Document and test 'let-alist' support for indexing

* etc/NEWS: Announce 'let-alist' support for indexing.
* test/lisp/emacs-lisp/let-alist-tests.el (let-alist-numbers):
Add a test for 'let-alist's support for indexing.
* doc/lispref/lists.texi (Association Lists): Document indexing
with 'let-alist'.  (Bug#66509)

doc/lispref/lists.texi

@@ -1934,6 +1934,19 @@ Nested association lists is supported:
 Nesting @code{let-alist} inside each other is allowed, but the code in
 the inner @code{let-alist} can't access the variables bound by the
 outer @code{let-alist}.
+
+Indexing into lists is also supported:
+
+@lisp
+(setq colors '((rose . red) (lily . (yellow pink))))
+(let-alist colors .lily.1)
+     @result{} pink
+@end lisp
+
+Note that forms like @samp{.0} or @samp{.3} are interpreted as numbers
+rather than as symbols, so they won't be bound to the corresponding
+values in ALIST.
+
 @end defmac
 
 @node Property Lists

etc/NEWS

@@ -2854,6 +2854,12 @@ function 'load-path-filter-cache-directory-files', calling 'load' will
 cache the directories it scans and their files, and the following
 lookups should be faster.
 
++++
+** 'let-alist' supports indexing into lists.
+The macro 'let-alist' now interprets symbols containing numbers as list
+indices.  For example, '.key.0' looks up 'key' in the alist and then
+returns its first element.
+
 ** Lexical binding
 
 ---

test/lisp/emacs-lisp/let-alist-tests.el

@@ -100,4 +100,15 @@ See Bug#24641."
                    `[,(+ .a) ,(+ .a .b .b)])
                  [1 5])))
 
+(ert-deftest let-alist-numbers ()
+  "Check that .num indexes into lists."
+  (should (equal
+           (let-alist
+               '(((a . val1) (b . (nil val2)))
+                 (c . (val3)))
+             (list .0 .0.a .0.b.1 .c.0))
+           ;; .0 is interpreted as a number, so we can't use `let-alist'
+           ;; to do indexing alone.  Everything else works though.
+           '(0.0 val1 val2 val3))))
+
 ;;; let-alist-tests.el ends here