A rest must not cross a beat boundary unless it starts on one and fills whole beats (or is a whole-bar rest)

Rest durations and placement must keep the metric beat visible — a rest that starts mid-beat and continues past the next beat boundary hides where beat 2, 3, etc. fall.

A single rest glyph spanning, e.g., beat 2 through beat 3.5 in 4/4 — the classic dotted-quarter-rest-off-the-beat case.

Gould, Behind Bars, ch. on rests: rests should be split at beat boundaries so the underlying pulse stays legible — the same principle behind why a dotted-quarter rest from beat 2 to 3.5 is conventionally written as a quarter rest + eighth rest instead of one glyph.

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