Today I learned:

• --emit-llvm silently does nothing if you're compiling to an executable, and requires that you provide a -o filename that ends in .bc. Perhaps there should be errors instead.
• --save-temps and --emit-llvm only do anything if they come before the crate file name.

These things should probably be documented. I'm not sure where such documentation belongs (seems a bit verbose for the command-line usage notes).