You're right about that and i was just being naïve. 

Emacs' default prefix completion simply uses all available 
input for completion and jumps to the end of the line.
It also says "No Match" if the input isn't the prefix of an existing file or directory.

While this is a terrible solution for completing the name of a new file (i.e. when saving something)
this may be a better alternative than inserting a space. 

I'll open a new branch and implement that.