use ToOwned trait instead of inherent method

crates/ra_syntax/src/ast.rs

@@ -15,12 +15,13 @@ use crate::{
 /// conversion itself has zero runtime cost: ast and syntax nodes have exactly
 /// the same representation: a pointer to the tree root and a pointer to the
 /// node itself.
-pub trait AstNode: rowan::TransparentNewType<Repr = rowan::SyntaxNode<RaTypes>> {
+pub trait AstNode:
+    rowan::TransparentNewType<Repr = rowan::SyntaxNode<RaTypes>> + ToOwned<Owned = TreeArc<Self>>
+{
     fn cast(syntax: &SyntaxNode) -> Option<&Self>
     where
         Self: Sized;
     fn syntax(&self) -> &SyntaxNode;
-    fn to_owned(&self) -> TreeArc<Self>;
 }
 
 pub trait AstToken: AstNode {