feat: external completions for commands/flags (#6295)

* wip

* wip

* cleanup

* error message

* cleanup

* cleanup

* fix clippy

* add test

* fix span

* cleanup

* cleanup

* cleanup

* fixed completion

* push char

* wip

* small fixes

* fix remove last span

* fmt

* cleanup

* fixes + more tests

* fix test

* only complete for commands

* also complete flags

* change decl_id to block_id

* use nu completion first

* fix test

* ignore test

* update config section

crates/nu-cli/tests/external_completions.rs

@@ -0,0 +1,73 @@
+pub mod support;
+
+use nu_cli::NuCompleter;
+use nu_parser::parse;
+use nu_protocol::engine::StateWorkingSet;
+use reedline::{Completer, Suggestion};
+use support::new_engine;
+
+#[test]
+#[ignore]
+fn external_completer_trailing_space() {
+    // https://github.com/nushell/nushell/issues/6378
+    let block = "let external_completer = {|spans| $spans}";
+    let input = "gh alias ".to_string();
+
+    let suggestions = run_completion(&block, &input);
+    assert_eq!(3, suggestions.len());
+    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("alias", suggestions.get(1).unwrap().value);
+    assert_eq!("", suggestions.get(2).unwrap().value);
+}
+
+#[test]
+fn external_completer_no_trailing_space() {
+    let block = "let external_completer = {|spans| $spans}";
+    let input = "gh alias".to_string();
+
+    let suggestions = run_completion(&block, &input);
+    assert_eq!(2, suggestions.len());
+    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("alias", suggestions.get(1).unwrap().value);
+}
+
+#[test]
+fn external_completer_pass_flags() {
+    let block = "let external_completer = {|spans| $spans}";
+    let input = "gh api --".to_string();
+
+    let suggestions = run_completion(&block, &input);
+    assert_eq!(3, suggestions.len());
+    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("api", suggestions.get(1).unwrap().value);
+    assert_eq!("--", suggestions.get(2).unwrap().value);
+}
+
+fn run_completion(block: &str, input: &str) -> Vec<Suggestion> {
+    // Create a new engine
+    let (dir, _, mut engine_state, mut stack) = new_engine();
+    let (_, delta) = {
+        let mut working_set = StateWorkingSet::new(&engine_state);
+        let (block, err) = parse(&mut working_set, None, block.as_bytes(), false, &[]);
+        assert!(err.is_none());
+
+        (block, working_set.render())
+    };
+
+    assert!(engine_state.merge_delta(delta).is_ok());
+
+    // Merge environment into the permanent state
+    assert!(engine_state.merge_env(&mut stack, &dir).is_ok());
+
+    let latest_block_id = engine_state.num_blocks() - 1;
+
+    // Change config adding the external completer
+    let mut config = engine_state.get_config().clone();
+    config.external_completer = Some(latest_block_id);
+    engine_state.set_config(&config);
+
+    // Instatiate a new completer
+    let mut completer = NuCompleter::new(std::sync::Arc::new(engine_state), stack);
+
+    completer.complete(&input, input.len())
+}