Fix issue 6596 (#6603)

* Fix issue 6596

* add two unit tests

* fix pipe

* add cp test

* fix test on windows

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

@@ -123,7 +123,8 @@ impl NuCompleter {
         let offset = working_set.next_span_start();
         let (mut new_line, alias_offset) = try_find_alias(line.as_bytes(), &working_set);
         let initial_line = line.to_string();
-        new_line.push(b'a');
+        let alias_total_offset: usize = alias_offset.iter().sum();
+        new_line.insert(alias_total_offset + pos, b'a');
         let pos = offset + pos;
         let config = self.engine_state.get_config();
 
@@ -169,9 +170,10 @@ impl NuCompleter {
                             }
                         };
 
-                        // Parses the prefix
+                        // Parses the prefix. Completion should look up to the cursor position, not after.
                         let mut prefix = working_set.get_span_contents(flat.0).to_vec();
-                        prefix.remove(pos - (flat.0.start - span_offset));
+                        let index = pos - (flat.0.start - span_offset);
+                        prefix.drain(index..);
 
                         // Variables completion
                         if prefix.starts_with(b"$") || most_left_var.is_some() {

crates/nu-cli/tests/completions.rs

@@ -63,7 +63,7 @@ fn variables_single_dash_argument_with_flagcompletion(mut completer: NuCompleter
 
 #[rstest]
 fn variables_command_with_commandcompletion(mut completer_strings: NuCompleter) {
-    let suggestions = completer_strings.complete("my-command ", 9);
+    let suggestions = completer_strings.complete("my-c ", 4);
     let expected: Vec<String> = vec!["my-command".into()];
     match_suggestions(expected, suggestions);
 }
@@ -672,3 +672,63 @@ fn unknown_command_completion() {
 
     match_suggestions(expected_paths, suggestions)
 }
+
+#[rstest]
+fn flagcompletion_triggers_after_cursor(mut completer: NuCompleter) {
+    let suggestions = completer.complete("tst -h", 5);
+    let expected: Vec<String> = vec!["--help".into(), "--mod".into(), "-h".into(), "-s".into()];
+    match_suggestions(expected, suggestions);
+}
+
+#[rstest]
+fn customcompletion_triggers_after_cursor(mut completer_strings: NuCompleter) {
+    let suggestions = completer_strings.complete("my-command c", 11);
+    let expected: Vec<String> = vec!["cat".into(), "dog".into(), "eel".into()];
+    match_suggestions(expected, suggestions);
+}
+
+#[rstest]
+fn customcompletion_triggers_after_cursor_piped(mut completer_strings: NuCompleter) {
+    let suggestions = completer_strings.complete("my-command c | ls", 11);
+    let expected: Vec<String> = vec!["cat".into(), "dog".into(), "eel".into()];
+    match_suggestions(expected, suggestions);
+}
+
+#[rstest]
+fn flagcompletion_triggers_after_cursor_piped(mut completer: NuCompleter) {
+    let suggestions = completer.complete("tst -h | ls", 5);
+    let expected: Vec<String> = vec!["--help".into(), "--mod".into(), "-h".into(), "-s".into()];
+    match_suggestions(expected, suggestions);
+}
+
+#[test]
+fn filecompletions_triggers_after_cursor() {
+    let (_, _, engine, stack) = new_engine();
+
+    let mut completer = NuCompleter::new(std::sync::Arc::new(engine), stack);
+
+    let suggestions = completer.complete("cp   test_c", 3);
+
+    #[cfg(windows)]
+    let expected_paths: Vec<String> = vec![
+        "nushell".to_string(),
+        "test_a\\".to_string(),
+        "test_b\\".to_string(),
+        "another\\".to_string(),
+        "custom_completion.nu".to_string(),
+        ".hidden_file".to_string(),
+        ".hidden_folder\\".to_string(),
+    ];
+    #[cfg(not(windows))]
+    let expected_paths: Vec<String> = vec![
+        "nushell".to_string(),
+        "test_a/".to_string(),
+        "test_b/".to_string(),
+        "another/".to_string(),
+        "custom_completion.nu".to_string(),
+        ".hidden_file".to_string(),
+        ".hidden_folder/".to_string(),
+    ];
+
+    match_suggestions(expected_paths, suggestions);
+}