ast: Introduce some traits to get AST node properties generically

And use them to avoid constructing some artificial `Nonterminal` tokens during expansion

compiler/rustc_ast_pretty/src/lib.rs

@@ -1,5 +1,7 @@
+#![feature(associated_type_bounds)]
 #![feature(crate_visibility_modifier)]
 #![feature(box_patterns)]
+#![feature(with_negative_coherence)]
 #![recursion_limit = "256"]
 
 mod helpers;

compiler/rustc_ast_pretty/src/pprust/mod.rs

@@ -4,12 +4,42 @@ mod tests;
 pub mod state;
 pub use state::{print_crate, AnnNode, Comments, PpAnn, PrintState, State};
 
-use rustc_ast as ast;
 use rustc_ast::token::{Nonterminal, Token, TokenKind};
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::{self as ast, AstDeref};
 
 use std::borrow::Cow;
 
+pub trait AstPrettyPrint {
+    fn pretty_print(&self) -> String;
+}
+
+impl<T: AstDeref<Target: AstPrettyPrint>> AstPrettyPrint for T {
+    fn pretty_print(&self) -> String {
+        self.ast_deref().pretty_print()
+    }
+}
+
+macro_rules! impl_ast_pretty_print {
+    ($($T:ty => $method:ident),+ $(,)?) => {
+        $(
+            impl AstPrettyPrint for $T {
+                fn pretty_print(&self) -> String {
+                    State::new().$method(self)
+                }
+            }
+        )+
+    };
+}
+
+impl_ast_pretty_print! {
+    ast::Item => item_to_string,
+    ast::AssocItem => assoc_item_to_string,
+    ast::ForeignItem => foreign_item_to_string,
+    ast::Expr => expr_to_string,
+    ast::Stmt => stmt_to_string,
+}
+
 pub fn nonterminal_to_string(nt: &Nonterminal) -> String {
     State::new().nonterminal_to_string(nt)
 }

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -858,6 +858,14 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         Self::to_string(|s| s.print_item(i))
     }
 
+    fn assoc_item_to_string(&self, i: &ast::AssocItem) -> String {
+        Self::to_string(|s| s.print_assoc_item(i))
+    }
+
+    fn foreign_item_to_string(&self, i: &ast::ForeignItem) -> String {
+        Self::to_string(|s| s.print_foreign_item(i))
+    }
+
     fn generic_params_to_string(&self, generic_params: &[ast::GenericParam]) -> String {
         Self::to_string(|s| s.print_generic_params(generic_params))
     }

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -19,7 +19,7 @@ impl<'a> State<'a> {
         }
     }
 
-    fn print_foreign_item(&mut self, item: &ast::ForeignItem) {
+    crate fn print_foreign_item(&mut self, item: &ast::ForeignItem) {
         let ast::Item { id, span, ident, ref attrs, ref kind, ref vis, tokens: _ } = *item;
         self.ann.pre(self, AnnNode::SubItem(id));
         self.hardbreak_if_not_bol();
@@ -496,7 +496,7 @@ impl<'a> State<'a> {
         }
     }
 
-    fn print_assoc_item(&mut self, item: &ast::AssocItem) {
+    crate fn print_assoc_item(&mut self, item: &ast::AssocItem) {
         let ast::Item { id, span, ident, ref attrs, ref kind, ref vis, tokens: _ } = *item;
         self.ann.pre(self, AnnNode::SubItem(id));
         self.hardbreak_if_not_bol();