std.build

The program’s own build. A program that keeps a cache of work it did, such as data it parsed, can key the cache on its build ID. A new build of the same code finds the cache still good, and a build with any change to the code makes a new one, so a cache written by older code is never read by newer code.

CalxOutput

Tested

import std.build.{build_id}

fn main() {
    let id = build_id()
    println("{id.len()}")
    var hex = true
    for b in id.to_bytes() {
        if not ((b >= 48 and b <= 57) or (b >= 97 and b <= 102)) {
            hex = false
        }
    }
    println("{hex} {id == build_id()}")
}

Function	Gives
build_id() -> String	32 lowercase hex digits: a hash of the program’s generated C and the runtime it links

• The same code gives the same ID, built by the same compiler, in debug and in release, on any machine. The build’s flags aren’t in it.
• Any change gives a new ID: to the program’s own modules, to a std module it uses, to the runtime, or to the compiler, where that changes the C it writes.
• It costs nothing to run. The ID is a constant in a small file of its own, which the compiler writes and compiles only for programs that call build_id. When the code changes, that file is the one extra thing compiled.
• It isn’t a version number: two IDs only say whether the code was the same.

std.build is written in Calx: std/build.calx. The ID comes from the compiler’s build.