Added ~mixin, which checs that its argument is indeed a mixin. Mixins can still be used by themeselves, but there's always the risk that they'll be seen as a pattern variable, if for some reason their identifier is not bound to a mixin (forgot to export, wrong scopes…)

main.rkt

@@ -6,6 +6,7 @@
          "private/post.rkt"
          "private/global.rkt"
          "private/optional.rkt"
+         "private/mixin.rkt"
          (for-template "private/define-syntax+simple-api.rkt"))
 
 (provide #;define-splicing-syntax-class-with-eh-mixins
@@ -14,6 +15,7 @@
          (expander-out eh-mixin)
          ~seq-no-order
          ~no-order
+         ~mixin
          ~post-check
          ~post-fail
          ~nop

private/define-syntax+simple-api.rkt

@@ -63,7 +63,7 @@
               (define (name stx2)
                 (syntax-parameterize ([stx (make-rename-transformer #'stx2)])
                   (syntax-parse stx2
-                    [(name . colon-stxclass) . body])))
+                    [(_ . colon-stxclass) . body])))
               (define (private-simple-api stx/arg attr-name/arg …)
                 (syntax-parameterize ([stx (make-rename-transformer #'stx/arg)])
                   (syntax-parse #'nothing

private/mixin.rkt

@@ -0,0 +1,11 @@
+#lang racket
+
+(require "no-order.rkt"
+         (for-syntax syntax/parse))
+
+(provide ~mixin)
+
+(define-eh-mixin-expander ~mixin
+  (syntax-parser
+    [(_ (~var mixin (static eh-mixin-expander? "an eh-mixin expander")))
+     #'(mixin)]))

test/test-structure-options.rkt

@@ -73,11 +73,11 @@
                 {structure-kw-predicate-mixin}
                 {structure-kw-fields-mixin}}))
 
-;; ---------
-
 (define-splicing-syntax-class structure-kws
   (pattern {~seq-no-order {structure-kw-all-mixin}}))
 
+;; ---------
+
 (check-equal? (syntax-parse #'(#:instance #:? p)
                 [(:structure-kws)
                  (list* (attribute instance)
@@ -163,4 +163,4 @@ builder-style field declarations"
 ;;      body]))
 ;;
 ;; For speed, we could just copy the whole implementation in real-macro-name's
-;; definition, instead of calling simplified-macro-implementation.
+;; definition, instead of calling simplified-macro-implementation.