Insert a final newline if needed (microsoft#300)

Author: lhecker

src/buffer/mod.rs

@@ -204,6 +204,7 @@ pub struct TextBuffer {
     ruler: CoordType,
     encoding: &'static str,
     newlines_are_crlf: bool,
+    insert_final_newline: bool,
     overtype: bool,
 
     wants_cursor_visibility: bool,
@@ -249,7 +250,9 @@ impl TextBuffer {
             line_highlight_enabled: false,
             ruler: 0,
             encoding: "UTF-8",
-            newlines_are_crlf: cfg!(windows), // Unfortunately Windows users insist on CRLF
+            // Windows users want CRLF and no final newline.
+            newlines_are_crlf: cfg!(windows),
+            insert_final_newline: !cfg!(windows),
             overtype: false,
 
             wants_cursor_visibility: false,
@@ -621,6 +624,7 @@ impl TextBuffer {
         // * the logical line count
         // * the newline type (LF or CRLF)
         // * the indentation type (tabs or spaces)
+        // * whether there's a final newline
         {
             let chunk = self.read_forward(0);
             let mut offset = 0;
@@ -711,10 +715,13 @@ impl TextBuffer {
                 (_, lines) = unicode::newlines_forward(chunk, offset, lines, CoordType::MAX);
             }
 
+            let final_newline = chunk.ends_with(b"\n");
+
             // Add 1, because the last line doesn't end in a newline (it ends in the literal end).
             self.stats.logical_lines = lines + 1;
             self.stats.visual_lines = self.stats.logical_lines;
             self.newlines_are_crlf = newlines_are_crlf;
+            self.insert_final_newline = final_newline;
             self.indent_with_tabs = indent_with_tabs;
             self.tab_size = tab_size;
         }
@@ -1894,6 +1901,22 @@ impl TextBuffer {
             }
         }
 
+        // POSIX mandates that all valid lines end in a newline.
+        // This isn't all that common on Windows and so we have
+        // `self.final_newline` to control this.
+        //
+        // In order to not annoy people with this, we only add a
+        // newline if you just edited the very end of the buffer.
+        if self.insert_final_newline
+            && self.cursor.offset > 0
+            && self.cursor.offset == self.text_length()
+            && self.cursor.logical_pos.x > 0
+        {
+            let cursor = self.cursor;
+            self.edit_write(if self.newlines_are_crlf { b"\r\n" } else { b"\n" });
+            self.set_cursor_internal(cursor);
+        }
+
         self.edit_end();
     }