Implement the FTCS_PROMPT sequence for marking the start of the prompt

[zadjii-msft]

Implements the FTCS_PROMPT sequence, OSC 133 ; A ST. In this PR, it's just used to set a simple Prompt mark on the current line, in the same way that the iTerm2 sequence works.

There's rumination in #11000 on how to implement the rest of the FTCS sequences.

This is broken into its own PR at the moment. Quoth j4james:

That should be just as easy, and I've noticed a couple of other terminals that are doing that, so it's not unprecedented. If we don't have any immediate use for the other options, there shouldn't be any harm in ignoring them initially.

And the benefit of going with the more widely supported sequence is that we're more likely to benefit from any shells that have this functionality built in. Otherwise they're forced to try and detect the terminal, which is practically impossible for Windows Terminal. Even iTerm2 supports the OSC 133 sequence, so we'd probably be the only odd one out.

This part of the plumbing is super easy, so I thought it would be valuable to add regardless if we get to the whole of FTCS in 1.15.

• I work here
• Tested manually - in my pwsh $PROFILE:

  function prompt {
    $loc = $($executionContext.SessionState.Path.CurrentLocation);
    $out = "PS $loc$('>' * ($nestedPromptLevel + 1)) ";
    $out += "$([char]27)]9;9;`"$loc`"$([char]07)";
    $out += "$([char]27)]133;A;$([char]7)"; # add the FTCS_PROMPT to the... well, end, but you get the point
    return $out
  }

• See also megathread: Scrollbar Marks #11000

[lhecker]

I don't believe this code requires a string copy. This avoids one:

Suggested change

	const auto action{ parts[0] };
	const auto& action = parts[0];

[DHowett]

I see you prefer he not use the uniform initialization syntax (since you switched it to =) 😉

Suggested change

	const auto action{ parts[0] };
	const auto& action{ parts[0] };

I had to read the rules about const auto& locals and how they extend lifetime -- I still think it's a bad pattern to encourage, but we use it all over the place \_(shrug)_/

[lhecker]

I see you prefer he not use the uniform initialization syntax (since you switched it to =) 😉

The rest of the file uses classic = assignments. I like consistency. 👉👈

Of course you probably already figured out my deepest secret that - entirely unrelated to this - I personally kinda dislike the uniform initialization syntax. 😅 This is what I write:

auto bar = bar_producer();  // good
auto bar{ bar_producer() }; // "bad"
Foo foo{ bar };             // good

Isn't the uniform initialization syntax nothing but a bolted on language hack, because people were afraid of breaking existing code by making round brackets better? I use it because it's very useful, but would you write this if it was an equivalent thing?

auto bar(bar_producer());

I wouldn't and so I don't use curly brackets either, because I consider curly brackets stricter round brackets. In my mind I'm not constructing an object there, I'm assigning an unnamed rvalue a name via variable assignment. And so I write auto result = foo().

I had to read the rules about const auto& locals and how they extend lifetime -- I still think it's a bad pattern to encourage, but we use it all over the place \_(shrug)_/

I think about it differently: Here we "bind to the memory address" of the first item in an array.

[zadjii-msft]

ultimately, Audit will complain:

error C26445: Do not assign gsl::span or std::string_view to a reference. They are cheap to construct and are not owners of the underlying data.

so, const auto action = ... it is

[lhecker]

error C26445: Do not assign gsl::span or std::string_view to a reference.

But passing it as a by-reference argument is fine? smh

[DHowett]

I guess the underlying point is: this is not a string copy, it's a string_view copy ;)

[DHowett]

Minor nits

[DHowett]

My nits are just comment changes? Huh.

[ghost]

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[ghost]

🎉Windows Terminal Preview v1.15.186 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download

[TBBle]

Quick note since this is currently the Windows Terminal bug-tracker example for adding FTCS_PROMPT to one's shell: It seems from the diagram in #11000 and the iTerm docs, that FTCS_PROMPT should be at the start of the prompt.

There also seems to be an extraneous ; after A in the test example. The docs give OSC 133 ; A ST, although I'm having trouble getting it going either way, debug tap shows ␛]133;A␛\ and I have experimental.showMarksOnScrollbar set in my top-level settings.json, but I can't seem to see any actual effect. So I don't know if that extraneous ; is a problem (I added it and it didn't help, so I don't have a known-good state to experiment from).

Edit: I did eventually get it working ("experimental.showMarksOnScrollbar": true in my default profile, I'm sure I'd tried that before... I also added FTCS_COMMAND_START to my prompt, but it still worked after I removed that again) and confirmed it works with and without the extraneous ;, i.e. ␛]133;A;␛\ does also work. (whether it should is an OSC-parser question).