std.Build: support passing optionals to dependency args

[Lzard]

Allows conditionally setting dependency options.

Example

a's build.zig:

const std = @import("std");

pub fn build(b: *std.Build) !void {
    const target = b.standardTargetOptions(.{});
    std.debug.print("target = {s}\n", .{ try target.result.zigTriple(b.allocator) });

    const optimize = b.standardOptimizeOption(.{});
    std.debug.print("optimize = {}\n", .{ optimize });

    const x = b.option(i32, "x", "");
    std.debug.print("x = {?}\n", .{ x });
}

b's build.zig:

const std = @import("std");

pub fn build(b: *std.Build) void {
    _ = b.standardOptimizeOption(.{});
    _ = b.dependency("a", .{
        .target = b.standardTargetOptions(.{}),
        .optimize = @as(?std.builtin.OptimizeMode, null),
        .x = b.option(i32, "x", ""),
    });
}

b zig build output:

target = x86_64-windows.win10_cu...win10_cu-gnu
optimize = builtin.OptimizeMode.Debug
x = null

b zig build -Dtarget=aarch64-linux -Doptimize=ReleaseSafe -Dx=1 output:

target = aarch64-linux.4.19...6.11.5-musl
optimize = builtin.OptimizeMode.Debug    
x = 1

[GalaxyShard]

I feel like this is an odd pattern to assign a seperate boolean for the sole purpose of determining whether or not to skip this iteration of the inline loop on the next line.

Could it be replaced with this:

inline for (@typeInfo(@TypeOf(args)).@"struct".fields) |field| blk: {
    const maybe_value = @field(args, field.name);
    const value = switch (@typeInfo(@TypeOf(maybe_value))) {
        .optional => maybe_value orelse break :blk,
        else => maybe_value,
    };
    switch (@TypeOf(value)) {
        ...

[GalaxyShard]

Also, what should the behavior be when just specifying null instead of something like @as(?std.builtin.OptimizeMode, null) as in your example? With both my suggested change and your current code, I believe this would just be a compile error (null has it's own type; it's not covered by the .optional case). I think it may be better to just treat plain null as not specified to be consistent with optionals.

Example fix:

inline for (@typeInfo(@TypeOf(args)).@"struct".fields) |field| blk: {
    const maybe_value = @field(args, field.name);
    const value = switch (@typeInfo(@TypeOf(maybe_value))) {
        .optional => maybe_value orelse break :blk,
        .null => break :blk,    // <-- Note this added case
        else => maybe_value,
    };
    switch (@TypeOf(value)) {
        ...