Build.zig can't produce LLVM bytecode without outputting the full binary

[gwenzek]

Zig Version

0.13.0

Steps to Reproduce and Observed Behavior

Create a hello world file at "hello.zig"
Next to it create the following "build.zig" file.

const std = @import("std");

pub fn build(b: *std.Build) void {
    const hello = b.addObject(.{
        .name = "hello_world",
        .root_source_file = b.path("hello.zig"),
        .optimize = .ReleaseFast,
        .target = b.standardTargetOptions(.{}),
        .use_llvm = true,
    });

    const install_bc = b.addInstallFile(hello.getEmittedLlvmBc(), "hello.bc");
    b.getInstallStep().dependOn(&install_bc.step);
}

    // If this two lines are commented out, the bytecode file doesn't get installed (but still emitted).
    // const install_bin = b.addInstallFile(zig_kernel.getEmittedBin(), "hello.bin");
    // b.getInstallStep().dependOn(&install_bin.step);
}

Then zig build fails with the following error:

getPath() was called on a GeneratedFile that wasn't built yet.
  Is there a missing Step dependency on step 'zig build-obj hello_world ReleaseFast native'?

Expected Behavior

Zig should accept to compile the program and only output llvm bytecode, not the full binary.

Note this is still present on zig_0.14.0-dev.363, and it was also present a few months ago when I wrote this https://gist.github.com/gwenzek/7e77e592320b9b1971b95fb0d6429424 (totally forgot about it in the meantime).

This issue is not only about Llvm bytecode, it's also reproducible with other outputs kind such as -femit-asm

Note that directly calling zig build-obj -fno-emit-bin -femit-llvm-bytecode works fine it's really an issue with the build system.

[Snektron]

Should be fixed by #21115

[Snektron]

Above pr is merged and contains a test for this exact case