RE: diffs between a buffer and the underlying file

["Drew Adams" <drew.adams@oracle.com>]

    I sometimes get into the following situation:  I open a file, make
    some changes, then I want to see the changes before saving the file.
    The best way to do this seems to be to set the region to the whole
    buffer, write the region to some temporary file, and run diff on the
    two files.  But I find this very unconvenient.

    I'd like to have a function in emacs which can be applied to a buffer,
    say buffer A, that opens a new buffer displaying all the changes
    between the buffer A and the disk file, opened in buffer A.

    Searching the emacs documentation I haven't found anything like this.
    Only functions for diff'ing two buffers or two files.

You can always use ediff to compare any two buffers. So, with nothing extra,
you can 1) rename buffer A as foo, 2) re-read file A into buffer A, and 3)
compare buffers foo and A. The commands for this are 1) M-x rename-buffer,
2) M-x revert-buffer, and 3) M-x ediff-buffers.

Ediff  is your friend, generally.