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

pub const Foo = struct {
    bar: struct {
        baz: void, // Illegal here
    } = .{ .baz = {} },
};

pub fn main() !void {
    var arr = std.ArrayList(u8).init(std.heap.smp_allocator);
    defer arr.deinit();

    try std.zon.stringify.serialize(@as(Foo, .{}), .{}, arr.writer());
}
const std = @import("std");

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

Observe:

/nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/debug.zig:518:14: error: reached unreachable code
    if (!ok) unreachable; // assertion failure
             ^~~~~~~~~~~
/nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/zon/stringify.zig:493:28: note: called from here
            comptime assert(canSerializeType(@TypeOf(val)));
                     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    value__anon_3073: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/zon/stringify.zig:473:44
    serialize__anon_2007: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/zon/stringify.zig:51:17
    main: mwe.zig:11:36
    posixCallMainAndExit: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/start.zig:656:37
    _start: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/start.zig:464:40
    comptime: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/start.zig:91:63
    start: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/lib/std/std.zig:97:27
    comptime: /nix/store/4h7cpfzzsczh6g8ivdvaz5k9dygy5z6p-zig-0.14.0-dev.3241+55c46870b/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.

[robbielyman]

i wondered if maybe ?union(enum) { b } was the problem (i mistakenly forgot that union(enum) can have variants implicitly with value of type void), so i took this for a spin with 0.14.0-dev.3328+b6a1fdd3f and was unable to reproduce the error (on macOS).

instead i got

% zig run test.zig
.{ .foo = .b }

[NicoElbers]

Yes, at the time of making this issue this was a bug. I'll update the example but the principal remains