Painter::draw_twice(child, first, |used| second) becomes Painter::measure
plus an ordinary draw. Iris's objection was the shape it forced on the
caller rather than the two draws themselves: the arithmetic that picks
the real region had to happen inside a closure, and anything it wanted to
keep came back out through a captured &mut. LazySpan::place was the only
caller, and it now reads as the three statements it is.
DrawMode::Measure is that draw with everything it writes switched off --
no arena slot, no mask, no move slot, nothing left in `active`, nothing
marked dirty. Only the returned Size survives, and the widget is left
exactly as it was, so the real draw that follows is an ordinary first
draw or redraw. That last part is load-bearing: a measurement that left
an ActiveData behind would let the following draw hit draw_inner's
"already at this region" fast path and return having drawn nothing.
A measurement also does not consume a redraw mark, since it is not the
redraw the mark asked for, and it takes none of the fast paths, since
"already drawn here" cannot report a size.
Every Painter method that writes now returns early on the mode -- a
widget's own draw never checks, which is the point. A debug_assert at the
end of draw_inner catches one that forgot, because the failure otherwise
is a single leaked primitive per measured widget per frame, which a
screen redrawn every frame turns into an arena that grows without bound.
What this is worth, and what it is not. The amplification it applies to,
measured on a streamed frame: 1,083 Widget::draw calls over 113 distinct
widgets, with the worst drawn 11 times at nesting depth 7-8 -- it is not
two draws but two to the power of how many measuring ancestors a widget
has. Only the writes go away; the walk and the region arithmetic still
happen 11 times, and removing those needs a size answerable without a
draw, which LAYOUT.md section 5 rules out. Streamed frame p50 1.39ms ->
1.22ms, p99 4.75ms -> 3.58ms. The upload numbers do not move, because
slot recycling had already made the discarded writes free in arena terms.
Also extracts move_slot_for from draw_inner, since measuring must not
allocate one and the reuse-in-place rule wanted saying once.
Verified: run-tests.sh, iris's suite, clippy and rustfmt clean, and the
headless phone render is byte-identical to the previous commit's on the
real GPU (Venus, RX 7900 XT -- checked, not llvmpipe).
iris
18c5f9aaac