Change approach to handling older Racket w/o new index structs

racket/scribble.rkt

@@ -3,7 +3,8 @@
 
 #lang racket/base
 
-(require racket/contract
+(require (for-syntax racket/base)
+         racket/contract
          racket/format
          racket/match
          racket/promise
@@ -13,28 +14,7 @@
                   tag?
                   content->string)
          scribble/blueboxes
-         (only-in scribble/manual-struct
-                  ;; i.e. Not the newer exported-index-desc*
-                  ;; accessors, which we need to dynamic-require below
-                  ;; when running on Racket <= 8.14.0, else provide
-                  ;; stubs. We can't import them here, too. Note that
-                  ;; except-in errors for items that aren't actually
-                  ;; exported, so we can't use that here.
-                  constructor-index-desc?
-                  exported-index-desc?
-                  exported-index-desc-name
-                  module-path-index-desc?
-                  language-index-desc?
-                  reader-index-desc?
-                  form-index-desc?
-                  procedure-index-desc?
-                  thing-index-desc?
-                  struct-index-desc?
-                  class-index-desc?
-                  interface-index-desc?
-                  mixin-index-desc?
-                  method-index-desc?
-                  exported-index-desc-from-libs)
+         scribble/manual-struct
          scribble/xref
          scribble/tag
          setup/xref
@@ -43,6 +23,14 @@
          "elisp.rkt"
          "util.rkt")
 
+;; Fallbacks when new index structs aren't available, before c. Racket
+;; 8.14.0.6.
+(define-fallbacks scribble/manual-struct
+  [(exported-index-desc*? _) #f]
+  [(exported-index-desc*-extras _) #hasheq()]
+  [(index-desc? _) #f]
+  [(index-desc-extras _) #hasheq()])
+
 (provide binding->path+anchor
          identifier->bluebox
          bluebox-command
@@ -132,20 +120,6 @@
 ;; making _another_ 30K+ list, by returning a thunk for elisp-write
 ;; to call, to do "streaming" writes.
 
-;; When available use new index structs circa Racket 8.14.0.6.
-(define-polyfill (exported-index-desc*? _)
-  #:module scribble/manual-struct
-  #f)
-(define-polyfill (exported-index-desc*-extras _)
-  #:module scribble/manual-struct
-  #hasheq())
-(define-polyfill (index-desc? _)
-  #:module scribble/manual-struct
-  #f)
-(define-polyfill (index-desc-extras _)
-  #:module scribble/manual-struct
-  #hasheq())
-
 (define (hide-desc? desc)
   ;; Don't show doc for constructors; class doc suffices.
   (or (constructor-index-desc? desc)

racket/util.rkt

@@ -4,7 +4,6 @@
 #lang racket/base
 
 (require (for-syntax racket/base)
-         syntax/stx
          syntax/parse/define
          racket/format)
 
@@ -21,7 +20,8 @@
          log-racket-mode-fatal
          time-apply/log
          with-time/log
-         define-polyfill)
+         define-polyfill
+         define-fallbacks)
 
 (define (string->namespace-syntax str)
   (namespace-syntax-introduce
@@ -57,7 +57,7 @@
 (define-simple-macro (with-time/log what e ...+)
   (time-apply/log what (λ () e ...) '()))
 
-;; dynamic-require with backup implementation
+;;; dynamic-require with backup implementation
 
 (define-simple-macro (define-polyfill (id:id arg:expr ...)
                        #:module mod:id
@@ -66,3 +66,25 @@
     (with-handlers ([exn:fail? (λ (_exn)
                                  (λ (arg ...) body ...))])
       (dynamic-require 'mod 'id))))
+
+;;; similar, but defining fallbacks only for items not imported by a
+;;; normal require
+
+;; advantage: Things like go-to definition work as expected in the
+;; normal, non-fallback case. Whereas with define-polyfill, the
+;; identifier _always_ results from the dynamic-require, and go-to def
+;; always goes there.
+
+(define-syntax-parser define-fallback
+  [(_ mod:id (id:id arg:expr ...) body:expr ...+)
+   (if (with-handlers ([exn:fail? (λ _ #f)])
+         (dynamic-require (syntax-e #'mod) (syntax-e #'id)))
+       #'(void)
+       #'(define (id arg ...)
+           body ...))])
+
+(define-syntax-parser define-fallbacks
+  [(_ mod:id [(id:id arg:expr ...) body:expr ...+] ...+)
+   #'(begin
+       (define-fallback mod (id arg ...) body ...) ...)])
+