zig build on platforms with !Watch.have_impl regressed

[linusg]

Zig Version

0.15.0-dev.1380+e98aeeb73

Steps to Reproduce and Observed Behavior

Zig fails to compile the build runner because it tries to unconditionally call update() on a possibly void type:

zig/lib/compiler/build_runner.zig

Line 505 in 4932654

try w.update(gpa, run.step_stack.keys());

zig/lib/std/Build/Watch.zig

Lines 903 to 905 in 4932654

	pub fn update(w: *Watch, gpa: Allocator, steps: []const *Step) !void {
	return Os.update(w, gpa, steps);
	}

zig/lib/std/Build/Watch.zig

Line 848 in 4932654

else => void,

Expected Behavior

zig build without watch functionality should still work.

[linusg]

I think this worked before #24588 due to this comptime-known conditional just before w.update():

if (!Watch.have_impl) fatal("--watch not yet implemented for {s}", .{@tagName(builtin.os.tag)});

Now it has moved here:

zig/lib/compiler/build_runner.zig

Lines 444 to 448 in 4932654

	var w: Watch = w: {
	if (!watch) break :w undefined;
	if (!Watch.have_impl) fatal("--watch not yet implemented for {s}", .{@tagName(builtin.os.tag)});
	break :w try .init();
	};

[mlugg]

Oops, sorry for the regression! Will get on this shortly.