A slur's first and last note must not be a rest

A slur connects two sounding notes in a legato phrase; a rest has nothing to legato-connect to or from. A phrase mark MAY visually span over an intervening rest inside its range, but its two ENDPOINTS must land on actual notes.

Gould ('Slurs') and ('Slurs and phrasing') discuss slurs strictly in terms of note-to-note spans; Read states a slur's ends are always on notes, with any interior rest simply passed under the arc.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.