Properly align comments in unicode lines

For an end user, the number of characters in a string is most likely
interpreted as the number of [grapheme cluster](http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries)s in the string,
which may be different than either the number of unicode codepoints or
bytes. Use the number of graphemes to determine list comment alignment
rather than the byte length of the line.

Closes rust-lang#4151

rustfmt-core/rustfmt-lib/src/lists.rs

@@ -4,6 +4,7 @@ use std::cmp;
 use std::iter::Peekable;
 
 use rustc_span::BytePos;
+use unicode_segmentation::UnicodeSegmentation;
 
 use crate::comment::{find_comment_end, rewrite_comment, FindUncommented};
 use crate::config::lists::*;
@@ -464,8 +465,10 @@ where
 
             if !starts_with_newline(comment) {
                 if formatting.align_comments {
-                    let mut comment_alignment =
-                        post_comment_alignment(item_max_width, inner_item.len());
+                    let mut comment_alignment = post_comment_alignment(
+                        item_max_width,
+                        UnicodeSegmentation::graphemes(inner_item.as_str(), true).count(),
+                    );
                     if first_line_width(&formatted_comment)
                         + last_line_width(&result)
                         + comment_alignment
@@ -474,8 +477,10 @@ where
                     {
                         item_max_width = None;
                         formatted_comment = rewrite_post_comment(&mut item_max_width)?;
-                        comment_alignment =
-                            post_comment_alignment(item_max_width, inner_item.len());
+                        comment_alignment = post_comment_alignment(
+                            item_max_width,
+                            UnicodeSegmentation::graphemes(inner_item.as_str(), true).count(),
+                        );
                     }
                     for _ in 0..=comment_alignment {
                         result.push(' ');
@@ -533,7 +538,7 @@ where
     let mut first = true;
     for item in items.clone().into_iter().skip(i) {
         let item = item.as_ref();
-        let inner_item_width = item.inner_as_ref().len();
+        let inner_item_width = UnicodeSegmentation::graphemes(item.inner_as_ref(), true).count();
         if !first
             && (item.is_different_group()
                 || item.post_comment.is_none()

rustfmt-core/rustfmt-lib/tests/target/issue-4151.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let mappings = [
+        ("gamma", 'ɡ'), // comment 1
+        ("sqrt", '√'),  // comment 2
+        ("a", 'a'),     // comment 3
+        ("eye", 'ಠ'),   // comment 4
+    ];
+}