feat(pretty): Preserve the frontmatter

Author: epage

Diff for: compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -123,6 +123,15 @@ fn gather_comments(sm: &SourceMap, path: FileName, src: String) -> Vec<Comment>
         pos += shebang_len;
     }
 
+    if let Some(frontmatter_len) = rustc_lexer::strip_frontmatter(&text[pos..]) {
+        comments.push(Comment {
+            style: CommentStyle::Isolated,
+            lines: vec![text[pos..frontmatter_len].to_string()],
+            pos: start_bpos + BytePos(pos as u32),
+        });
+        pos += frontmatter_len;
+    }
+
     for token in rustc_lexer::tokenize(&text[pos..]) {
         let token_text = &text[pos..pos + token.len as usize];
         match token.kind {

Diff for: tests/ui/unpretty/frontmatter.stdout

@@ -1,3 +1,5 @@
+---
+---
 
 //@ compile-flags: -Zunpretty=normal
 //@ check-pass