Compile error when trying to load a ZON file with an enum literal member

[dk949]

Zig Version

0.14.0

Steps to Reproduce and Observed Behavior

Compile the following file:

const std = @import("std");

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const alloc = gpa.allocator();
    defer _ = gpa.deinit();

    const ZonType = struct {
        comptime field: @TypeOf(.value) = .value,
    };
    const zon_txt =
        \\.{
        \\    .field = .value,
        \\}
    ;

    const zon_parsed = try std.zon.parse.fromSlice(ZonType, alloc, zon_txt, null, .{});
    defer std.zon.parse.free(alloc, zon_parsed);
}

Observed behaviour

Compiler reaches unreachable code in a switch in std.zon.parse.free.

This also happens if I delete the call to std.zon.parse.free.

Expected Behavior

Program compiles with no error.

Parsing a ZON file with enum literals is required to fully read a build.zig.zon file.

(It's still possible to read it partially by using the ignore_unknown_fields option when parsing)

[MasonRemaley]

The reason this doesn't work is that @TypeOf(.value) is enum_literal which isn't supported as the value must be comptime known, and the parsing here is happening at runtime.

This is working as intended but it's hard to understand as a user. It's worth considering whether comptime unreachable should be replaced with something that prints "type T not supported by ZON".

IIRC I did this initially, but changed it because the standard library tries to avoid custom type errors like this. I'm not sure I understand why we avoid this though--my guess is that it's because it's better to improve the built in errors than to try to patch over them, especially since it's hard to test custom errors like this.

If that is the motivation maybe this is a reasonable exception.