It
is unwise to actually do this.
target_arch
Key-value option set once with the target's CPU architecture. The value is similar to the first element of the platform's target triple, but not identical.
Example values:
"x86"
"x86_64"
"mips"
"powerpc"
"powerpc64"
"arm"
"aarch64"
target_feature
Key-value option set for each platform feature available for the current compilation target.
Example values:
"avx"
"avx2"
"crt-static"
"rdrand"
"sse"
"sse2"
"sse4.1"
See the target_feature
attribute for more details on the available
features. An additional feature of crt-static
is available to the
target_feature
option to indicate that a static C runtime is available.
target_os
Key-value option set once with the target's operating system. This value is similar to the second and third element of the platform's target triple.
Example values:
"windows"
"macos"
"ios"
"linux"
"android"
"freebsd"
"dragonfly"
"openbsd"
"netbsd"
"none"
(typical for embedded targets)
target_family
Key-value option providing a more generic description of a target, such as the family of the
operating systems or architectures that the target generally falls into. Any number of
target_family
key-value pairs can be set.
Example values:
"unix"
"windows"
"wasm"
unix
and windows
unix
is set if target_family = "unix"
is set and windows
is set if
target_family = "windows"
is set.
target_env
Key-value option set with further disambiguating information about the target
platform with information about the ABI or libc
used. For historical reasons,
this value is only defined as not the empty-string when actually needed for
disambiguation. Thus, for example, on many GNU platforms, this value will be
empty. This value is similar to the fourth element of the platform's target
triple. One difference is that embedded ABIs such as gnueabihf
will simply
define target_env
as "gnu"
.
Example values:
""
"gnu"
"msvc"
"musl"
"sgx"
target_endian
Key-value option set once with either a value of "little" or "big" depending on the endianness of the target's CPU.
target_pointer_width
Key-value option set once with the target's pointer width in bits.
Example values:
"16"
"32"
"64"
target_vendor
Key-value option set once with the vendor of the target.
Example values:
"apple"
"fortanix"
"pc"
"unknown"
target_has_atomic
Key-value option set for each bit width that the target supports atomic loads, stores, and compare-and-swap operations.
When this cfg is present, all of the stable core::sync::atomic
APIs are available for
the relevant atomic width.
Possible values:
"8"
"16"
"32"
"64"
"128"
"ptr"
test
Enabled when compiling the test harness. Done with rustc
by using the
--test
flag. See Testing for more on testing support.
debug_assertions
Enabled by default when compiling without optimizations.
This can be used to enable extra debugging code in development but not in
production. For example, it controls the behavior of the standard library's
debug_assert!
macro.
proc_macro
Set when the crate being compiled is being compiled with the proc_macro
crate type.
panic
Key-value option set depending on the panic strategy. Note that more values may be added in the future.
Example values:
"abort"
"unwind"
Forms of conditional compilation
The cfg
attribute
Syntax
CfgAttrAttribute :
cfg
(
ConfigurationPredicate)
The cfg
attribute conditionally includes the thing it is attached to based
on a configuration predicate.
It is written as cfg
, (
, a configuration predicate, and finally )
.
If the predicate is true, the thing is rewritten to not have the cfg
attribute
on it. If the predicate is false, the thing is removed from the source code.
When a crate-level cfg
has a false predicate, the behavior is slightly
different: any crate attributes preceding the cfg
are kept, and any crate
attributes following the cfg
are removed. This allows #![no_std]
and
#![no_core]
crates to avoid linking std
/core
even if a #![cfg(...)]
has
removed the entire crate.
Some examples on functions:
#![allow(unused)] fn main() { // The function is only included in the build when compiling for macOS #[cfg(target_os = "macos")] fn macos_only() { // ... } // This function is only included when either foo or bar is defined #[cfg(any(foo, bar))] fn needs_foo_or_bar() { // ... } // This function is only included when compiling for a unixish OS with a 32-bit // architecture #[cfg(all(unix, target_pointer_width = "32"))] fn on_32bit_unix() { // ... } // This function is only included when foo is not defined #[cfg(not(foo))] fn needs_not_foo() { // ... } // This function is only included when the panic strategy is set to unwind #[cfg(panic = "unwind")] fn when_unwinding() { // ... } }
The cfg
attribute is allowed anywhere attributes are allowed.
The cfg_attr
attribute
Syntax
CfgAttrAttribute :
cfg_attr
(
ConfigurationPredicate,
CfgAttrs?)
The cfg_attr
attribute conditionally includes attributes based on a
configuration predicate.
When the configuration predicate is true, this attribute expands out to the
attributes listed after the predicate. For example, the following module will
either be found at linux.rs
or windows.rs
based on the target.
#[cfg_attr(target_os = "linux", path = "linux.rs")]
#[cfg_attr(windows, path = "windows.rs")]
mod os;
Zero, one, or more attributes may be listed. Multiple attributes will each be expanded into separate attributes. For example:
#[cfg_attr(feature = "magic", sparkles, crackles)]
fn bewitched() {}
// When the `magic` feature flag is enabled, the above will expand to:
#[sparkles]
#[crackles]
fn bewitched() {}
Note: The
cfg_attr
can expand to anothercfg_attr
. For example,#[cfg_attr(target_os = "linux", cfg_attr(feature = "multithreaded", some_other_attribute))]
is valid. This example would be equivalent to#[cfg_attr(all(target_os = "linux", feature ="multithreaded"), some_other_attribute)]
.
The cfg_attr
attribute is allowed anywhere attributes are allowed.
The cfg
macro
The built-in cfg
macro takes in a single configuration predicate and evaluates
to the true
literal when the predicate is true and the false
literal when
it is false.
For example:
#![allow(unused)] fn main() { let machine_kind = if cfg!(unix) { "unix" } else if cfg!(windows) { "windows" } else { "unknown" }; println!("I'm running on a {} machine!", machine_kind); }