Set libc.txt file for a whole tree in build.zig

[pfgithub]

Targeting android, zig doesn't provide a libc, so my application needs to use a libc.txt file:

// my_app/build.zig:
    const libc_file = b.path("android_libc.txt");
    const harfbuzz_dep = b.dependency("harfbuzz", .{ .target = target, .optimize = optimize });
    my_app.root_module.linkLibrary(harfbuzz_dep.artifact("harfbuzz");
    my_app.setLibCFile(libc_file);
    my_app.linkLibC();

// harfbuzz/build.zig:
    const harfbuzz = b.addStaticLibrary(.{ .name = "harfbuzz", ... });
    harfbuzz.linkLibC();
    b.installArtifact(harfbuzz);

The problem is that the harfbuzz module does not get the libc file, and then fails to compile.

Allowing my_app to pass a libc file would require adding a LazyPath option to harfbuzz and using it to setLibCFile:

// my_app/build.zig:
    const libc_file = b.path("android_libc.txt");
-   const harfbuzz_dep = b.dependency("harfbuzz", .{ .target = target, .optimize = optimize });
+   const harfbuzz_dep = b.dependency("harfbuzz", .{ .target = target, .optimize = optimize, .libc = libc_file });
    my_app.root_module.linkLibrary(harfbuzz_dep.artifact("harfbuzz");
    my_app.setLibCFile(libc_file);
    my_app.linkLibC();

// harfbuzz/build.zig:
+   const libc_file = b.option(std.Build.LazyPath, "libc", "libc file for compilation");
    const harfbuzz = b.addStaticLibrary(.{ .name = "harfbuzz", ... });
+   if(libc_file) |f| harfbuzz.setLibCFile(f);
    harfbuzz.linkLibC();
    b.installArtifact(harfbuzz);

Currently, zero libraries in allyourcodebase do this. This seems like an unresonable ask for libraries to implement.

Proposal

Instead, libc.txt could be part of 'target', so it gets automatically passed down correctly without opt-in from dependencies.

// my_app/build.zig:
-   const libc_file = b.path("android_libc.txt");
+   const target = b.resolveTargetQuery(.{
+       ...
+       .libc = b.path("android_libc.txt"),
+   });
    const harfbuzz_dep = b.dependency("harfbuzz", .{ .target = target, .optimize = optimize });
    my_app.root_module.linkLibrary(harfbuzz_dep.artifact("harfbuzz");
-   my_app.setLibCFile(libc_file);
    my_app.linkLibC();

// harfbuzz/build.zig:
    const harfbuzz = b.addStaticLibrary(.{ .name = "harfbuzz", ... });
    harfbuzz.linkLibC();
    b.installArtifact(harfbuzz);

This way, no changes are needed to any dependencies. It could also possibly replace the --libc option in system integration options if libc file is added to the end of the target quadruple, like aarch64-linux-android-lp64-libc.txt or something

Alternatives

• Using --libc to set the libc file for the whole build could maybe work, but any tools which want to run on the host system (.target = b.resolveTargetQuery(.{}) + b.addRunArtifact()) get their libc file replaced which makes them fail to build.
• Update every dependency to add a libc option, like described above.
• Currently, I'm using a hack where I explore the b.graph.dependency_cache, find every installed artifact, and set the libc file on each one: https://github.com/pfgithub/blockeditor/blob/9c4bf03fe706e3cf679b8ff41dd626637807eb74/packages/beui_impl_android/build.zig#L135-L143
• b.linkLibC() -> b.linkLibrary(b.standardLibCOption(.{})) & update all b.dependency() to .{ .target, .optimize, .libc = b.standardLibCOption(.{}) }
• Something else?