`-L` and `-o` rustc arguments can be used unintuitively

[wesleywiser]

A developer reported that --emit mir wasn't creating a .mir file next to their source file:

$ rustc foobar.rs --emit mir -Cdebuginfo=2 -Csymbol-mangling-version -optimize -Ccodegen-units=1
$ ls
foobar.rs    ptimize

Obviously the issue is that -optimize is treated as -o ptimize but it would be nice if we could do something better here. -L has a similar kind of issue in that -Link-everything-statically is treated as -L ink-everything-statically but that one we could probe and issue a warning if the specified path doesn't exist.

One possibility might be to start issuing a warning (but never an error) if -o is specified without a space between the argument and the filename.

[xizheyin]

@rustbot claim