Can we disable it for Darwin instead?

Clang LTO on Linux should be fine.

And add a comment there why its disabled on Darwin.

Have you tested it? I would suspect it is more likely to be LLVM exposing UB than a Darwin issue. And the Nix derivation uses isGNU here.

Tried building lix from master with clangStdenv and it built fine. (x86_64-linux)

Interesting. What about with libc++?

This does work if that's what you are asking:

❯  nix build --impure --expr '(builtins.getFlake "github:nixos/nixpkgs/master").legacyPackages."${builtins.currentSystem}".libcxx.override {stdenv = (builtins.getFlake "github:nixos/nixpkgs/master").legacyPackages."${builtins.currentSystem}".clangStdenv;}' --print-out-paths 
/nix/store/nqpv4xbh5zqnvfn80702gahiklxxjcr3-libcxx-18.1.8

PS: if you have a shorthand for these overrides in cli, let me know!

what in the hell. maybe we have a missing include but I'm guessing those sys constants are just broken. (though libcxxStdenv is SURELY not using an alternative libc right? just libc++?)

it may also be broken toolchain on the nixpkgs side for all i know, like this release blocker for lix 2.92: #177129

This does seem to only be a missing include. Sent a patch upstream in https://gerrit.lix.systems/c/lix/+/2286

Using this override still fails when linking with things like the aws-sdk though (probably because they're using glibc?), and being a bit more forceful with libcxx usage like so

let
  pkgs = import <nixpkgs> {
    config = {
      replaceStdenv = { pkgs }: pkgs.libcxxStdenv;
    };
    overlays = [ ];
  };
in

pkgs.lix

results in the always fun error: infinite recursion encountered, so I don't think we can determine whether this is a libcxx or Darwin issue just yet

I'm inclined to say we should change this to stdenv.cc.libcxx != null for now, as that would cover Darwin and the possibly broken (and currently un-evaluatable) libcxx Linux build -- which can always be revisited later

Also let me know if there's anything I can help with to get this merged. Would love to see a fix after all this time :)

i believe there is a libcxxStdenvPackages somewhere that is a nixpkgs staged from libcxxStdenv but yeah you can't really mix libc++ and libstdc++ in one binary at least if you're linking c++ symbols between them.

if this pr is acceptable i can just like, merge it. i have commit rights. i think it's more important that lix works than that it has LTO in all configurations possible (including weird ones) given that iirc LTO doesn't make that huge a difference anyway.

i think it's more important that lix works than that it has LTO in all configurations possible

Agreed.

I'm inclined to say we should change this to stdenv.cc.libcxx != null for now, as that would cover Darwin and the possibly broken (and currently un-evaluatable) libcxx Linux build -- which can always be revisited later

Also agreed.

if this pr is acceptable

I'd say it 100% is. My suggestions were by no means a blocker, just a way to hopefully get things going

i believe there is a libcxxStdenvPackages somewhere that is a nixpkgs staged from libcxxStdenv

TIL that is pkgsLLVM...I thought that used clangStdenv. Time to build :)