Common Standard Macros

cfg!

The cfg! macro evaluates a compile-time configuration predicate (target_os, target_arch, feature, debug_assertions, unix, windows, and all() / any() / not() combinators) and expands to a compile-time bool literal (true or false).

cfg!(...) Macro vs. #[cfg(...)] Attribute

Rust provides two ways to branch on compile-time configuration flags, and their difference in type-checking is critical:

Construct Syntax What Happens to the Inactive Branch? Can Reference Platform-Specific APIs?
#[cfg(...)] attribute #[cfg(target_os = "macos")] fn f() {} Completely stripped from AST before name resolution and type checking Yes (inactive branch can reference Windows-only or Unix-only crates/types)
cfg!(...) macro if cfg!(target_os = "macos") { ... } Parsed and type-checked on ALL platforms, then dead-code eliminated by LLVM No (both branches must compile and type-check on the current target) 
#[cfg(unix)]                           if cfg!(unix) {
fn os_only() { ... }                       // Checked by rustc on BOTH Unix and Windows!
// Stripped on Windows                     // Eliminated by optimizer when false

Common Configuration Predicates

Predicate Meaning
cfg!(target_os = "macos") / "linux" / "windows" Target operating system
cfg!(target_arch = "aarch64") / "x86_64" Target CPU architecture
cfg!(target_family = "unix") or cfg!(unix) Any Unix-like target (macOS, Linux, BSD)
cfg!(target_endian = "little") Byte endianness of target CPU
cfg!(target_pointer_width = "64") Pointer width in bits
cfg!(debug_assertions) true in dev/test builds, false in --release by default
cfg!(feature = "serde") true when Cargo feature "serde" is enabled
cfg!(all(unix, target_pointer_width = "64")) Logical AND (all), OR (any), NOT (not) 
const IS_64_BIT_UNIX: bool = cfg!(all(unix, target_pointer_width = "64"));

fn main() {
    let os_family = if cfg!(unix) {
        "unix-like"
    } else if cfg!(windows) {
        "windows"
    } else {
        "other"
    };

    println!("OS family:         {os_family}");
    println!("64-bit Unix build: {IS_64_BIT_UNIX}");
    println!("Debug assertions:  {}", cfg!(debug_assertions));
}

OS family:         unix-like
64-bit Unix build: true
Debug assertions:  true