fix: Include frontmatter in -Zunpretty output

In the implementation (rust-lang#140035), this was left as an open question for
the tracking issue (rust-lang#136889).  My assumption is that this should be
carried over.

Thankfully, either way, `-Zunpretty` is unstable and we can always
change it even if we stabilize frontmatter.

Author: epage

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -120,7 +120,7 @@ fn gather_comments(sm: &SourceMap, path: FileName, src: String) -> Vec<Comment>
         pos += shebang_len;
     }
 
-    for token in rustc_lexer::tokenize(&text[pos..], rustc_lexer::FrontmatterAllowed::No) {
+    for token in rustc_lexer::tokenize(&text[pos..], rustc_lexer::FrontmatterAllowed::Yes) {
         let token_text = &text[pos..pos + token.len as usize];
         match token.kind {
             rustc_lexer::TokenKind::Whitespace => {
@@ -171,6 +171,14 @@ fn gather_comments(sm: &SourceMap, path: FileName, src: String) -> Vec<Comment>
                     })
                 }
             }
+            rustc_lexer::TokenKind::Frontmatter { .. } => {
+                code_to_the_left = false;
+                comments.push(Comment {
+                    style: CommentStyle::Isolated,
+                    lines: vec![token_text.to_string()],
+                    pos: start_bpos + BytePos(pos as u32),
+                });
+            }
             _ => {
                 code_to_the_left = true;
             }

tests/ui/unpretty/frontmatter.stdout

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