Use expr_field in builtin derives and assert that we aren't accessing _

Author: clubby789

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -1578,14 +1578,12 @@ impl<'a> TraitDef<'a> {
                     // `unwrap_or_else` case otherwise the hygiene is wrong and we get
                     // "field `0` of struct `Point` is private" errors on tuple
                     // structs.
-                    let mut field_expr = cx.expr(
+                    let mut field_expr = cx.expr_field(
                         sp,
-                        ast::ExprKind::Field(
-                            selflike_arg.clone(),
-                            struct_field.ident.unwrap_or_else(|| {
-                                Ident::from_str_and_span(&i.to_string(), struct_field.span)
-                            }),
-                        ),
+                        selflike_arg.clone(),
+                        struct_field.ident.unwrap_or_else(|| {
+                            Ident::from_str_and_span(&i.to_string(), struct_field.span)
+                        }),
                     );
                     if is_packed {
                         // In general, fields in packed structs are copied via a

compiler/rustc_expand/src/build.rs

@@ -259,6 +259,7 @@ impl<'a> ExtCtxt<'a> {
     }
 
     pub fn expr_field(&self, span: Span, expr: P<Expr>, field: Ident) -> P<ast::Expr> {
+        debug_assert_ne!(field.name, kw::Underscore);
         self.expr(span, ast::ExprKind::Field(expr, field))
     }