compileLog can print 'runtime_value' even when its not

[daurnimator]

const std = @import("std");

fn parseFile(contents:[]const u8) u8 {
    var acc:u8 = 0;
    for (contents) |byte| {
        acc |= byte;
    }
    return acc;
}

fn compileTimeParseFile(comptime filename:[]const u8) u8 {
    comptime {
        const contents = @embedFile(filename);
        return parseFile(contents[0..contents.len]);
    }
}

pub fn main() void {
    const x = compileTimeParseFile("sample-file");
    @compileLog(x);
    std.debug.warn("FILE={}\n", x);
}

$ zig build-exe embedfile-test.zig 
| (runtime value)
embedfile-test.zig:20:5: error: found compile log statement
    @compileLog(x);
    ^

Now I comment out the @compileLog line:

$ echo foo >> sample-file
$ zig build-exe embedfile-test.zig 
$ ./embedfile-test 
FILE=111
$ strings embedfile-test | grep foo

[ghost]

If you want it to work now you could change the one line to this:

const x = comptime compileTimeParseFile("sample-file");

[andrewrk]

This is working as designed. Function calls are executed at runtime, unless the type signature requires a comptime execution, or unless the function call is in a comptime scope. @dbandstra's suggestion puts the function call into a comptime scope, which is why it solves the problem.

I think the somewhat counter-intuitive thing here is that return control flow is unaffected by whether it is in a comptime scope or not. However the return expression is comptime evaluated. So your compileTimeParseFile ends up being code generated to return 42 (or whatever the result of the computation is) which is then called at runtime from main.

A related issue is #425.

I'm happy to consider proposals to adjust semantics if you have a clear idea of how to improve this.

[daurnimator]

Thankyou for the explanation!