Optimise let and let* whose body is constant or the last variable

Simplify  (let ((X1 E1) ... (Xn En)) Xn)
      =>  (progn E1 ... En)

and       (let* ((X1 E1) ... (Xn En)) Xn)
      =>  (let* ((X1 E1) ... (Xn-1 En-1)) En)

and similarly the case where the body is a constant, extending a
previous optimisation that only applied to the constant nil.
This reduces the number of bound variables, shortens the code, and
enables further optimisations.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-letX): Rewrite using
`pcase` and add the aforementioned transformations.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.

1 files changed, 18 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index ee0f931c192..5aa853c7212 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -509,6 +509,24 @@
                               ((member x '("b" "c")) 2)
                               ((not x) 3)))
             '("a" "b" "c" "d" nil))
+
+    ;; `let' and `let*' optimisations with body being constant or variable
+    (let* (a
+           (b (progn (setq a (cons 1 a)) 2))
+           (c (1+ b))
+           (d (list a c)))
+      d)
+    (let ((a nil))
+      (let ((b (progn (setq a (cons 1 a)) 2))
+            (c (progn (setq a (cons 3 a))))
+            (d (list a)))
+        d))
+    (let* ((_a 1)
+           (_b 2))
+      'z)
+    (let ((_a 1)
+          (_b 2))
+      'z)
     )
   "List of expressions for cross-testing interpreted and compiled code.")