Add 'read-extended-command-predicate'

* doc/emacs/m-x.texi (M-x): Document it.
* doc/lispref/commands.texi (Interactive Call): Document it further.

* lisp/simple.el (read-extended-command-predicate): New user option.
(read-extended-command-predicate): Use it.
(completion-in-mode-p): New function (the default predicate).

2 files changed, 14 insertions, 0 deletions

diff --git a/doc/emacs/m-x.texi b/doc/emacs/m-x.texi
index 865220fb218..689125e7b4a 100644
--- a/doc/emacs/m-x.texi
+++ b/doc/emacs/m-x.texi
@@ -94,3 +94,8 @@ the command is followed by arguments.
   @kbd{M-x} works by running the command
 @code{execute-extended-command}, which is responsible for reading the
 name of another command and invoking it.
+
+@vindex read-extended-command-predicate
+  This command heeds the @code{read-extended-command-predicate}
+variable, which will (by default) filter out commands that are not
+applicable to the current major mode (or enabled minor modes).
diff --git a/doc/lispref/commands.texi b/doc/lispref/commands.texi
index d60745a825b..b3bcdf35c9f 100644
--- a/doc/lispref/commands.texi
+++ b/doc/lispref/commands.texi
@@ -773,6 +773,15 @@ part of the prompt.
      @result{} t
 @end group
 @end example
+
+@vindex read-extended-command-predicate
+This command heeds the @code{read-extended-command-predicate}
+variable, which will (by default) filter out commands that are not
+applicable to the current major mode (or enabled minor modes).
+@code{read-extended-command-predicate} will be called with two
+parameters: The symbol that is to be included or not, and the current
+buffer.  If should return non-@code{nil} if the command is to be
+included when completing.
 @end deffn
 
 @node Distinguish Interactive