Function placed_semantics_from_applier 

pub fn placed_semantics_from_applier(
    applier: &mut MemoryApplier,
    root: NodeId,
    size: Size,
) -> Result<Option<PlacedSemanticsNode>, NodeError>

Lay root out at size and read back its semantics with geometry.

The applier must already be carrying a runtime handle (MemoryApplier::set_runtime_handle) — a subcomposing widget cannot be measured without one. crate::testing::ComposeTestRule::placed_semantics does that part; this is for a caller driving a TestComposition by hand.

None means the composition placed nothing at all, which for a root that composed content is itself the answer to a bug hunt.