4 ms·

I am going to ask a question that is is definitely not the place for, but I am not involved with Zig in any way and am curious, so I hope you'll indulge me. I

by skavi 7mo ago

I am going to ask a question that is is definitely not the place for, but I am not involved with Zig in any way and am curious, so I hope you'll indulge me. I noticed the following comment was added to lib/std/multi_array_list.zig [0] with this change: /// This pointer is always aligned to the boundary `sizes.big_align`; this is not specified /// in the type to avoid `MultiArrayList(T)` depending on the alignment of `T` because this /// can lead to dependency loops. See `allocatedBytes` which `@alignCast`s this pointer to /// the correct type. How could relying on `@alignOf(T)` in the definition of `MultiArrayList(T)` cause a loop? Even with `T` itself being a MultiArrayList, surely that is a fully distinct, monomorphized type? I expect I am missing something obvious. [0]: https://codeberg.org/ziglang/zig/pulls/31403/files#diff-a6fcdd8d64d09b77e76edcc89ea4c2881cebda7f https://codeberg.org/ziglang/zig/pulls/31403/files#diff-a6fc...

messe 7mo ago
I had to search for this, but managed to find the relevant mlugg@ comment[0] on the ZSF zulip: > i had to change the bytes field from [*]align(@alignOf(T)) u8 to just [*]u8 (and cast the alignment back in the like one place that field is accessed). this wasn't necessary for MultiArrayList in and of itself, but it was necessary for embedding a MultiArrayList(T) inside of T without a dependency loop, like const T = struct { children: MultiArrayList(T), }; // reproduced for completeness: fn MultiArrayList(comptime T: type) type { return struct { bytes: [*]align(@alignOf(T)) u8, // ... }; } [0]: https://zsf.zulipchat.com/#narrow/channel/454360-compiler/topic/type.20resolution/near/533972426 https://zsf.zulipchat.com/#narrow/channel/454360-compiler/to...
- skavi 7mo ago
  Ah, that makes sense. Thanks for pulling this up!
  - messe 7mo ago
    He went into it a bit more over on Ziggit today too. I only noticed it way after I went digging: https://ziggit.dev/t/devlog-type-resolution-redesign-with-language-changes-to-taste/14579/5 https://ziggit.dev/t/devlog-type-resolution-redesign-with-la...

by skavi 7mo ago

messe 7mo ago

I had to search for this, but managed to find the relevant mlugg@ comment[0] on the ZSF zulip: > i had to change the bytes field from [*]align(@alignOf(T)) u8 to just [*]u8 (and cast the alignment back in the like one place that field is accessed). this wasn't necessary for MultiArrayList in and of itself, but it was necessary for embedding a MultiArrayList(T) inside of T without a dependency loop, like const T = struct { children: MultiArrayList(T), }; // reproduced for completeness: fn MultiArrayList(comptime T: type) type { return struct { bytes: [*]align(@alignOf(T)) u8, // ... }; } [0]: https://zsf.zulipchat.com/#narrow/channel/454360-compiler/topic/type.20resolution/near/533972426 https://zsf.zulipchat.com/#narrow/channel/454360-compiler/to...

skavi 7mo ago
Ah, that makes sense. Thanks for pulling this up!
- messe 7mo ago
  He went into it a bit more over on Ziggit today too. I only noticed it way after I went digging: https://ziggit.dev/t/devlog-type-resolution-redesign-with-language-changes-to-taste/14579/5 https://ziggit.dev/t/devlog-type-resolution-redesign-with-la...