fix: preserve path when completing intermediate directory (#10831)

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- fixes #10766

# Description
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If the partial supplied to the completion function is shorter than the
span, the cursor is in between the path, we are trying to complete an
intermediate directory. In such a case we:
- only suggest directory names
- don't append the slash since it is already present
- only complete the path till the component the cursor is on

# User-Facing Changes
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helps us keep track of breaking changes. -->

Intermediate directories can be completed without erasing the rest of
the path.

# Tests + Formatting
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crates/nu-cli/src/completions/completion_common.rs

@@ -1,5 +1,6 @@
 use crate::completions::{matches, CompletionOptions};
 use nu_path::home_dir;
+use nu_protocol::{engine::StateWorkingSet, Span};
 use std::path::{is_separator, Component, Path, PathBuf, MAIN_SEPARATOR as SEP};
 
 fn complete_rec(
@@ -160,3 +161,36 @@ pub fn escape_path(path: String, dir: bool) -> String {
         path
     }
 }
+
+pub struct AdjustView {
+    pub prefix: String,
+    pub span: Span,
+    pub readjusted: bool,
+}
+
+pub fn adjust_if_intermediate(
+    prefix: &[u8],
+    working_set: &StateWorkingSet,
+    mut span: nu_protocol::Span,
+) -> AdjustView {
+    let span_contents = String::from_utf8_lossy(working_set.get_span_contents(span)).to_string();
+    let mut prefix = String::from_utf8_lossy(prefix).to_string();
+
+    // A difference of 1 because of the cursor's unicode code point in between.
+    // Using .chars().count() because unicode and Windows.
+    let readjusted = span_contents.chars().count() - prefix.chars().count() > 1;
+    if readjusted {
+        let remnant: String = span_contents
+            .chars()
+            .skip(prefix.chars().count() + 1)
+            .take_while(|&c| !is_separator(c))
+            .collect();
+        prefix.push_str(&remnant);
+        span = Span::new(span.start, span.start + prefix.chars().count() + 1);
+    }
+    AdjustView {
+        prefix,
+        span,
+        readjusted,
+    }
+}

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

@@ -1,4 +1,7 @@
-use crate::completions::{completion_common::complete_item, Completer, CompletionOptions, SortBy};
+use crate::completions::{
+    completion_common::{adjust_if_intermediate, complete_item, AdjustView},
+    Completer, CompletionOptions, SortBy,
+};
 use nu_protocol::{
     engine::{EngineState, StateWorkingSet},
     levenshtein_distance, Span,
@@ -21,19 +24,19 @@ impl DirectoryCompletion {
 impl Completer for DirectoryCompletion {
     fn fetch(
         &mut self,
-        _: &StateWorkingSet,
+        working_set: &StateWorkingSet,
         prefix: Vec<u8>,
         span: Span,
         offset: usize,
         _: usize,
         options: &CompletionOptions,
     ) -> Vec<Suggestion> {
-        let partial = String::from_utf8_lossy(&prefix).to_string();
+        let AdjustView { prefix, span, .. } = adjust_if_intermediate(&prefix, working_set, span);
 
         // Filter only the folders
         let output: Vec<_> = directory_completion(
             span,
-            &partial,
+            &prefix,
             &self.engine_state.current_work_dir(),
             options,
         )

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

@@ -1,4 +1,7 @@
-use crate::completions::{completion_common::complete_item, Completer, CompletionOptions, SortBy};
+use crate::completions::{
+    completion_common::{adjust_if_intermediate, complete_item, AdjustView},
+    Completer, CompletionOptions, SortBy,
+};
 use nu_protocol::{
     engine::{EngineState, StateWorkingSet},
     levenshtein_distance, Span,
@@ -21,15 +24,21 @@ impl FileCompletion {
 impl Completer for FileCompletion {
     fn fetch(
         &mut self,
-        _: &StateWorkingSet,
+        working_set: &StateWorkingSet,
         prefix: Vec<u8>,
         span: Span,
         offset: usize,
         _: usize,
         options: &CompletionOptions,
     ) -> Vec<Suggestion> {
-        let prefix = String::from_utf8_lossy(&prefix).to_string();
-        let output: Vec<_> = file_path_completion(
+        let AdjustView {
+            prefix,
+            span,
+            readjusted,
+        } = adjust_if_intermediate(&prefix, working_set, span);
+
+        let output: Vec<_> = complete_item(
+            readjusted,
             span,
             &prefix,
             &self.engine_state.current_work_dir(),