6.11. Compile-time file embedding
6.11.1. include_str! / include_bytes!
const TEMPLATE: str = include_str!("templates/page.html");
static ICON: [u8; 1406] = include_bytes!("res/icon.png");
include_str!("path") evaluates to the contents of the file path as a str, and include_bytes!("path") to its bytes as a [u8; N] (N the file's length; a byte string, 1.7.5, including its Slice<u8> rule). The argument is exactly one string literal. The path is relative to the directory of the source file that contains the call. The file is read at compile time; the result is a constant and may appear anywhere a literal may, including const and static initializers.
It is a compile error if the file cannot be read, or, for include_str!, if its contents are not valid UTF-8. The included file is an input of the build: changing it rebuilds what uses it (9.2.16).
Like cfg! (6.3.5), these are compiler built-in forms, not user-defined macros.