Rust-specific emacs compilation regexp

[pnkfelix]

Fix #6887

[huonw]

What happens if a filename does have a space in it?

(Note: I have no idea how this is supposed to work.)

[pnkfelix]

Then the regexp will not match.

I was about to claim that the 'gnu regexp makes the same trade-off, but I decided to actually check that claim. Which is good, because the claim would have been false.

(The docs for the gnu regexp claim that it matches file names composed of any non-newline char, apart from some particular exceptions, and direct experimentation seems to confirm this claim, though I cannot vouch for the regexp there since it is too big for me to decode. You can see that regexp with its documentation at mirrors/emacs.)

I'll see if I can improve upon this to support spaces as well.

[pnkfelix]

(I even tested it out; it seems to work, at least for a couple minimal of files and parent directories with embedded spaces that I threw into /tmp.)

[pnkfelix]

(also, for anyone who wants to play around with this stuff, I left copious notes in the comments for #6887 that should explain how you can experiment interactively in the Emacs *scratch* buffer to find out how different regular-expressions behave. There is also M-x re-builder option, which was also very interesting to play with, but even with that I could not figure out a direct way to fix the original 'gnu regexp.)