Support @ marker in tree! macro to append subtrees

Author: RJ

src/lib.rs

@@ -822,6 +822,22 @@ pub mod iter;
 /// };
 /// # }
 /// ```
+/// Compose trees using the `@` marker:
+/// ```
+/// #[macro_use] extern crate ego_tree;
+/// # fn main() {
+/// let subtree = tree! {
+///     "foo" => { "bar", "baz" }
+/// };
+/// let new_tree = tree! {
+///     "root" => {
+///         "child x",
+///         "child y",
+///         @ subtree,
+///     }
+/// };
+/// # }
+/// ```
 #[macro_export]
 macro_rules! tree {
     (@ $n:ident { }) => { };
@@ -858,6 +874,13 @@ macro_rules! tree {
         }
     };
 
+    // Append subtree from expression.
+    (@ $n:ident { @ $subtree:expr $(, $($tail:tt)*)? }) => {{
+        $n.append_subtree($subtree);
+        $( tree!(@ $n { $($tail)* }); )?
+    }};
+
+
     ($root:expr) => { $crate::Tree::new($root) };
 
     ($root:expr => $children:tt) => {

tests/macro.rs

@@ -163,3 +163,27 @@ fn mixed() {
 
     assert_eq!(manual_tree, macro_tree);
 }
+
+#[test]
+fn subtree() {
+    let subtree = tree! {
+        'x' => { 'y' => {'z'}}
+    };
+    let tree = tree! {
+        'a' => {
+            'b',
+            'c' => {'d', 'e'},
+            @ subtree.clone(),
+            'f' => { @subtree },
+        }
+    };
+    let expected_tree = tree! {
+        'a' => {
+            'b',
+            'c' => {'d', 'e'},
+            'x' => { 'y' => {'z'}},
+            'f' => {'x' => { 'y' => {'z'}} },
+        }
+    };
+    assert_eq!(tree, expected_tree);
+}