Conditional compilation

^Syntax
ConfigurationPredicate :
      ConfigurationOption
   | ConfigurationAll
   | ConfigurationAny
   | ConfigurationNot

ConfigurationOption :
   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" and foo = "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.

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"

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"

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.

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.

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() {
  // ...
}
}

The cfg attribute is allowed anywhere attributes are allowed.

The cfg_attr attribute

^Syntax
CfgAttrAttribute :
   cfg_attr ( ConfigurationPredicate , CfgAttrs^? )

CfgAttrs :
   Attr (, Attr)^* ,^?

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 another cfg_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);
}