Subject: bug#52459: 28.0.90; prin1-to-string does not escape bidi control characters despite print-escape-control-characters=t

From: Daniel Mendler <[email-masked]>
Date: Sun, 12 Dec 2021 22:11:38 +0100

>> 1. Start emacs -Q
>> 2. Enter the following in the scratch buffer:
>> (let ((print-escape-control-characters t))
>>   (prin1-to-string bidi-directional-controls-chars))
>> 3. Evaluate. The bidi control characters are not escaped despite
>> print-escape-control-characters=t.
>>
>> The bidi characters should probably be treated as control characters
>> since they have the Bidi_Control property according to the Unicode
>> standard.
> 
> print-escape-control-characters is about ASCII control characters, not
> about Unicode fomatting controls.

I see, but this is not a satisfactory answer.

>> If it is undesirable to treat bidi control characters like other control
>> characters it may make sense to introduce another print configuration
>> variable, print-escape-all-control-characters or
>> print-escape-bidi-control-characters?
> 
> I don't think it's desirable.  Those formatting controls have starkly
> different roles that ASCII control characters, and we already have
> features to make them stand out on display.  Moreover, escape
> sequences are not well-defined for codepoints beyond a single byte.
> 
> So I don't think we should do anything here, and we should close the
> bug.

I disagree. I would like to turn this report into a feature request
then. I've observed that multiple packages which escape strings and
print them for debugging or help purposes have difficulties with bidi
characters. All I would like to have is a way to print strings safely,
such that control characters (any kind of control characters, ascii or
unicode) do not affect the output.

Example packages which are affected by this issue are for example the
Helpful package, which provides an enhanced Help buffer. Another package
affected by the issue is the Marginalia package which adds annotations
to `describe-variable` in the minibuffer. The annotations show the
variable value. I would like to print the variable values in a safe way
which does not mess up the display. Instead of "string" these packages
show "str"gnandallthatfollowsisgarbage. How is this supposed to be done?

I propose the addition of an additional variable which configures
prin1-string such that all control characters which affect the display
in special ways are escaped. Is there an alternative approach to achieve
this goal?