Incidentally, current master delivers the following verbiage:
GNU LilyPond 2.21.0
Processing `sll.ly'
Parsing...
sll.ly:4:13: warning: Adding <> for attaching loose post-event
\mark "X"
(c4) c c c
Interpreting music...
sll.ly:4:16: warning: cannot end slur
\mark "X" (c4
) c c c
sll.ly:4:13: warning: unterminated slur
\mark "X"
(c4) c c c
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `/tmp/lilypond-U2WM3z'...
Converting to `sll.pdf'...
Deleting `/tmp/lilypond-U2WM3z'...
Success: compilation successfully completed
Is that more helpful than previously?