if-let*/when-let*/and-let*: Don't recommend (VALUEFORM) form

* doc/lispref/control.texi (Conditionals):
* lisp/subr.el (if-let*): Document '(_ VALUEFORM)' instead of
'(VALUEFORM)'.

doc/lispref/control.texi

@@ -317,7 +317,7 @@ following way instead:
   (do-something result2))
 @end example
 
-There's a number of variations on this theme, and they're briefly
+There are a number of variations on this theme, and they're briefly
 described below.
 
 @defmac if-let* varlist then-form else-forms...
@@ -328,10 +328,9 @@ Evaluate each binding in @var{varlist}, stopping if a binding value is
 Each element of @code{varlist} has the form @w{@code{(@var{symbol}
 @var{value-form})}}: @var{value-form} is evaluated and @var{symbol} is
 locally bound to the result.  Bindings are sequential, as in @code{let*}
-(@pxref{Local Variables}).  As a special case, @var{symbol} can be
+(@pxref{Local Variables}).  If only the test result of @var{value-form}
-omitted if only the test result of @var{value-form} is of interest:
+is of interest, use @code{_} for @var{symbol}.  Then @var{value-form} is
-@var{value-form} is evaluated and checked for @code{nil}, but its value
+evaluated and checked for @code{nil}, but its value is not bound.
-is not bound.
 @end defmac
 
 @defmac when-let* varlist then-forms...
@@ -343,8 +342,8 @@ form in @var{then-forms}.
 @code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
 in which @var{value-form} is evaluated and @var{symbol} is locally bound
 to the result.  Bindings are sequential, as in @code{let*} (@pxref{Local
-Variables}).  As a special case, @var{symbol} can be omitted if only the
+Variables}).  If only the test result of @var{value-form}
-test result of @var{value-form} is of interest: @var{value-form} is
+is of interest, use @code{_} for @var{symbol}.  Then @var{value-form} is
 evaluated and checked for @code{nil}, but its value is not bound.
 @end defmac
 
@@ -358,8 +357,8 @@ the value of the last binding.
 @code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
 in which @var{value-form} is evaluated and @var{symbol} is locally bound
 to the result.  Bindings are sequential, as in @code{let*} (@pxref{Local
-Variables}).  As a special case, @var{symbol} can be omitted if only the
+Variables}).  If only the test result of @var{value-form} is of
-test result of @var{value-form} is of interest: @var{value-form} is
+interest, use @code{_} for @var{symbol}.  Then @var{value-form} is
 evaluated and checked for @code{nil}, but its value is not bound.
 @end defmac
 
@@ -382,8 +381,8 @@ established anew.
 @code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
 in which @var{value-form} is evaluated and @var{symbol} is locally bound
 to the result.  Bindings are sequential, as in @code{let*} (@pxref{Local
-Variables}).  As a special case, @var{symbol} can be omitted if only the
+Variables}).  If only the test result of @var{value-form} is of
-test result of @var{value-form} is of interest: @var{value-form} is
+interest, use @code{_} for @var{symbol}.  Then @var{value-form} is
 evaluated and checked for @code{nil}, but its value is not bound.
 
 The return value of @code{while-let} is always @code{nil}.

lisp/subr.el

@@ -2632,6 +2632,9 @@ Affects only hooks run in the current buffer."
                 binding))
             bindings)))
 
+;; FIXME: Once Emacs 29 is ancient history we can consider a
+;; byte-compiler warning.  This is because Emacs 29 and older will warn
+;; about unused variables with (_ VALUEFORM).
 (defmacro if-let* (varlist then &rest else)
   "Bind variables according to VARLIST and evaluate THEN or ELSE.
 Evaluate each binding in turn, as in `let*', stopping if a
@@ -2639,12 +2642,18 @@ binding value is nil.  If all are non-nil return the value of
 THEN, otherwise the value of the last form in ELSE, or nil if
 there are none.
 
-Each element of VARLIST is a list (SYMBOL VALUEFORM) that binds
+Each element of VARLIST is a list (SYMBOL VALUEFORM) that binds SYMBOL
-SYMBOL to the value of VALUEFORM.  An element can additionally be
+to the value of VALUEFORM.  If only the test result is of interest, use
-of the form (VALUEFORM), which is evaluated and checked for nil;
+`_' as SYMBOL, i.e. (_ VALUEFORM), in which case VALUEFORM is evaluated
-i.e. SYMBOL can be omitted if only the test result is of
+and checked for nil but the result is not bound.
-interest.  It can also be of the form SYMBOL, then the binding of
+An element of VARLIST can also be of the form SYMBOL, in which case the
-SYMBOL is checked for nil."
+binding of SYMBOL is checked for nil, only.
+
+An older form for entries of VARLIST is also supported, where SYMBOL is
+omitted, i.e. (VALUEFORM).  This means the same as (_ VALUEFORM).
+This form is not recommended because many programmers find it
+significantly less readable.  A future release of Emacs may introduce a
+byte-compiler warning for uses of (VALUEFORM) in VARLIST."
   (declare (indent 2)
            (debug ((&rest [&or symbolp (symbolp form) (form)])
                    body)))