Heads up that @bobvanderlinden left a note at 75b18d2#r111783708 mentioning that this should probably be set in enterShell instead of via env, as otherwise this solution relies on the DEVENV_PROFILE environment variable being properly set in the user's shell at all times.

I plan to tweak the solution in #559 to do the same.

Thanks for the heads-up! The PR that fixed the earlier issue is here for Python: #573

See that PR also for the ability to add tests in examples using .test.sh. This can be used to avoid regressions in the future.

@bobvanderlinden Oh, I hadn't seen that. Thank you for doing that!

Did anyone test if this works?

I worked around this on macOS via

      env =
        {}
        // lib.mkIf pkgs.stdenv.isDarwin {
          RUSTFLAGS = lib.mkForce ["-L framework=$DEVENV_PROFILE/Library/Frameworks"];
          RUSTDOCFLAGS = lib.mkForce ["-L framework=$DEVENV_PROFILE/Library/Frameworks"];
          CFLAGS = lib.mkForce ["-iframework $DEVENV_PROFILE/Library/Frameworks"];
        };

which is equivalent to this PR.

I think this will never work with the current implementation. Using $DEVENV_PROFILE inside env.* will not be expanded (or has that been changed in the meantime?). Adding it to enterShell does work.
Best would be to add a test, so that these kinds of problems won't pop up again and it is clear for others how to replicate them.

@bobvanderlinden thanks you are absolutely correct. I'll open up a PR that mimics your solution for python. Thank you!

This one won't fix it, there's a new PR and I'm testing a more general fix.