output: more big refactors

output/src/content.rs

@@ -0,0 +1,40 @@
+use crate::*;
+
+/// Composable renderable with static dispatch.
+pub trait Content<E: Out>: Sized {
+    /// Return opaque [Draw]able.
+    fn content (&self) -> impl Draw<E> + Layout<E> + '_ { () }
+}
+
+/// The platonic ideal unit of [Content]:
+/// total emptiness at dead center (e=1vg^sqrt(-1))
+impl<E: Out> Content<E> for () {}
+
+impl<E: Out, T: Draw<E> + Layout<E>> Content<E> for fn()->T {
+    fn content (&self) -> impl Draw<E> + Layout<E> + '_ {
+        self()
+    }
+}
+
+/// Implement composable content for a struct.
+#[macro_export] macro_rules! content {
+
+    // Implement for all [Out]s.
+    (|$self:ident:$Struct:ty| $content:expr) => {
+        impl<E: Out> Content<E> for $Struct {
+            fn content (&$self) -> impl Draw<E> + Layout<E> + '_ { Some($content) }
+        }
+    };
+
+    // Implement for specific [Out].
+    ($Out:ty:|
+        $self:ident:
+        $Struct:ident$(<$($($L:lifetime)? $($T:ident)? $(:$Trait:path)?),+>)?
+    |$content:expr) => {
+        impl $(<$($($L)? $($T)? $(:$Trait)?),+>)? Content<$Out>
+        for $Struct $(<$($($L)? $($T)?),+>)? {
+            fn content (&$self) -> impl Draw<$Out> + Layout<$Out> + '_ { $content }
+        }
+    };
+
+}