Fix some char boundary violation bugs.

src/formatter.rs

@@ -512,9 +512,9 @@ pub fn find_good_break(slice: &RopeSlice, lower_limit: usize, byte_idx: usize) -
     let mut prev = if i == slice_len {
         None
     } else {
-        Some(slice.char(byte_idx))
+        Some(slice.char(i))
     };
-    let mut char_itr = slice.chars_at(byte_idx);
+    let mut char_itr = slice.chars_at(i);
     while i > lower_limit {
         let c = char_itr.prev();
         if WS_CHARS.contains(&c.unwrap()) && prev.map(|pc| !WS_CHARS.contains(&pc)).unwrap_or(true)

src/graphemes.rs

@@ -41,6 +41,8 @@ pub fn prev_grapheme_boundary(slice: &RopeSlice, byte_idx: usize) -> usize {
     // Bounds check
     debug_assert!(byte_idx <= slice.len());
 
+    let byte_idx = slice.ceil_char_boundary(byte_idx);
+
     // Get the chunk with our byte index in it.
     let (mut chunk, mut chunk_byte_idx) = slice.chunk(byte_idx);
 
@@ -82,6 +84,8 @@ pub fn next_grapheme_boundary(slice: &RopeSlice, byte_idx: usize) -> usize {
     // Bounds check
     debug_assert!(byte_idx <= slice.len());
 
+    let byte_idx = slice.floor_char_boundary(byte_idx);
+
     // Get the chunk with our byte index in it.
     let (mut chunk, mut chunk_byte_idx) = slice.chunk(byte_idx);
 
@@ -115,6 +119,11 @@ pub fn is_grapheme_boundary(slice: &RopeSlice, byte_idx: usize) -> bool {
     // Get the chunk with our byte index in it.
     let (chunk, chunk_byte_idx) = slice.chunk(byte_idx);
 
+    // Can't be a grapheme boundary if it's not a char boundary.
+    if !chunk.is_char_boundary(chunk_byte_idx) {
+        return false;
+    }
+
     // Set up the grapheme cursor.
     let mut gc = GraphemeCursor::new(byte_idx, slice.len(), true);