How can I add line numbers to a range of lines in a file opened in Vim? Not as in :set nu—this just displays line numbers—but actually have them be prepended to each line in the file?
With
:%s/^/\=line('.')/
EDIT: to sum up the comments.
This command can be tweaked as much as you want.
Let's say you want to add numbers in front of lines from a visual selection (V + move), and you want the numbering to start at 42.
:'<,'>s/^/\=(line('.')-line("'<")+42)/
If you want to add a string between the number and the old text from the line, just concatenate (with . in VimL) it to the number-expression:
:'<,'>s/^/\=(line('.')-line("'<")+42).' --> '/
If you need this to sort as text, you may want to zero pad the results, which can be done using printf for 0001, 0002 ... instead of 1, 2... eg:
:%s/^/\=printf('%04d', line('.'))/
Anyway, if you want more information, just open vim help: :h :s and follow the links (|subreplace-special|, ..., |submatch()|)