Use rewrite buffer to determine if comment should be on a newline

Rewriting may change the context of a later item (in this case, a
comment), so looking at the original snippet to determine the context
may not be correct. Since for any given item, all previous items should
already be rewritten, it is enough to determine the item's context in
the new source but looking back at the rewrite buffer.

This commit also gets rid of variables no longer used due to the change.

Closes #4227

Author: ayazhafiz

src/formatting/missed_spans.rs

@@ -1,4 +1,4 @@
-use rustc_span::{BytePos, Pos, Span};
+use rustc_span::{BytePos, Span};
 
 use crate::config::{file_lines::FileLines, FileName};
 
@@ -140,28 +140,15 @@ impl<'a> FmtVisitor<'a> {
     where
         F: Fn(&mut FmtVisitor<'_>, &str, &str),
     {
-        // Get a snippet from the file start to the span's hi without allocating.
-        // We need it to determine what precedes the current comment. If the comment
-        // follows code on the same line, we won't touch it.
-        let big_span_lo = self.snippet_provider.start_pos();
-        let big_snippet = self.snippet_provider.entire_snippet();
-        let big_diff = (span.lo() - big_span_lo).to_usize();
-
         let snippet = self.snippet(span);
 
         debug!("write_snippet `{}`", snippet);
 
-        self.write_snippet_inner(big_snippet, snippet, big_diff, span, process_last_snippet);
+        self.write_snippet_inner(snippet, span, process_last_snippet);
     }
 
-    fn write_snippet_inner<F>(
-        &mut self,
-        big_snippet: &str,
-        snippet: &str,
-        big_diff: usize,
-        span: Span,
-        process_last_snippet: F,
-    ) where
+    fn write_snippet_inner<F>(&mut self, snippet: &str, span: Span, process_last_snippet: F)
+    where
         F: Fn(&mut FmtVisitor<'_>, &str, &str),
     {
         // Trim whitespace from the right hand side of each line.
@@ -200,13 +187,7 @@ impl<'a> FmtVisitor<'a> {
             let newline_count = lf_count + crlf_count;
             if CodeCharKind::Comment == kind && within_file_lines_range {
                 // 1: comment.
-                self.process_comment(
-                    &mut status,
-                    snippet,
-                    &big_snippet[..(offset + big_diff)],
-                    offset,
-                    subslice,
-                );
+                self.process_comment(&mut status, snippet, offset, subslice);
             } else if subslice.trim().is_empty() && newline_count > 0 && within_file_lines_range {
                 // 2: blank lines.
                 self.push_vertical_spaces(newline_count);
@@ -233,11 +214,11 @@ impl<'a> FmtVisitor<'a> {
         &mut self,
         status: &mut SnippetStatus,
         snippet: &str,
-        big_snippet: &str,
         offset: usize,
         subslice: &str,
     ) {
-        let last_char = big_snippet
+        let last_char = self
+            .buffer
             .chars()
             .rev()
             .find(|rev_c| ![' ', '\t'].contains(rev_c));

tests/source/issue-4227.rs

@@ -0,0 +1,6 @@
+fn main() {
+    /*
+    Block comment
+    */ // Line comment
+    println!("Hello");
+}

tests/target/issue-4227.rs

@@ -0,0 +1,7 @@
+fn main() {
+    /*
+    Block comment
+    */
+    // Line comment
+    println!("Hello");
+}