** New function 'hash-table-contains-p'.

[minad]

This function returns non-nil if a given key is present in a hash table.

[minad]

Just added in https://github.com/emacs-mirror/emacs/commit/dd0dd87e3aaf3116c400fba858cbe35ced15f04e. The implementation looks unnecessarily inefficient though since it always allocates a new symbol.

(defsubst hash-table-contains-p (key table)
  "Return non-nil if TABLE has an element with KEY."
  (declare (side-effect-free t)
           (important-return-value t))
  (let ((missing (make-symbol "missing")))
    (not (eq (gethash key table missing) missing))))

Maybe the following is a better alternative? It works well enough as long as hash table users do use the private hash-table--missing.

(defconst hash-table--missing (make-symbol "hash-table--missing"))
(defsubst hash-table-contains-p (key table)
  "Return non-nil if TABLE has an element with KEY."
  (declare (side-effect-free t)
           (important-return-value t))
  (not (eq (gethash key table hash-table--missing) hash-table--missing)))

cc @phikal @skangas

[minad]

Is it possible to leverage eval-when-compile?

(defsubst hash-table-contains-p (key table)
  "Return non-nil if TABLE has an element with KEY."
  (declare (side-effect-free t)
           (important-return-value t))
  (let ((missing (eval-when-compile (make-symbol "missing"))))
    (not (eq (gethash key table missing) missing))))

(I lack the understanding of the fine details of uninterned symbols and their behavior during compilation.)

[minad]

Reported upstream as https://debbugs.gnu.org/cgi/bugreport.cgi?bug=78162

[minad]

Done in https://github.com/emacs-compat/compat/commit/cfe89a74a63e8205782b7217ae01145b54a27fcc