Allow creating some environment variables (compatibility gtk, pxthon…)

[tobiasBora]

Some app will not work out of the box with nix-ld like gtk-based app (see https://discourse.nixos.org/t/nix-ld-no-gsettings-schemas-are-installed-on-the-system/74064/6) or python (see doc of this module) and require extra environment variables. Unfortunately, setting them globally may harm other nixos app (e.g. LD_PRELOAD may have dangerous effects) so I'd need a way to set these environment variables direcly inside nix-ld (I shown a while ago that this is an elegant way to create less-intrusive wrappers).

In term of implementation, we could for instance look for environment variables like NIX_LD_ENV_CREATE_FOO and export them as FOO. To allow prefixing/suffixing an existing environment variable we could prefix to FOO (similarly for suffix) env called like NIX_LD_ENV_PREFIX_FOO.

Would you consider adding this feature?

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-ld-no-gsettings-schemas-are-installed-on-the-system/74064/8

[tobiasBora]

I started to give it a look, and for efficiency reasons I wanted to use a HashMap to check which environment variable is defined and which environment variable is not… Sadly the code is marked with no_std and hash maps are from the std library. Why is it the case that you don't use std, is it just for efficiency reasons, or is std just impossible to use because of how loaders work?