Packed tagged union in zig

Packed tagged.zig

const E = enum(u12) { 
    Int = 0xFFF,
    String = 0xFFE,
    Pointer = 0xFFD,
}

const P = packed union(?e){
    const this = @This();

    Int: packed struct {
        number: u32,
        _padding: u20,
        @tag(this) : e,
    },

    String: packed struct{
        string: [6]u8,
        @tag(this) : e,
    },

    Pointer: packed struct {
        pointer: u48,
        @tag(this) : e,
    }

    null: f64,
}

const std = @import("std");
const expect = std.testing.expect;

fn tag_of(p: P) u12 {return @intCast(u12, (@bitCast(u64, p) | 0xFFF0_0000_0000_0000) >> 52)}

test "NaN-boxed float" {
    var with_int = P{.Int = .{number = 1234}};
    var with_string = P{.String = .{string = "foo"}};
    var as_float = P{.null = 1.234};

    try expect(with_int.Int.number == 1234);
    try expectEqual(with_string.String.string, "foo");
    try expect(as_float.null == 1.234);

    try expect(tag_of(with_int) == 0xFFF);
    try expect(tag_of(with_string) == 0xFFE)
}

• tag location is always comptime checked. It must be in the same place as all non-null tags.
• might be possible to infer the location of the tag (first available high order locations - this would make "padding" unnecessary for "int")
• it is considered safety-checked ub if the null enum writes an enumerated value into the respective bits of the tag location