xml/xexpr: pcdata structs are not X-expressions

Fix `xexpr/c`, `xexpr?`, `correct-xexpr?`, and `validate-xexpr`
to reject instances of the `pcdata` struct, consistent with
the documentation and other parts of the implementation.

Trying to use `pcdata` structs as X-expressions was already
very broken, e.g.:

```
$ racket
Welcome to Racket v8.17 [cs].
> (require xml)
> (xexpr->string (pcdata #f #f "pcdata"))
""
```

Author: LiberalArtist

pkgs/racket-doc/xml/xml.scrbl

@@ -173,12 +173,21 @@ A @racket[valid-char?] represents a numeric entity. For example,
 
 A @racket[_cdata] is an instance of the @racket[cdata] structure type,
 and a @racket[_misc] is an instance of the @racket[comment] or
-@racket[p-i] structure types.}
+@racket[p-i] structure types.
+
+@history[
+ #:changed "8.17.0.5"
+ @elem{Fixed a bug that had accepted instances of the @racket[pcdata] structure type.}
+]}
 
 @defthing[xexpr/c contract?]{
  A contract that is like @racket[xexpr?] except produces a better error
  message when the value is not an @tech{X-expression}.
-}
+
+@history[
+ #:changed "8.17.0.5"
+ @elem{Fixed a bug that had accepted instances of the @racket[pcdata] structure type.}
+]}
 
 @; ----------------------------------------------------------------------
 
@@ -400,7 +409,11 @@ is the part of @racket[v] that caused the exception.
   (validate-xexpr '(doc () "over " (em () "9000") "!"))
   (validate-xexpr #\newline)
 ]
-}
+
+@history[
+ #:changed "8.17.0.5"
+ @elem{Fixed a bug that had accepted instances of the @racket[pcdata] structure type.}
+]}
 
 @defproc[(correct-xexpr? [v any/c]
                          [success-k (-> any/c)]
@@ -411,7 +424,12 @@ Like @racket[validate-xexpr], except that @racket[success-k] is called
 on each valid leaf, and @racket[fail-k] is called on invalid leaves;
 the @racket[fail-k] may return a value instead of raising an exception
 or otherwise escaping. Results from the leaves are combined with
-@racket[and] to arrive at the final result.}
+@racket[and] to arrive at the final result.
+
+@history[
+ #:changed "8.17.0.5"
+ @elem{Fixed a bug that had accepted instances of the @racket[pcdata] structure type.}
+]}
 
 @; ----------------------------------------------------------------------
 

pkgs/racket-test/tests/xml/test.rkt

@@ -153,7 +153,8 @@ END
      (test-not-xexpr? '(a ((b)) c))
      (test-xexpr? (make-cdata #f #f "unquoted <b>"))
      (test-xexpr? (make-comment "Comment!"))
-     (test-xexpr? (make-pcdata #f #f "quoted <b>"))
+     (test-xexpr? "quoted <b>")
+     (test-not-xexpr? (make-pcdata #f #f "quoted <b>"))
 
      (test-not-xexpr? (list 'a (list (list 'href)) "content"))
 
@@ -922,7 +923,7 @@ XML
          (test-validate-xexpr 64)
          (test-validate-xexpr 'nbsp)
          (test-validate-xexpr "string")
-         (test-validate-xexpr (make-pcdata #f #f "pcdata"))
+         (test-validate-xexpr/exn (make-pcdata #f #f "pcdata") (make-pcdata #f #f "pcdata"))
          (test-validate-xexpr (make-cdata #f #f "cdata"))
          (test-validate-xexpr (make-comment "comment"))
          (test-validate-xexpr (make-p-i #f #f 's1 "s2"))

racket/collects/xml/private/core.rkt

@@ -18,9 +18,6 @@
 ; also represents XMLDecl
 (define-struct (p-i source) (target-name instruction) #:transparent)
 
-; Pcdata = (make-pcdata Location Location String)
-(define-struct (pcdata source) (string) #:transparent)
-
 ; Cdata = (make-cdata Location Location String)
 (define-struct (cdata source) (string) #:transparent)
 

racket/collects/xml/private/structures.rkt

@@ -37,6 +37,9 @@
 ; Misc = Comment
 ;      |  Processing-instruction
 
+; Pcdata = (make-pcdata Location Location String)
+(define-struct (pcdata source) (string) #:transparent)
+
 ; Entity = (make-entity Location Location (U Nat Symbol))
 (define-struct (entity source) (text) #:transparent)
 

racket/collects/xml/private/xexpr-core.rkt

@@ -24,12 +24,6 @@
 ;;        |  Cdata
 ;; Attribute-srep ::= (list Symbol String)
 
-;; sorting is no longer necessary, since xt3d uses xml->zxexpr, which sorts.
-
-(define xexpr-datum/c
-  (or/c string? symbol? valid-char?
-        comment? p-i? cdata? pcdata?))
-
 (define (xexpr? x)
   (not (incorrect-xexpr? x)))
 
@@ -86,7 +80,6 @@
     [(comment? x) #f]
     [(p-i? x) #f]
     [(cdata? x) #f]
-    [(pcdata? x) #f]
     [(list? x)     
      (cond [(null? x) 
             (make-exn:invalid-xexpr

racket/collects/xml/private/xexpr.rkt

@@ -71,7 +71,7 @@
     [(string? x) (make-pcdata 'racket 'racket x)]
     [(or (symbol? x) (exact-nonnegative-integer? x))
      (make-entity 'racket 'racket x)]
-    [(or (comment? x) (p-i? x) (cdata? x) (pcdata? x)) x]
+    [(or (comment? x) (p-i? x) (cdata? x)) x]
     [else ;(error 'xexpr->xml "malformed xexpr ~e" x)
      x]))