Comment by messe

With regard to the casting example, you could always wrap the cast in a function:

    fn signExtendCast(comptime T: type, x: anytype) T {
        const ST = std.meta.Int(.signed, @bitSizeOf(T));
        const SX = std.meta.Int(.signed, @bitSizeOf(@TypeOf(x)));
        return @bitCast(@as(ST, @as(SX, @bitCast(x))));
    }

    export fn addi8(addr: u16, offset: u8) u16 {
        return addr +% signExtendCast(u16, offset);
    }

This compiles to the same assembly, is reusable, and makes the intent clear.