Implement and-let*

This also includes changes to if-let and when-let.  The single tuple
special case is ambiguous, and binding a symbol to nil is not as
useful as binding it to its value outside the lexical scope of the
binding.  (Bug#28254)
* etc/NEWS: Mention.
* lisp/emacs-lisp/subr-x.el (internal--listify):
(internal--build-binding-value-form): Extend to account for
solitary symbols and (EXPR) items in binding varlist.
(if-let*, when-let*): Nix single tuple case and incumbent
bind-symbol-to-nil behavior.
(and-let*): New macro.
(if-let, when-let): Mark obsolete.  Redefine in terms of if-let*, so
they implicitly gain the new features without breaking existing code.
* test/lisp/emacs-lisp/subr-x-tests.el: Adjust tests for: lack of
single-tuple special case, lack of binding solitary symbols to nil,
and the introduction of uninterned symbols for (EXPR) bindings.  Add
SRFI-2 test suite adapted to Elisp.

lisp/emacs-lisp/subr-x.el

@@ -83,10 +83,15 @@ threading."
   `(internal--thread-argument nil ,@forms))
 
 (defsubst internal--listify (elt)
-  "Wrap ELT in a list if it is not one."
-  (if (not (listp elt))
-      (list elt)
-    elt))
+  "Wrap ELT in a list if it is not one.
+If ELT is of the form ((EXPR)), listify (EXPR) with a dummy symbol."
+  (cond
+   ((symbolp elt) (list elt elt))
+   ((and (null (cdr elt))
+         (let ((form (car elt)))
+           (or (listp form) (atom form))))
+    (list (make-symbol "s") (car elt)))
+   (t elt)))
 
 (defsubst internal--check-binding (binding)
   "Check BINDING is properly formed."
@@ -98,7 +103,10 @@ threading."
 
 (defsubst internal--build-binding-value-form (binding prev-var)
   "Build the conditional value form for BINDING using PREV-VAR."
-  `(,(car binding) (and ,prev-var ,(cadr binding))))
+  (let ((var (car binding)))
+    (if (and (null (cdr binding)) (atom (car binding)) (not (symbolp (car binding))))
+        `(,var (and ,prev-var ,var))
+      `(,var (and ,prev-var ,(cadr binding))))))
 
 (defun internal--build-binding (binding prev-var)
   "Check and build a single BINDING with PREV-VAR."
@@ -117,44 +125,68 @@ threading."
                 binding))
             bindings)))
 
-(defmacro if-let* (bindings then &rest else)
+(defmacro if-let* (varlist then &rest else)
   "Bind variables according to VARLIST and eval THEN or ELSE.
-Each binding is evaluated in turn with `let*', and evaluation
-stops if a binding value is nil.  If all are non-nil, the value
-of THEN is returned, or the last form in ELSE is returned.
-Each element of VARLIST is a symbol (which is bound to nil)
-or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
-In the special case you only want to bind a single value,
-VARLIST can just be a plain tuple.
-\n(fn VARLIST THEN ELSE...)"
+Each binding is evaluated in turn, and evaluation stops if a
+binding value is nil.  If all are non-nil, the value of THEN is
+returned, or the last form in ELSE is returned.
+
+Each element of VARLIST is a list (SYMBOL VALUEFORM) which binds
+SYMBOL to the value of VALUEFORM).
+An element can additionally be of the form (VALUEFORM), which is
+evaluated and checked for nil."
   (declare (indent 2)
-           (debug ([&or (&rest [&or symbolp (symbolp form)]) (symbolp form)]
+           (debug ((&rest [&or symbolp (symbolp form) (sexp)])
                    form body)))
-  (when (and (<= (length bindings) 2)
-             (not (listp (car bindings))))
-    ;; Adjust the single binding case
-    (setq bindings (list bindings)))
-  `(let* ,(internal--build-bindings bindings)
-     (if ,(car (internal--listify (car (last bindings))))
-         ,then
-       ,@else)))
+  (if varlist
+      `(let* ,(setq varlist (internal--build-bindings varlist))
+         (if ,(caar (last varlist))
+             ,then
+           ,@else))
+    `(let* () ,@else)))
 
-(defmacro when-let* (bindings &rest body)
+(defmacro when-let* (varlist &rest body)
   "Bind variables according to VARLIST and conditionally eval BODY.
-Each binding is evaluated in turn with `let*', and evaluation
-stops if a binding value is nil.  If all are non-nil, the value
-of the last form in BODY is returned.
-Each element of VARLIST is a symbol (which is bound to nil)
-or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
-In the special case you only want to bind a single value,
-VARLIST can just be a plain tuple.
-\n(fn VARLIST BODY...)"
-  (declare (indent 1) (debug if-let))
-  (list 'if-let bindings (macroexp-progn body)))
+Each binding is evaluated in turn, and evaluation stops if a
+binding value is nil.  If all are non-nil, the value of the last
+form in BODY is returned.
 
-(defalias 'if-let 'if-let*)
-(defalias 'when-let 'when-let*)
-(defalias 'and-let* 'when-let*)
+VARLIST is the same as in `if-let*'."
+  (declare (indent 1) (debug if-let*))
+  (list 'if-let* varlist (macroexp-progn body)))
+
+(defmacro and-let* (varlist &rest body)
+  "Bind variables according to VARLIST and conditionally eval BODY.
+Like `when-let*', except if BODY is empty and all the bindings
+are non-nil, then the result is non-nil."
+  (declare (indent 1) (debug when-let*))
+  (let (res)
+    (if varlist
+        `(let* ,(setq varlist (internal--build-bindings varlist))
+           (if ,(setq res (caar (last varlist)))
+               ,@(or body `(,res))))
+      `(let* () ,@(or body '(t))))))
+
+(defmacro if-let (spec then &rest else)
+  "Bind variables according to SPEC and eval THEN or ELSE.
+Like `if-let*' except SPEC can have the form (SYMBOL VALUEFORM)."
+  (declare (indent 2)
+           (debug ([&or (&rest [&or symbolp (symbolp form) (sexp)])
+                        (symbolp form)]
+                   form body))
+           (obsolete "use `if-let*' instead." "26.1"))
+  (when (and (<= (length spec) 2)
+             (not (listp (car spec))))
+    ;; Adjust the single binding case
+    (setq spec (list spec)))
+  (list 'if-let* spec then (macroexp-progn else)))
+
+(defmacro when-let (spec &rest body)
+  "Bind variables according to SPEC and conditionally eval BODY.
+Like `when-let*' except SPEC can have the form (SYMBOL VALUEFORM)."
+  (declare (indent 1) (debug if-let)
+           (obsolete "use `when-let*' instead." "26.1"))
+  (list 'if-let spec (macroexp-progn body)))
 
 (defsubst hash-table-empty-p (hash-table)
   "Check whether HASH-TABLE is empty (has 0 elements)."