std.Build: accept comptime string as option value

[thiago-negri]

Small quality of life improvement for std.Build interface: accept comptime string as option value without the need to cast it to []const u8.

Problem

On one project I have a build option declared as such:

const str_opt = b.option([]const u8, "str_opt", "my str opt");

Then on another project I want to set that option:

const dep = b.dependency("other-project", .{
    .str_opt = @as([]const u8, "the str value")
});

I'm forced to cast it to []const u8.

If I try this:

const dep = b.dependency("other-project", .{
    .str_opt = "the str value"
});

I get the error:

Build.zig:503:25: error: option 'opt_str' has unsupported type: *const [13:0]u8

Expectation

Zig compiler automatically casts comptime strings to []const u8 if they are, for example, declared as the type of a function argument. I'd expect the same here.

Diagnosis

std.Build.userInputOptionsFromArgs is a comptime function that evaluates its args using @TypeOf information. So the compiler has no way to know whether that requires an auto-cast or not.

Solution

This PR tries to address the issue by forcing the []const u8 type when it encounters a value that looks like a comptime string. As the inner UserValue.value uses a []const u8, I suppose the behavior does not change.

I'm not sure if that's the correct way to infer if a value can be considered a "comptime string". But it works for my use case.