Shrink Nonterminal.

By heap allocating the argument within `NtPath`, `NtVis`, and `NtStmt`.
This slightly reduces cumulative and peak allocation amounts, most
notably on `deep-vector`.

compiler/rustc_ast/src/attr/mod.rs

@@ -439,7 +439,7 @@ impl MetaItem {
             }
             Some(TokenTree::Token(Token { kind: token::Interpolated(nt), .. })) => match *nt {
                 token::Nonterminal::NtMeta(ref item) => return item.meta(item.path.span),
-                token::Nonterminal::NtPath(ref path) => path.clone(),
+                token::Nonterminal::NtPath(ref path) => (**path).clone(),
                 _ => return None,
             },
             _ => return None,

compiler/rustc_ast/src/mut_visit.rs

@@ -772,7 +772,9 @@ pub fn visit_interpolated<T: MutVisitor>(nt: &mut token::Nonterminal, vis: &mut
         token::NtBlock(block) => vis.visit_block(block),
         token::NtStmt(stmt) => visit_clobber(stmt, |stmt| {
             // See reasoning above.
-            vis.flat_map_stmt(stmt).expect_one("expected visitor to produce exactly one item")
+            stmt.map(|stmt| {
+                vis.flat_map_stmt(stmt).expect_one("expected visitor to produce exactly one item")
+            })
         }),
         token::NtPat(pat) => vis.visit_pat(pat),
         token::NtExpr(expr) => vis.visit_expr(expr),

compiler/rustc_ast/src/token.rs

@@ -668,7 +668,7 @@ impl PartialEq<TokenKind> for Token {
 pub enum Nonterminal {
     NtItem(P<ast::Item>),
     NtBlock(P<ast::Block>),
-    NtStmt(ast::Stmt),
+    NtStmt(P<ast::Stmt>),
     NtPat(P<ast::Pat>),
     NtExpr(P<ast::Expr>),
     NtTy(P<ast::Ty>),
@@ -677,13 +677,13 @@ pub enum Nonterminal {
     NtLiteral(P<ast::Expr>),
     /// Stuff inside brackets for attributes
     NtMeta(P<ast::AttrItem>),
-    NtPath(ast::Path),
-    NtVis(ast::Visibility),
+    NtPath(P<ast::Path>),
+    NtVis(P<ast::Visibility>),
 }
 
 // `Nonterminal` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(Nonterminal, 48);
+rustc_data_structures::static_assert_size!(Nonterminal, 16);
 
 #[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable)]
 pub enum NonterminalKind {