fix(completion): invalid prefix for external path argument with spaces (#15998)

Fixes the default behavior of #15790 

# Description

As for the mentioned carapace version: `cat ~"/Downloads/Obsidian
Vault/"`, the problem lies in the unexpanded home directory `~`. Either
we encourage users to manually expand that in
`$env.config.completions.external.completer` or open an issue on the
carapace project.

# User-Facing Changes

bug fix

# Tests + Formatting

Adjusted

# After Submitting

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

@@ -466,6 +466,14 @@ impl NuCompleter {
                                 return suggestions;
                             }
                         }
+                        // for external path arguments with spaces, please check issue #15790
+                        if suggestions.is_empty() {
+                            let (new_span, prefix) =
+                                strip_placeholder_if_any(working_set, &span, strip);
+                            let ctx = Context::new(working_set, new_span, prefix, offset);
+                            suggestions.extend(self.process_completion(&mut FileCompletion, &ctx));
+                            return suggestions;
+                        }
                         break;
                     }
                 }

crates/nu-cli/tests/completions/mod.rs

@@ -9,14 +9,16 @@ use std::{
 use nu_cli::NuCompleter;
 use nu_engine::eval_block;
 use nu_parser::parse;
-use nu_path::expand_tilde;
+use nu_path::{AbsolutePathBuf, expand_tilde};
 use nu_protocol::{Config, PipelineData, debugger::WithoutDebug, engine::StateWorkingSet};
 use nu_std::load_standard_library;
+use nu_test_support::fs;
 use reedline::{Completer, Suggestion};
 use rstest::{fixture, rstest};
 use support::{
     completions_helpers::{
-        new_dotnu_engine, new_external_engine, new_partial_engine, new_quote_engine,
+        new_dotnu_engine, new_engine_helper, new_external_engine, new_partial_engine,
+        new_quote_engine,
     },
     file, folder, match_suggestions, match_suggestions_by_string, new_engine,
 };
@@ -716,6 +718,16 @@ fn external_completer_fallback() {
     let expected = [folder("test_a"), file("test_a_symlink"), folder("test_b")];
     let suggestions = run_external_completion(block, input);
     match_suggestions_by_string(&expected, &suggestions);
+
+    // issue #15790
+    let input = "foo `dir with space/`";
+    let expected = vec!["`dir with space/bar baz`", "`dir with space/foo`"];
+    let suggestions = run_external_completion_within_pwd(
+        block,
+        input,
+        fs::fixtures().join("external_completions"),
+    );
+    match_suggestions(&expected, &suggestions);
 }
 
 /// Fallback to external completions for flags of `sudo`
@@ -2103,11 +2115,15 @@ fn alias_of_another_alias() {
     match_suggestions(&expected_paths, &suggestions)
 }
 
-fn run_external_completion(completer: &str, input: &str) -> Vec<Suggestion> {
+fn run_external_completion_within_pwd(
+    completer: &str,
+    input: &str,
+    pwd: AbsolutePathBuf,
+) -> Vec<Suggestion> {
     let completer = format!("$env.config.completions.external.completer = {completer}");
 
     // Create a new engine
-    let (_, _, mut engine_state, mut stack) = new_engine();
+    let (_, _, mut engine_state, mut stack) = new_engine_helper(pwd);
     let (block, delta) = {
         let mut working_set = StateWorkingSet::new(&engine_state);
         let block = parse(&mut working_set, None, completer.as_bytes(), false);
@@ -2131,6 +2147,10 @@ fn run_external_completion(completer: &str, input: &str) -> Vec<Suggestion> {
     completer.complete(input, input.len())
 }
 
+fn run_external_completion(completer: &str, input: &str) -> Vec<Suggestion> {
+    run_external_completion_within_pwd(completer, input, fs::fixtures().join("completions"))
+}
+
 #[test]
 fn unknown_command_completion() {
     let (_, _, engine, stack) = new_engine();