On Mac OS, I was compiling a source file in a directory where I didn't have permission to create a new file like so:

$ rustc -o hello hello.rs
error: Could not write output: x86_64-a

It wasn't clear that this was an file IO issue and I could find no mention of this error in docs. The subject of the message was misleading and it looked to me like a code generation error due to the mention of "x86_64-a".

The error would be more clear if it printed the name of the file it was attempting to write instead of the architecture.