Introduce owned ast nodes

ast::FooNode is an owned 'static counterpart to ast::Foo<'a>

crates/ra_syntax/src/ast/generated.rs.tera

@@ -7,11 +7,25 @@ the below applies to the result of this template
 
 use crate::{
     ast,
-    SyntaxNodeRef, AstNode,
+    SyntaxNode, SyntaxNodeRef, AstNode,
     SyntaxKind::*,
 };
 {% for node, methods in ast %}
 // {{ node }}
+
+#[derive(Debug, Clone)]
+pub struct {{ node }}Node(SyntaxNode);
+
+impl {{ node }}Node {
+    pub fn new(&self, ast: {{ node }}) -> {{ node }}Node {
+        let syntax = ast.syntax().owned();
+        {{ node }}Node(syntax)
+    }
+    pub fn ast(&self) -> {{ node }} {
+        {{ node }}::cast(self.0.borrowed()).unwrap()
+    }
+}
+
 {%- if methods.enum %}
 #[derive(Debug, Clone, Copy)]
 pub enum {{ node }}<'a> {