[MacPorts] #58848: Error: Failed to build rust: command execution failed

[Chris Jones]

> It could be that switching to the latest compiler would help. You can
> try building with
>      sudo port -v build rust configure.compiler=macports-clang-7.0
> or something like that (I'm not 100% sure if the command is exactly
> right as I'm not behind my mac right now).

Tried that, doesn't help.

   = note: dyld: Symbol not found: __ZTIN4llvm10CallbackVHE
             Referenced from: /opt/local/libexec/llvm-7.0/bin/clang
             Expected in: 
/opt/local/var/macports/build/_Users_chris_Projects_MacPorts_ports_lang_rust/rust/work/rustc-1.36.0-src/build/stage0-x86_64/lib/libLLVM.dylib
            in /opt/local/libexec/llvm-7.0/bin/clang

which looks nasty. It appears rust is borking its build environment in a 
way that causes /opt/local/libexec/llvm-7.0/bin/clang to pick up an 
inconsistent internal build of libLLVM.dylib. nice....

Chris