Remove references to tail arrays in the documentation

* doc/ref/api-data.texi (Vtables, Structure Basics): Update to remove
  references to tail arrays, in preparation for deprecation.

doc/ref/api-data.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000-2004, 2006-2016
+@c Copyright (C)  1996, 1997, 2000-2004, 2006-2017
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -8757,7 +8757,6 @@ records in Guile are implemented with structures.
 * Vtable Contents::
 * Meta-Vtables::
 * Vtable Example::
-* Tail Arrays::
 @end menu
 
 @node Vtables
@@ -8808,8 +8807,7 @@ Scheme level.  This can be used for fields which should only be used
 from C code.
 @end itemize
 
-Here are some examples.  @xref{Tail Arrays}, for information on the
+Here are some examples.
-legacy tail array facility.
 
 @example
 (make-vtable "pw")      ;; one writable field
@@ -8840,12 +8838,11 @@ structure.
 @node Structure Basics
 @subsubsection Structure Basics
 
-This section describes the basic procedures for working with
+This section describes the basic procedures for working with structures.
-structures.  @code{make-struct} creates a structure, and
+@code{make-struct/no-tail} creates a structure, and @code{struct-ref}
-@code{struct-ref} and @code{struct-set!} access its fields.
+and @code{struct-set!} access its fields.
 
-@deffn {Scheme Procedure} make-struct vtable tail-size init @dots{}
+@deffn {Scheme Procedure} make-struct/no-tail vtable init @dots{}
-@deffnx {Scheme Procedure} make-struct/no-tail vtable init @dots{}
 Create a new structure, with layout per the given @var{vtable}
 (@pxref{Vtables}).
 
@@ -8855,25 +8852,22 @@ put values in read-only fields.  If there are fewer @var{init}
 arguments than fields then the defaults are @code{#f} for a Scheme
 field (type @code{p}) or 0 for an uninterpreted field (type @code{u}).
 
-Structures also have the ability to allocate a variable number of
+The name is a bit strange, we admit.  The reason for it is that Guile
-additional cells at the end, at their tails.  However, this legacy
+used to have a @code{make-struct} that took an additional argument;
-@dfn{tail array} facilty is confusing and inefficient, and so we do not
+while we deprecate that old interface, @code{make-struct/no-tail} is the
-recommend it.  @xref{Tail Arrays}, for more on the legacy tail array
+new name for this functionality.
-interface.
 
-Type @code{s} self-reference fields, permission @code{o} opaque
+Type @code{s} self-reference fields and permission @code{o} opaque
-fields, and the count field of a tail array are all ignored for the
+fields are ignored for the @var{init} arguments, ie.@: an argument is
-@var{init} arguments, ie.@: an argument is not consumed by such a
+not consumed by such a field.  An @code{s} is always set to the
-field.  An @code{s} is always set to the structure itself, an @code{o}
+structure itself and an @code{o} is always set to @code{#f} or 0 (with
-is always set to @code{#f} or 0 (with the intention that C code will
+the intention that C code will do something to it later).
-do something to it later), and the tail count is always the given
-@var{tail-size}.
 
 For example,
 
 @example
 (define v (make-vtable "prpwpw"))
-(define s (make-struct v 0 123 "abc" 456))
+(define s (make-struct/no-tail v 123 "abc" 456))
 (struct-ref s 0) @result{} 123
 (struct-ref s 1) @result{} "abc"
 @end example
@@ -8886,6 +8880,8 @@ There are a few ways to make structures from C.  @code{scm_make_struct}
 takes a list, @code{scm_c_make_struct} takes variable arguments
 terminated with SCM_UNDEFINED, and @code{scm_c_make_structv} takes a
 packed array.
+
+For all of these, @var{tail_size} should be zero (as a SCM value).
 @end deftypefn
 
 @deffn {Scheme Procedure} struct? obj
@@ -9197,53 +9193,6 @@ cases, the records facility is usually sufficient.  But sometimes you
 need to make new kinds of data abstractions, and for that purpose,
 structs are here.
 
-@node Tail Arrays
-@subsubsection Tail Arrays
-
-Guile's structures have a facility whereby each instance of a vtable can
-contain a variable-length tail array of values.  The length of the tail
-array is stored in the structure.  This facility was originally intended
-to allow C code to expose raw C structures with word-sized tail arrays
-to Scheme.
-
-However, the tail array facility is confusing and doesn't work very
-well.  It is very rarely used, but it insinuates itself into all
-invocations of @code{make-struct}.  For this reason the clumsily-named
-@code{make-struct/no-tail} procedure can actually be more elegant in
-actual use, because it doesn't have a random @code{0} argument stuck in
-the middle.
-
-Tail arrays also inhibit optimization by allowing instances to affect
-their shapes.  In the absence of tail arrays, all instances of a given
-vtable have the same number and kinds of fields.  This uniformity can be
-exploited by the runtime and the optimizer.  The presence of tail arrays
-make some of these optimizations more difficult.
-
-Finally, the tail array facility is ad-hoc and does not compose with the
-rest of Guile.  If a Guile user wants an array with user-specified
-length, it's best to use a vector.  It is more clear in the code, and
-the standard optimization techniques will do a good job with it.
-
-That said, we should mention some details about the interface.  A vtable
-that has tail array has upper-case permission descriptors: @code{W},
-@code{R} or @code{O}, correspoding to tail arrays of writable,
-read-only, or opaque elements.  A tail array permission descriptor may
-only appear in the last element of a vtable layout.
-
-For exampple, @samp{pW} indicates a tail of writable Scheme-valued
-fields.  The @samp{pW} field itself holds the tail size, and the tail
-fields come after it.
-
-@example
-(define v (make-vtable "prpW")) ;; one fixed then a tail array
-(define s (make-struct v 6 "fixed field" 'x 'y))
-(struct-ref s 0) @result{} "fixed field"
-(struct-ref s 1) @result{} 2    ;; tail size
-(struct-ref s 2) @result{} x    ;; tail array ...
-(struct-ref s 3) @result{} y
-(struct-ref s 4) @result{} #f
-@end example
-
 
 @node Dictionary Types
 @subsection Dictionary Types