Clear line on `<ret>` when line is all whitespace (#4854)

This matches the insert-mode behavior for Vim and Kakoune: if the
current line is empty except for whitespace, `<ret>` should insert a
line ending at the beginning of the line, moving any indentation to the
next line.

helix-term/src/commands.rs

@@ -3085,6 +3085,22 @@ pub mod insert {
             let curr = contents.get_char(pos).unwrap_or(' ');
 
             let current_line = text.char_to_line(pos);
+            let line_is_only_whitespace = text
+                .line(current_line)
+                .chars()
+                .all(|char| char.is_ascii_whitespace());
+
+            let mut new_text = String::new();
+
+            // If the current line is all whitespace, insert a line ending at the beginning of
+            // the current line. This makes the current line empty and the new line contain the
+            // indentation of the old line.
+            let (from, to, local_offs) = if line_is_only_whitespace {
+                let line_start = text.line_to_char(current_line);
+                new_text.push_str(doc.line_ending.as_str());
+
+                (line_start, line_start, new_text.chars().count())
+            } else {
                 let indent = indent::indent_for_newline(
                     doc.language_config(),
                     doc.syntax(),
@@ -3095,7 +3111,7 @@ pub mod insert {
                     pos,
                     current_line,
                 );
-            let mut text = String::new();
+
                 // If we are between pairs (such as brackets), we want to
                 // insert an additional line which is indented one level
                 // more and place the cursor there
@@ -3107,18 +3123,21 @@ pub mod insert {
 
                 let local_offs = if on_auto_pair {
                     let inner_indent = indent.clone() + doc.indent_style.as_str();
-                text.reserve_exact(2 + indent.len() + inner_indent.len());
-                text.push_str(doc.line_ending.as_str());
-                text.push_str(&inner_indent);
-                let local_offs = text.chars().count();
-                text.push_str(doc.line_ending.as_str());
-                text.push_str(&indent);
+                    new_text.reserve_exact(2 + indent.len() + inner_indent.len());
+                    new_text.push_str(doc.line_ending.as_str());
+                    new_text.push_str(&inner_indent);
+                    let local_offs = new_text.chars().count();
+                    new_text.push_str(doc.line_ending.as_str());
+                    new_text.push_str(&indent);
                     local_offs
                 } else {
-                text.reserve_exact(1 + indent.len());
-                text.push_str(doc.line_ending.as_str());
-                text.push_str(&indent);
-                text.chars().count()
+                    new_text.reserve_exact(1 + indent.len());
+                    new_text.push_str(doc.line_ending.as_str());
+                    new_text.push_str(&indent);
+                    new_text.chars().count()
+                };
+
+                (pos, pos, local_offs)
             };
 
             let new_range = if doc.restore_cursor {
@@ -3139,9 +3158,9 @@ pub mod insert {
             // range.replace(|range| range.is_empty(), head); -> fn extend if cond true, new head pos
             // can be used with cx.mode to do replace or extend on most changes
             ranges.push(new_range);
-            global_offs += text.chars().count();
+            global_offs += new_text.chars().count();
 
-            (pos, pos, Some(text.into()))
+            (from, to, Some(new_text.into()))
         });
 
         transaction = transaction.with_selection(Selection::new(ranges, selection.primary_index()));