Conditional compilation changes what goes into a Rust program according to conditions known at compile time. A common condition is the target operating system; CPU capabilities can matter too. Put a `#[cfg(...)]` attribute before an item to include that item only when its condition matches the compilation target. section 7.6.1
Give each operating system the right usage string
A command-line tool can define `USAGE` twice: `#[cfg(target_os = "windows")]` selects instructions containing an `.exe` filename, while `#[cfg(not(target_os = "windows"))]` selects instructions without it. The resulting binary contains only the usage data for its target. Write negation with `not(...)`, not `target_os != "windows"`. section 7.6.1
Choose a condition that matches what the code needs:
- Use target settings such as `target_os`, `target_arch`, `target_pointer_width`, or `target_has_atomic` to distinguish platforms and CPU facilities. section 7.6.1
- Use `not(...)`, `all(...)`, and `any(...)` to combine conditions. Configuration can also be adjusted when invoking the build tools with `--cfg ATTRIBUTE`. section 7.6.1
The same technique helps a cross-platform clock tool use different implementations for setting the time, because operating systems provide different mechanisms. For smaller differences, `cfg!(debug_assertions)` can guard diagnostic printing: a development run shows debug output, while the shown release run does not. section 9.7 section 1.5
Listing 7.9 Demonstrating the use of conditional compilation
3 #[cfg(target_os = "windows")] 4 const USAGE: &str = " 5 Usage: 6 akv_mem.exe FILE get KEY 7 akv_mem.exe FILE delete KEY 8 akv_mem.exe FILE insert KEY VALUE 9 akv_mem.exe FILE update KEY VALUE 10 "; 11 12 #[cfg(not(target_os = "windows"))] 13 const USAGE: &str = " 14 Usage: 15 akv_mem FILE get KEY 16 akv_mem FILE delete KEY 17 akv_mem FILE insert KEY VALUE 18 akv_mem FILE update KEY VALUE 19 ";
Compare the two `USAGE` definitions and their complementary `#[cfg(...)]` attributes. section 7.6.1
