Auto merge of #96924 - petrochenkov:nonon, r=nnethercote

ast: Introduce some traits to get AST node properties generically

And use them to avoid constructing some artificial `Nonterminal` tokens during expansion
r? `@nnethercote` `@Aaron1011`

compiler/rustc_ast/src/ast.rs

@@ -929,16 +929,6 @@ pub struct Stmt {
 }
 
 impl Stmt {
-    pub fn tokens(&self) -> Option<&LazyTokenStream> {
-        match self.kind {
-            StmtKind::Local(ref local) => local.tokens.as_ref(),
-            StmtKind::Item(ref item) => item.tokens.as_ref(),
-            StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => expr.tokens.as_ref(),
-            StmtKind::Empty => None,
-            StmtKind::MacCall(ref mac) => mac.tokens.as_ref(),
-        }
-    }
-
     pub fn has_trailing_semicolon(&self) -> bool {
         match &self.kind {
             StmtKind::Semi(_) => true,
@@ -2684,13 +2674,6 @@ impl Item {
     }
 }
 
-impl<K: Into<ItemKind>> Item<K> {
-    pub fn into_item(self) -> Item {
-        let Item { attrs, id, span, vis, ident, kind, tokens } = self;
-        Item { attrs, id, span, vis, ident, kind: kind.into(), tokens }
-    }
-}
-
 /// `extern` qualifier on a function item or function type.
 #[derive(Clone, Copy, Encodable, Decodable, Debug)]
 pub enum Extern {