wip: big flat pt.4: extract layout crate

layout/src/logic.rs

@@ -0,0 +1,43 @@
+use crate::*;
+
+/// Conditional rendering, in unary and binary forms.
+pub struct Cond;
+
+impl Cond {
+    /// Render `item` when `cond` is true.
+    pub fn when <E: Engine, A: Render<E>> (cond: bool, item: A) -> When<E, A> {
+        When(cond, item, Default::default())
+    }
+    /// Render `item` if `cond` is true, otherwise render `other`.
+    pub fn either <E: Engine, A: Render<E>, B: Render<E>> (cond: bool, item: A, other: B) -> Either<E, A, B> {
+        Either(cond, item, other, Default::default())
+    }
+}
+
+/// Renders `self.1` when `self.0` is true.
+pub struct When<E: Engine, A: Render<E>>(bool, A, PhantomData<E>);
+
+impl<E: Engine, A: Render<E>> Render<E> for When<E, A> {
+    fn min_size (&self, to: E::Size) -> Perhaps<E::Size> {
+        let Self(cond, item, ..) = self;
+        if *cond { item.min_size(to) } else { Ok(Some([0.into(), 0.into()].into())) }
+    }
+    fn render (&self, to: &mut E::Output) -> Usually<()> {
+        let Self(cond, item, ..) = self;
+        if *cond { item.render(to) } else { Ok(()) }
+    }
+}
+
+/// Renders `self.1` when `self.0` is true, otherwise renders `self.2`
+pub struct Either<E: Engine, A: Render<E>, B: Render<E>>(bool, A, B, PhantomData<E>);
+
+impl<E: Engine, A: Render<E>, B: Render<E>> Render<E> for Either<E, A, B> {
+    fn min_size (&self, to: E::Size) -> Perhaps<E::Size> {
+        let Self(cond, item, other, ..) = self;
+        if *cond { item.min_size(to) } else { other.min_size(to) }
+    }
+    fn render (&self, to: &mut E::Output) -> Usually<()> {
+        let Self(cond, item, other, ..) = self;
+        if *cond { item.render(to) } else { other.render(to) }
+    }
+}