fix and coverage for layout modifiers

src/layout.rs

@@ -96,6 +96,53 @@ impl Sizer {
     }
 }
 
+macro_rules! def_layout_modifier {
+    (
+        $Trait:ident ($fn_x:ident $fn_y:ident $fn_xy:ident),
+        $Struct:ident { $X:ident $Y:ident $XY:ident } $kw_name:ident $name:literal
+        |$self:ident, $to:ident| $body:block
+    ) => {
+        impl<S: Screen, T: Draw<S>> $Trait<S> for T {}
+
+        pub trait $Trait<S: Screen>: Draw<S> + Sized {
+            fn $fn_x <N: Into<Option<S::Unit>> + Copy> (self, x: N)
+                -> $Struct<S, Self, N> { $Struct::$X(self, x) }
+            fn $fn_y <N: Into<Option<S::Unit>> + Copy> (self, y: N)
+                -> $Struct<S, Self, N> { $Struct::$Y(self, y) }
+            fn $fn_xy <N: Into<Option<S::Unit>> + Copy> (self, x: N, y: N)
+                -> $Struct<S, Self, N> { $Struct::$XY(self, x, y) }
+        }
+
+        pub enum $Struct<S: Screen, I: Draw<S>, X: Into<Option<S::Unit>>> {
+            __(PhantomData<S>), $X(I, X), $Y(I, X), $XY(I, X, X),
+        }
+
+        impl <S: Screen, I: Draw<S>, X: Into<Option<S::Unit>> + Copy> Draw<S> for $Struct<S, I, X> {
+            fn draw (&$self, $to: &mut S) -> Perhaps<XYWH<S::Unit>> {
+                $body
+            }
+        }
+
+        fn_kw_layout!($kw_name |state, output, expr| {
+            let head = expr.head()?;
+            let mut frags = head.src()?.unwrap_or_default().split("/");
+            Ok(matches!(expr.head()?, Some("exact")).then(||{
+                let args  = expr.tail();
+                let arg0  = args.head();
+                let tail0 = args.tail();
+                let arg1  = tail0.head();
+                let tail1 = tail0.tail();
+                let arg2  = tail1.head();
+                eval_xy!(
+                    $name => expr, head, output, state, frags.next(),
+                    $fn_xy, $fn_x, $fn_y,
+                    arg0, arg1, arg2,
+                )
+            }).transpose()?.flatten())
+        });
+    }
+}
+
 mod area;    pub use self::area::*;
 mod axis;    pub use self::axis::*;
 mod azimuth; pub use self::azimuth::*;