zon: canSerializeType provides no information as to what it cannot serialize

[NicoElbers]

Zig Version

0.14.0-dev.3213+53216d2f2

Steps to Reproduce and Observed Output

mwe.zig

const Foo = struct {
    foo: ?union(enum) { b },
};

pub fn main() !void {
    const stdout = std.io.getStdOut();
    const writer = stdout.writer();

    const foo: Foo = .{ .foo = .b };

    try std.zon.stringify.serialize(foo, .{}, writer);
}

const std = @import("std");

run zig build-exe -freference-trace mwe.zig

Observe:

/nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/debug.zig:518:14: error: reached unreachable code
    if (!ok) unreachable; // assertion failure
             ^~~~~~~~~~~
/nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:493:28: note: called from here
            comptime assert(canSerializeType(@TypeOf(val)));
                     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    fieldArbitraryDepth__anon_19074: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:1005:56
    fieldArbitraryDepth__anon_18003: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:907:55
    valueArbitraryDepth__anon_16985: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:584:83
    valueArbitraryDepth__anon_14342: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:593:49
    fieldArbitraryDepth__anon_10402: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:1005:56
    fieldArbitraryDepth__anon_7688: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:907:55
    valueArbitraryDepth__anon_5065: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:571:62
    value__anon_2935: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:473:44
    serialize__anon_1989: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/zon/stringify.zig:51:17
    main: mwe.zig:11:36
    posixCallMainAndExit: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/start.zig:656:37
    _start: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/start.zig:464:40
    comptime: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/start.zig:91:63
    start: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/std.zig:97:27
    comptime: /nix/store/w0x7fxnl703niapxdabvwjhbpmcrz5b5-zig-0.14.0-dev.3237+ddff1fa4c/lib/std/std.zig:168:9

Expected Output

I expect canSerializeType to fail in a spot that shows me why I cannot serialize this type, or otherwise provides more detail what exactly cannot be serialized.