preserve comments within headers but don't reformat the header

These changes would allow rustfmt to retain the existing comments and
still allow it to format whatever content comes after the AST node.

Author: ytmimi

src/header.rs

@@ -11,7 +11,7 @@ use rustc_ast as ast;
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
 
-use crate::comment::combine_strs_with_missing_comments;
+use crate::comment::{combine_strs_with_missing_comments, contains_comment};
 use crate::rewrite::RewriteContext;
 use crate::shape::Shape;
 use crate::utils::rewrite_ident;
@@ -24,6 +24,13 @@ pub(crate) fn format_header(
     debug!(?parts, "format_header");
     let shape = shape.infinite_width();
 
+    // FIXME(ytmimi) Don't apply any formatting if the header contains comments.
+    // the rustfmt team needs to decide how to handle comments within the header
+    if comments_in_header(context, &parts) {
+        let span = header_span(&parts).expect("If we have comments we have headers");
+        return context.snippet(span).to_owned();
+    }
+
     // Empty `HeaderPart`s are ignored.
     let mut parts = parts.into_iter().filter(|x| !x.snippet.is_empty());
     let Some(part) = parts.next() else {
@@ -51,6 +58,23 @@ pub(crate) fn format_header(
     result
 }
 
+/// Get the Span of the entire header
+fn header_span(parts: &[HeaderPart]) -> Option<Span> {
+    let first = parts.first()?;
+    let last = parts.last()?;
+    Some(first.span.with_hi(last.span.hi()))
+}
+
+/// Check if there are any comments in the header parts
+fn comments_in_header(context: &RewriteContext<'_>, parts: &[HeaderPart]) -> bool {
+    let Some(span) = header_span(parts) else {
+        return false;
+    };
+
+    let snippet = context.snippet(span);
+    contains_comment(snippet)
+}
+
 #[derive(Debug)]
 pub(crate) struct HeaderPart {
     /// snippet of this part without surrounding space