Warn about overly long docstring in lambdascratch/bug-44858

* lisp/emacs-lisp/bytecomp.el
(byte-compile-docstring-length-warn): Warn about overly long
docstring in lambda.  (Bug#44858)

(byte-compile--wide-docstring-p): Improve comment.
* test/lisp/emacs-lisp/bytecomp-tests.el
("warn-wide-docstring-defun.el"): Update to test for the above new
warning.

2 files changed, 9 insertions, 16 deletions

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index a3a370134be..091fba44a59 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1666,7 +1666,10 @@ URLs."
                       (seq "(" (* (not ")")) ")")))
               ")")))
     ""
-    ;; Heuristic: assume these substitutions are of some length N.
+    ;; Heuristic: We assume that these `subsititute-command-keys'
+    ;; substitutions are of some length N (more precisely, N=1).  We
+    ;; can't reliably do these replacements, since the value of the
+    ;; keymaps in general can't be known at compile time.
     (replace-regexp-in-string
      (rx "\\" (or (seq "[" (* (not "]")) "]")))
      (make-string byte-compile--wide-docstring-substitution-len ?x)
@@ -1686,13 +1689,6 @@ value, it will override this variable."
   "Warn if documentation string of FORM is too wide.
 It is too wide if it has any lines longer than the largest of
 `fill-column' and `byte-compile-docstring-max-column'."
-  ;; This has some limitations that it would be nice to fix:
-  ;; 1. We don't try to handle defuns.  It is somewhat tricky to get
-  ;;    it right since `defun' is a macro.  Also, some macros
-  ;;    themselves produce defuns (e.g. `define-derived-mode').
-  ;; 2. We assume that any `subsititute-command-keys' command replacement has a
-  ;;    given length.  We can't reliably do these replacements, since the value
-  ;;    of the keymaps in general can't be known at compile time.
   (when (byte-compile-warning-enabled-p 'docstrings)
     (let ((col (max byte-compile-docstring-max-column fill-column))
           kind name docs)
@@ -1703,12 +1699,10 @@ It is too wide if it has any lines longer than the largest of
          (setq kind (nth 0 form))
          (setq name (nth 1 form))
          (setq docs (nth 3 form)))
-        ;; Here is how one could add lambda's here:
-        ;; ('lambda
-        ;;   (setq kind "")   ; can't be "function", unfortunately
-        ;;   (setq docs (and (stringp (nth 2 form))
-        ;;                   (nth 2 form))))
-        )
+        ('lambda
+          (setq kind "")          ; can't be "function", unfortunately
+          (setq docs (and (stringp (nth 2 form))
+                          (nth 2 form)))))
       (when (and (consp name) (eq (car name) 'quote))
         (setq name (cadr name)))
       (setq name (if name (format " `%s'" name) ""))
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index a76038938f5..4f32e8fcc36 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -892,10 +892,9 @@ byte-compiled.  Run with dynamic binding."
  "warn-wide-docstring-define-obsolete-variable-alias.el"
  "defvaralias .foo. docstring wider than .* characters")
 
-;; TODO: We don't yet issue warnings for defuns.
 (bytecomp--define-warning-file-test
  "warn-wide-docstring-defun.el"
- "wider than .* characters" 'reverse)
+ "wider than .* characters")
 
 (bytecomp--define-warning-file-test
  "warn-wide-docstring-defvar.el"