fix(lsp): a panic caused by completion with decl_id out of range (#15576)

Fixes a bug caused by #15536 
Sorry about that, @fdncred 

# Description

I've made the panic reproducible in the test case.

TLDR: completer will sometimes return new decl_ids outside of the range
of the engine_state passed in.

# User-Facing Changes

bug fix

# Tests + Formatting

+1

# After Submitting

crates/nu-cli/src/completions/exportable_completions.rs

@@ -69,7 +69,8 @@ impl Completer for ExportableCompletion<'_> {
                     wrapped_name(name),
                     Some(cmd.description().to_string()),
                     None,
-                    SuggestionKind::Command(cmd.command_type(), Some(*decl_id)),
+                    // `None` here avoids arguments being expanded by snippet edit style for lsp
+                    SuggestionKind::Command(cmd.command_type(), None),
                 );
             }
         }

crates/nu-lsp/src/completion.rs

@@ -68,13 +68,19 @@ impl LanguageServer {
         let mut idx = 0;
         // use snippet as `insert_text_format` for command argument completion
         if let Some(SuggestionKind::Command(_, Some(decl_id))) = suggestion.kind {
+            // NOTE: for new commands defined in current context,
+            // which are not present in the engine state, skip the documentation and snippet.
+            if engine_state.num_decls() > decl_id.get() {
                 let cmd = engine_state.get_decl(decl_id);
                 doc_string = Some(Self::get_decl_description(cmd, true));
                 insert_text_format = Some(InsertTextFormat::SNIPPET);
                 let signature = cmd.signature();
                 // add curly brackets around block arguments
                 // and keywords, e.g. `=` in `alias foo = bar`
-            let mut arg_wrapper = |arg: &PositionalArg, text: String, optional: bool| -> String {
+                let mut arg_wrapper = |arg: &PositionalArg,
+                                       text: String,
+                                       optional: bool|
+                 -> String {
                     idx += 1;
                     match &arg.shape {
                         SyntaxShape::Block | SyntaxShape::MatchBlock => {
@@ -108,14 +114,16 @@ impl LanguageServer {
                 }
                 for optional in signature.optional_positional {
                     snippet_text.push(' ');
-                snippet_text
-                    .push_str(arg_wrapper(&optional, format!("{}?", optional.name), true).as_str());
+                    snippet_text.push_str(
+                        arg_wrapper(&optional, format!("{}?", optional.name), true).as_str(),
+                    );
                 }
                 if let Some(rest) = signature.rest_positional {
                     idx += 1;
                     snippet_text.push_str(format!(" ${{{}:...{}}}", idx, rest.name).as_str());
                 }
             }
+        }
         // no extra space for a command with args expanded in the snippet
         if idx == 0 && suggestion.suggestion.append_whitespace {
             snippet_text.push(' ');
@@ -329,6 +337,17 @@ mod tests {
             })
         ));
 
+        // fallback completion on a newly defined command,
+        // the decl_id is missing in the engine state, this shouldn't cause any panic.
+        let resp = send_complete_request(&client_connection, script.clone(), 13, 9);
+        assert_json_include!(
+            actual: result_from_message(resp),
+            expected: serde_json::json!([
+                // defined before the cursor
+                { "label": "config n foo bar", "detail": detail_str, "kind": 2 },
+            ])
+        );
+
         // inside parenthesis
         let resp = send_complete_request(&client_connection, script, 10, 34);
         assert!(result_from_message(resp).as_array().unwrap().contains(

tests/fixtures/lsp/completion/command.nu

@@ -10,3 +10,5 @@ def "config n foo bar" [
   echo "🤔🐘"
   | str substring (str substring -)
 }
+
+config n # don't panic!