Intentionally control when a space is appended for native completions

[epage]

A trailing space signifies that there is nothing else to complete for this argument and the next argument should be started.

Cases where a space is appropriate

• long flags
• values

Cases where a space is not appropriate

• options that allow =
• short flags
• delimiter-separated values

Open questions

• When there is only one option left for a completion and there is "something more", like --opt completing for --option=value, instead of completing to --option should we offer every --option=value possible?
• Can we do that for --option value as well?
• Should we prefer --option=value or --option value? (requires_equals and num_args being more than 1 are times when there isn't a choice)
• When the "something more" is optional (using num_args), should we offer with space and without?

Note: we might be able to defer some of these open questions as "parity with stable, static clap_complete" is the minimum bar

[shannmu]

A trailing space design

What do we support now

• subcommand
  • itself
  • aliases
• positional argument
• optional flags
  • long flags
  • short flags
• values
  • value hint, e.g. filepath
  • custom possible values

Cases where we should append the trailing space

• subcommand
  • itself
  • long alias(blocked on: Support subcommand flags in native completion engine #5284)
• positional argument
• long flags without Set or Append action
• long flags with Set or Append action & without requires_equals
  • I think that if the long flags does not need the =, we could append the trailing space for the completion to show users that this long flags is complete.

Cases where we should not append the trailing space

• subcommand
  • short alias(blocked on: Support subcommand flags in native completion engine #5284)
• short flags
• long flags with Set or Append action & with requires_equals(blocked on: Support is_require_equal_set in native completions #3923)
  • btw, i think that we could complete the long flags with format format!("--{}=", long_flag) rather than format!("--{}", long_flag) to show the users that this long flag needs = and value.

Cases that depend on the context

values

Note

What is the context when completing the values of flag
At the code level, it refers to the various settings of Arg.

• num_args
• value_delimiter
• trailing_var_arg refers to num_args

At the meaning of value level

• filepath
• custom possible values

• not append the trailing space
  • number of values are not matched the number of num_arg(blocked on: Support multiple values in native completions #3921)
    • In this way, users can better control whether to complete the next value.
    • Also when the number of values matches the num_args, we could append a trailing space to show user that the multi-values of flag are completed.
  • number of values matches the number of num_arg & value is a value hint(Blocked on: Support multiple values in native completions #3921)
    • We cannot be sure whether the user wants to use this filepath or create a new file based on this filepath. Other types of value hint are the same as filepath hint, because we cannot be sure whether the hint is complete.
  • delimiter-separated values(blocked on: Support delimited values for native completions #3922)
    • It's abvious that we can't known the end of the delimiter-separated values.
      NOTE: It seems that delimiter-separated values must be one raw_arg in command line. It means if the delimiter is space, we should pass the delimiter-separated values with the format "va1 val2 val3" rather than val1 val2 val3.
• append the trailing space
  • number of values matches the number of num_arg & value is custom possible value((blocked on: Support multiple values in native completions #3921))

BTW, when checking the completion for the values of the flag, I found that we might support ignore_case for possible values if the arg is set with ArgSettings::IgnoreCase. This will also provide a great user experience.

Open questions

When there is only one option left for a completion and there is "something more", like --opt completing for --option=value, instead of completing to --option should we offer every --option=value possible?

I think that we should not offer every --option=value possible. For example, if the completion of value comes from clap::ValueHint::FilePath, it may generate too many completions, which makes user confused. It would be better to let the user enter some leading prompt characters.

When the "something more" is optional (using num_args), should we offer with space and without

I think this question was mentioned above.

[shannmu]

Only the cases of the values is relatively complex and are blocked by other issues. The cases for others are relatively certain. We could get benefit from 67e31af, and add has_space to CompletionCandidate.

[shannmu]

Looking forward to your thoughts. Your feedback is really important!

[epage]

number of values are not matched the number of num_arg

This isn't a question of the number being matched as num_args only applies to 1 2 3 and not 1,2,3.

