transient--insert-suffix: Do not signal error by default

In [1: 8daf989] we started to signal an error instead of merely
showing a message on failure.  That was a mistake, see #374.
Add new option `transient-error-on-insert-failure', so users can
opt-in to fatal errors for minor issues.

1: 2025-03-24 8daf989
   transient--insert-suffix: Signal error if location is invalid

Author: tarsius

docs/transient.org

@@ -899,6 +899,16 @@ that tree are not objects but have the form {{{codevar((LEVEL CLASS PLIST))}}},
   This function edits the suffix or group at {{{var(LOC)}}} in {{{var(PREFIX)}}}, by setting
   the {{{var(PROP)}}} of its plist to {{{var(VALUE)}}}.
 
+Most of these functions do not signal an error if they cannot perform
+the requested modification.  The functions that insert new suffixes
+show a warning if {{{var(LOC)}}} cannot be found in {{{var(PREFIX,)}}} without signaling an
+error.  The reason for doing it like this is that establishing a key
+binding (and that is what we essentially are trying to do here) should
+not prevent the rest of the configuration from loading.  Among these
+functions only ~transient-get-suffix~ and ~transient-suffix-put~ signal
+an error by default.  If you really want the insert functions to also
+signal an error, set ~transient-error-on-insert-failure~ to ~t~.
+
 * Defining New Commands
 ** Technical Introduction
 

docs/transient.texi

@@ -1055,6 +1055,16 @@ This function edits the suffix or group at @var{LOC} in @var{PREFIX}, by setting
 the @var{PROP} of its plist to @var{VALUE}.
 @end defun
 
+Most of these functions do not signal an error if they cannot perform
+the requested modification.  The functions that insert new suffixes
+show a warning if @var{LOC} cannot be found in @var{PREFIX} without signaling an
+error.  The reason for doing it like this is that establishing a key
+binding (and that is what we essentially are trying to do here) should
+not prevent the rest of the configuration from loading.  Among these
+functions only @code{transient-get-suffix} and @code{transient-suffix-put} signal
+an error by default.  If you really want the insert functions to also
+signal an error, set @code{transient-error-on-insert-failure} to @code{t}.
+
 @node Defining New Commands
 @chapter Defining New Commands
 

lisp/transient.el

@@ -432,6 +432,16 @@ used."
   :group 'transient
   :type 'boolean)
 
+(defcustom transient-error-on-insert-failure nil
+  "Whether to signal an error when failing to insert a suffix.
+
+When `transient-insert-suffix' and `transient-append-suffix' fail
+to insert a suffix into an existing prefix, they usually just show
+a warning.  If this is non-nil, they signal an error instead."
+  :package-version '(transient . "0.8.8")
+  :group 'transient
+  :type 'boolean)
+
 (defcustom transient-align-variable-pitch nil
   "Whether to align columns pixel-wise in the popup buffer.
 
@@ -1455,14 +1465,16 @@ Intended for use in a group's `:setup-children' function."
     (setq suf (eval suf t))
     (cond
      ((not mem)
-      (error "Cannot insert %S into %s; %s not found"
-             suffix prefix loc))
+      (funcall (if transient-error-on-insert-failure #'error #'message)
+               "Cannot insert %S into %s; %s not found"
+               suffix prefix loc))
      ((or (and (vectorp suffix) (not (vectorp elt)))
           (and (listp   suffix) (vectorp elt))
           (and (stringp suffix) (vectorp elt)))
-      (error "Cannot place %S into %s at %s; %s"
-             suffix prefix loc
-             "suffixes and groups cannot be siblings"))
+      (funcall (if transient-error-on-insert-failure #'error #'message)
+               "Cannot place %S into %s at %s; %s"
+               suffix prefix loc
+               "suffixes and groups cannot be siblings"))
      (t
       (when-let* (((not (eq keep-other 'always)))
                   (bindingp (listp suf))