>> +@defvar safe-local-variable-directories
>> +This is a list of directories where local variables are always enabled.
>> +Directory-local variables loaded from these directories, such as the
>> +variables in @file{.dir-locals.el}, will be enabled even if they are
>> +risky.
>> +@end defvar
>
>This variable should also be documented in the Emacs user manual, not
>only in the ELisp Reference manual -- it's a user option, and a very
>important one at that.

Added to the manual in custom.texi "Safe File Variables" subsection.

>> ++++
>> +** New variable 'safe-local-variable-directories'.
>> +This variable is used to to permanently trust directories containing
>> +risky directory-local variables.
>
>I would rephrase:
>
>  This variable names directories in which Emacs will treat all
>  directory-local variables as safe.

Fixed in attached patch.

>>  ALL-VARS is the list of all variables to be set up.
>> @@ -3734,7 +3744,9 @@ n  -- to ignore the local variables list.")
>>  !  -- to apply the local variables list, and permanently mark these
>>        values (*) as safe (in the future, they will be set automatically.)
>>  i  -- to ignore the local variables list, and permanently mark these
>> -      values (*) as ignored\n\n")
>> +      values (*) as ignored
>> ++  -- to apply the local variables list, and permanently trust all
>> +      directory-local variables in this directory\n\n")
>
>I would remove the "permanently" part, it would just confuse here.

Fixed.

>> @@ -3908,6 +3924,7 @@ DIR-NAME is the name of the associated directory.  
>> Otherwise it is nil."
>>                 (null unsafe-vars)
>>                 (null risky-vars))
>>            (memq enable-local-variables '(:all :safe))
>> +             (member dir-name safe-local-variable-directories)
>
>If you use 'member' for this test, then (a) the documentation of
>safe-local-variable-directories should explicitly say that the
>directories in the list must be in full absolute form, and (b) we
>should consider the various issues with file names that are not
>'equal' as strings, but still name the same directory, such as
>letter-case differences on case-insensitive filesystems.  And what
>about equality of "foo/" "and "foo"?

Clarified the documentation. The directory path requires a trailing
separator, and is case-sensitive regardless of the filesystem (tested
on vFAT).

>Also, is 'dir-name' above guaranteed to be a fully-expanded absolute
>file name?

Yes. For TRAMP connections it's a string with text properties, but it's
the same equality-wise.

#("/ssh:user:/home/user/src/" 5 6 (tramp-default t))

>> +(ert-deftest files-tests-safe-local-variable-directories ()
>> +  ;; safe-local-variable-directories should be risky,
>> +  ;; so use it as an arbitrary risky variable.
>> +  (let ((test-alist '((safe-local-variable-directories . "some_val")))
>> +        (fakedir "test1/test2")
>> +        (enable-local-eval t))
>> +    (with-temp-buffer
>> +      (setq safe-local-variable-directories (list fakedir))
>
>The test should use absolute directory names for directories you put
>into safe-local-variable-directories.

Fixed.