>Where are you seeing evidence of that? As far as I can tell, HOMEBREW_ARCHFLAGS remains an empty string before and after rebar is run.

Environment variables are a per-process entity. Changes made by child processes are not reflected in the parent. `rebar` is removing empty environment variables before spawning the compiler, which causes it to be nil in our compiler wrapper:

```
/usr/local/Library/ENV/4.3/clang:241:in `archflags': private method `split' called for nil:NilClass (NoMethodError)
```