cs & ffi/unsafe: change base type of (_bytes o <n>) to _pointer

A base type of `_gcpointer` works ok for traditional Racket as a kind
of "maybe it's GCable, so treat it that way just in case". That
approach doesn't work for Racket CS, where `_gpointer` has to mean
"definitely GCable memory". Although the difference is unfortunate,
making Racket CS base `(_bytes o <n>)` on `_pointer` is consistent
with the way plain `_bytes` is like `_pointer` in Racket CS and like
`_gcpointer` in traditional Racket.

pkgs/racket-doc/scribblings/foreign/pointers.scrbl

@@ -57,8 +57,18 @@ offset is always in bytes.}
 @defproc[(cpointer-gcable? [cptr cpointer?]) boolean?]{
 
 Returns @racket[#t] if @racket[cptr] is treated as a reference to
-memory that is managed by the garbage collector, @racket[#f]
-otherwise.}
+memory that is (assumed to be) managed by the garbage collector,
+@racket[#f] otherwise.
+
+For a pointer based on @racket[_gcpointer] as a result type,
+@racket[cpointer-gcable?] will return @racket[#t]. In the @3m[] and
+@CGC[] variants of Racket, @racket[cpointer-gcable?] will return
+@racket[#f] for a pointer based on @racket[_pointer] as a result type.
+The @CS[] variant is mostly the sane, except that if a pointer is
+extracted using the @racket[_pointer] type from memory allocated as
+@racket['nonatomic], @racket[cpointer-gcable?] will report @racket[#t]
+for the extracted pointer.}
+
 
 @; ----------------------------------------------------------------------
 
@@ -466,12 +476,12 @@ use by such finalizers.}
 @defproc[(make-sized-byte-string [cptr cpointer?] [length exact-nonnegative-integer?]) 
          bytes?]{
 
-Returns a byte string made of the given pointer and the given length.
-No copying is performed. Beware that future implementations of Racket
-may not support this function (in case of a byte string representation
-that combines a size and byte-string content without an indirection).
+Returns a byte string made of the given pointer and the given length
+in the @3m[] and @CGC[] variants of Racket; no copying is performed.
+In the @CS[] variant, the @racket[exn:fail:unsupported] exception is
+raised.
 
-Beware also that the representation of a Racket byte string normally
+Beware that the representation of a Racket byte string normally
 requires a nul terminator at the end of the byte string (after
 @racket[length] bytes), but some functions work with a byte-string
 representation that has no such terminator---notably

pkgs/racket-doc/scribblings/foreign/types.scrbl

@@ -418,11 +418,13 @@ See @secref["foreign:tagged-pointers"] for creating pointer types that
 use these tags for safety. A @racket[#f] value is converted to
 @cpp{NULL} and vice versa.
 
-The address referenced by a @racket[_pointer] value must not refer to
+As a result type, the address referenced by a @racket[_pointer] value must not refer to
 memory managed by the garbage collector (unless the address
 corresponds to a value that supports interior pointers and that is
 otherwise referenced to preserve the value from garbage collection).
 The reference is not traced or updated by the garbage collector.
+As an argument type, @racket[_pointer] works for a reference to either
+GC-managed memory or not.
 
 The @racket[equal?] predicate equates C pointers (including pointers
 for @racket[_gcpointer] and possibly containing an offset) when they
@@ -433,11 +435,13 @@ case the equality rules of the relevant structure types apply.}
 
 @defthing[_gcpointer ctype?]{
 
-Like @racket[_pointer], but for a C pointer value that can refer to memory
-managed by the garbage collector.
+The same as @racket[_pointer] as an argument type, but as a result
+type, @racket[_gcpointer] corresponds to a C pointer value that refers
+to memory managed by the garbage collector.
 
-Although a @racket[_gcpointer] can reference to memory that is not
-managed by the garbage collector, beware of using an address that
+In the @3m[] and @CGC[] variants of Racket, a @racket[_gcpointer] result
+pointer can reference to memory that is not
+managed by the garbage collector, but beware of using an address that
 might eventually become managed by the garbage collector. For example,
 if a reference is created by @racket[malloc] with @racket['raw] and
 released by @racket[free], then the @racket[free] may allow the memory
@@ -445,9 +449,8 @@ formerly occupied by the reference to be used later by the garbage
 collector.
 
 The @racket[cpointer-gcable?] function returns @racket[#t] for a
-cpointer generated via the @racket[_gcpointer] type, while it
-generates @racket[#f] for a cpointer generated via the
-@racket[_cpointer] type.}
+cpointer generated via the @racket[_gcpointer] result type. See
+@racket[cpointer-gcable?] for more information.}
 
 
 @deftogether[(
@@ -1141,10 +1144,13 @@ type; a byte string is passed as @racket[_bytes] without any copying.
 Beware that a Racket byte string is not necessarily nul terminated;
 see also @racket[_bytes/nul-terminated].
 
-In the @3m[] and @CGC[] variants of Racket, a C non-NULL @cpp{char*}
-is converted to a Racket byte string without copying. In the @CS[]
-variant, conversion requires copying to represent a C @cpp{char*}
-result as a Racket byte string. In both cases, the C result must have
+In the @3m[] and @CGC[] variants of Racket, a C non-NULL result value
+is converted to a Racket byte string without copying; the pointer is
+treated as potentially managed by the garbage collector (see
+@racket[_gcpointer] for caveats). In the @CS[] variant of Racket,
+conversion requires copying to represent a C @cpp{char*}
+result as a Racket byte string, and the original pointer is @emph{not}
+treated as managed by the garbage collector. In both cases, the C result must have
 a nul terminator to determine the Racket byte string's length.
 
 A @racket[(_bytes o len-expr)] form is a @tech{custom function type}.
@@ -1182,6 +1188,7 @@ results.
 
 @history[#:added "6.12.0.2"]}
 
+
 @; ------------------------------------------------------------
 
 @section{C Struct Types}

racket/collects/ffi/unsafe.rkt

@@ -1114,6 +1114,13 @@
                  pre:  (x => (vector->cblock x t))
                  post: (x => (cblock->vector x t n)))]))
 
+;; Reflect the difference between 'racket and 'chez-scheme
+;; VMs for `_bytes` in `_bytes*`:
+(define _pointer/maybe-gcable
+  (if (eq? 'racket (system-type 'vm))
+      _gcpointer
+      _pointer))
+
 ;; _bytes or (_bytes o n) is for a memory block represented as a Scheme byte
 ;; string.  _bytes is just like a byte-string, and (_bytes o n) is for
 ;; pre-malloc of the string.  There is no need for other modes: i or io would
@@ -1123,7 +1130,7 @@
 (provide (rename-out [_bytes* _bytes]))
 (define-fun-syntax _bytes*
   (syntax-id-rules (o)
-    [(_ o n) (type: _gcpointer
+    [(_ o n) (type: _pointer/maybe-gcable
               pre:  (make-bytes-argument n)
               ;; post is needed when this is used as a function output type
               post: (x => (receive-bytes-result x n)))]

racket/collects/file/resource.rkt

@@ -268,9 +268,8 @@
 (define (to-utf-16 s)
   (let ([v (malloc _gcpointer)])
     (ptr-set! v _string/utf-16 s)
-    (let ([p (ptr-ref v _gcpointer)])
     (let ([len (* 2 (+ 1 (utf-16-length s)))])
-        (ptr-ref v (_bytes o len))))))
+      (ptr-ref v (_bytes o len)))))
 
 (define (utf-16-length s)
   (for/fold ([len 0]) ([c (in-string s)])