The way num_args ties into this is when no arguments are possible, like num_args(0..=1) for --color allowing --color to be a shortcut for --color=always.

So this is a distinct case from the others.

[epage]

I think that we should not offer every --option=value possible. For example, if the completion of value comes from clap::ValueHint::FilePath, it may generate too many completions, which makes user confused. It would be better to let the user enter some leading prompt characters.

This is a question of framing.

When we have

Arg::new("option")
    .value_name("value")
    .value_hint(ValueHint::FileType)
    .requires_equals(true)

If the user has foo --opt\t, do we view completion as being only for the current token or do we view completion as for the end result?

@ModProg would be good to get your input on this, if possible.

[epage]

add has_space to CompletionCandidate.

I think I'd prefer for this to be more descriptive, like incomplete. Depending on how we want to handle "maybe incomplete", an Option<bool> or an enum might be needed (where Incomplete is just one variant)

[epage]

Only the cases of the values is relatively complex and are blocked by other issues.

As mentioned in #5587 (comment), #3921 is not a blocker for this. #3922 is a soft-blocker. We could close this out for what is possible now and push responsibility for that case onto #3922 but I think I'd prefer to track this as a cohesive whole so we focus on what the entire user experience around this is.

[ModProg]

One thing to check is verify which shells actually support different behavior here, e.g., fish won't honer us providing the space or not, but uses a list of characters to decide (fish-shell/fish-shell#7832).

The list:
https://github.com/fish-shell/fish-shell/blob/ff72080aac0263048b2a9906c34736b40994f6c2/src/reader.rs#L5589
'/', '@', ':', '.', ',', '-' or a '='

In my testing, a trailing slash in the completion was actually escaped: example world\

So e.g. for an option that is numargs=0..=1, at least for the fish shell we should provide at least both --opt and --opt=.

I think that we should not offer every --option=value possible.

We could make this depend on two factors, number of possible values and whether there are other options left.

--opt\t:
--optimal, --option, --option=

--optio\t:
--option, --option=a, --option=b, --option=c

[shannmu]

I haven't found a way to handle fish's trailing space yet.
As for bash, zsh, elvish, and powershell, we can close the trailing space and handle trailing space in the rust native completion logic.

• bash

  complete -o nospace
  

  nospace Tell readline not to append a space (the default) to words completed at the end of the line.
  

• zsh

  compadd -S "" -a completions
  

  -P prefix
  This gives a string to be inserted before each match. The string given is not considered as part of the match and any shell metacharacters in it will not be quoted when the string is inserted.
  
  -S suffix
  Like -P, but gives a string to be inserted after each match.
  

• elvish & powershell
  They do not append the trailing space by default.
  Simple testing on:

  ~/clap> elvish --version
  0.17.0+Ubuntu-1
  

  PS C:\Users\shanmu\clap> $PSVersionTable
  
  Name                           Value
  ----                           -----
  PSVersion                      7.4.3
  PSEdition                      Core
  GitCommitId                    7.4.3
  OS                             Microsoft Windows 10.0.22631
  Platform                       Win32NT
  PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
  PSRemotingProtocolVersion      2.3
  SerializationVersion           1.1.0.1
  WSManStackVersion              3.0
  

[ModProg]

I think if we just extend CompletionCandidate to have a should_space() or get_with_space() don't care about the naming, the shell specifics shouldn't be an issue.

Though it does mean, that for lists in something like fish, it could make sense to offer completions with trailing commas? Either exclusively or both, producing both should probably also be limited to when there aren't too many completions on screen.

Either --a=1,2,| -> 1 1, 2 2, 3 3, or just 1, 2, 3, (I'm not sure if we support trailing commas in the argument parsing).

[epage]

(I'm not sure if we support trailing commas in the argument parsing).

Looks like we just do a basic split:

clap/clap_builder/src/parser/parser.rs

Lines 1164 to 1166 in 5efa52a

	} else {
	split_raw_vals.extend(raw_val.split(val_delim).map(|x| x.to_owned()));
	}