feat(complete): Add dynamic completion for nushell

[chklauser]

Adds an implementation of dynamic completion to clap_complete_nushell under the unstable-dynamic feature flag. Corresponding issue #5840. The issue description has more context and a more complete solution sketch. This PR doesn't implement the full solution sketched there. Ideally, we'd first agree that that solution is the way to go.

With this PR (and the unstable-dynamic feature flag), clap tools can generate a nushell module that offers three commands:

• complete, which performs the actual completion
• handles, which asks the module whether it is the correct completer for a particular command line
• install, which globally registers a completer that falls back to whatever completer was previously installed if handles rejects completing a command line.

The idea is that user's who already have a custom external completer can integrate the generated module's handles and complete commands into their completer.

Everyone else just puts

use my-app-completer.nu
my-app-completer install

into their nushell config.

To test it, I have integrated it into a local build of the relatively complex clap-based tool, JJ (Jujutsu), and it worked very well so far.

TODO

• Consensus for solution
• Automated tests

[chklauser]

I do have some questions:

1. Is feat (minor version bump) OK?
2. This is the first time I have had any contact with shell completion. What would you expect in terms of automated tests? I'm having a hard time telling the tests that cover the "static" completions apart from the ones that cover the dynamic completion shell integrations.

[epage]

Is feat (minor version bump) OK?

feat is right though we don't bump minor on features

What would you expect in terms of automated tests?

For dynamic completions, we focus on the shell integration

• Does it work at all
• What sort order is used (ours or the shells)
• How does escaping work

etc

e.g. see

clap/clap_complete/tests/testsuite/bash.rs

Lines 242 to 338 in fde45f9

	#[test]
	#[cfg(all(unix, feature = "unstable-dynamic"))]
	#[cfg(feature = "unstable-shell-tests")]
	fn register_dynamic_env() {
	common::register_example::<RuntimeBuilder>("dynamic-env", "exhaustive");
	}
	#[test]
	#[cfg(all(unix, feature = "unstable-dynamic"))]
	#[cfg(feature = "unstable-shell-tests")]
	fn complete_dynamic_env_toplevel() {
	if !common::has_command(CMD) {
	return;
	}
	let term = completest::Term::new();
	let mut runtime = common::load_runtime::<RuntimeBuilder>("dynamic-env", "exhaustive");
	let input = "exhaustive \t\t";
	let expected = snapbox::str![[r#"
	%
	action value last hint --global --help
	quote pacman alias help --generate --version
	"#]];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	}
	#[test]
	#[cfg(all(unix, feature = "unstable-dynamic"))]
	#[cfg(feature = "unstable-shell-tests")]
	fn complete_dynamic_env_quoted_help() {
	if !common::has_command(CMD) {
	return;
	}
	let term = completest::Term::new();
	let mut runtime = common::load_runtime::<RuntimeBuilder>("dynamic-env", "exhaustive");
	let input = "exhaustive quote \t\t";
	let expected = snapbox::str![[r#"
	%
	cmd-single-quotes cmd-backslash escape-help --global --backslash --choice
	cmd-double-quotes cmd-brackets help --double-quotes --brackets --help
	cmd-backticks cmd-expansions --single-quotes --backticks --expansions --version
	"#]];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	}
	#[test]
	#[cfg(all(unix, feature = "unstable-dynamic"))]
	#[cfg(feature = "unstable-shell-tests")]
	fn complete_dynamic_env_option_value() {
	if !common::has_command(CMD) {
	return;
	}
	let term = completest::Term::new();
	let mut runtime = common::load_runtime::<RuntimeBuilder>("dynamic-env", "exhaustive");
	let input = "exhaustive action --choice=\t\t";
	let expected = snapbox::str!["% "];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	let input = "exhaustive action --choice=f\t";
	let expected = snapbox::str!["exhaustive action --choice=f % exhaustive action --choice=f"];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	}
	#[test]
	#[cfg(all(unix, feature = "unstable-dynamic"))]
	#[cfg(feature = "unstable-shell-tests")]
	fn complete_dynamic_env_quoted_value() {
	if !common::has_command(CMD) {
	return;
	}
	let term = completest::Term::new();
	let mut runtime = common::load_runtime::<RuntimeBuilder>("dynamic-env", "exhaustive");
	let input = "exhaustive quote --choice \t\t";
	let expected = snapbox::str![[r#"
	%
	another shell bash fish zsh
	"#]];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	let input = "exhaustive quote --choice an\t";
	let expected =
	snapbox::str!["exhaustive quote --choice an % exhaustive quote --choice another shell "];
	let actual = runtime.complete(input, &term).unwrap();
	assert_data_eq!(actual, expected);
	}

Note that I'm going to focus first on the issue to see if there is any points there ti work out before coming back to the PR to review the implementation.

[chklauser]

re:tests: Thanks! I'll have a look at these. Getting them up and running for nushell should be relatively independent of the concrete implementation.

Note that I'm going to focus first on the issue to see if there is any points there ti work out before coming back to the PR to review the implementation.

Yep, let's nail down the conceptual aspects first.