- Zig's comptime allows running standard Zig code at compile time, without side effects, for conditional compilation.
- Conditional code disabling is useful for platform-specific code, debugging, and build configuration, avoiding runtime conditionals and reducing binary size.
- Unlike C's preprocessor (separate language) and Go's file-level build tags (external build system), Zig uses normal Zig code with comptime.
- The compiler automatically evaluates comptime conditions; the `comptime` keyword is optional but can make intent explicit.
- Example: `if (comptime builtin.os.tag == .macos) { ... }` omits non-macOS code when targeting macOS.
- Comptime can mix with runtime conditions; false comptime conditions cause the compiler to skip the entire block, avoiding errors from missing libraries or types.
- Gotcha: comptime doesn't cross function boundaries by default; use `comptime` keyword before the function call or mark the function as `inline`.
- Comptime is a killer feature for cross-platform code, eliminating the need for complex build systems or external tools.