to_markdown: fix another crash on non-ascii

Related-to: GH-170.

Author: wooorm

mdast_util_to_markdown/src/state.rs

@@ -535,6 +535,12 @@ impl<'a> State<'a> {
             };
         }
 
+        // Some of the operations above seem to end up right in a utf8 boundary
+        // (see GH-170 for more info).
+        // Move back.
+        while !value.is_char_boundary(start) {
+            start -= 1;
+        }
         result.push_str(&escape_backslashes(&value[start..end], config.after));
 
         result

mdast_util_to_markdown/tests/paragraph.rs

@@ -104,4 +104,17 @@ fn paragraph() {
         "a\t	\n	\tb\n",
         "should encode spaces around line endings in paragraphs"
     );
+
+    assert_eq!(
+        to(&Node::Paragraph(Paragraph {
+            children: vec![Node::Text(Text {
+                value: String::from("я_я"),
+                position: None
+            })],
+            position: None
+        }))
+        .unwrap(),
+        "яяя\n",
+        "should support escaping around non-ascii"
+    );
 }