Remove multiple trailing lines in comments

Previously, rustfmt would remove one blank, trailing line in a comment
every time it ran, so formatting was not idempotent if a comment had
multiple trailing lines. Comments are only reformatted in this way if
`comment::rewrite_comment_inner` is called, which is enabled by any of the
config options `normalize_comments`, `wrap_comments`, or
`format_code_in_doc_comments` (for doc comments only). Absent those
config options, no trailing line reformatting occurs at all.

In this commit, when the existing condition that detects a blank, trailing
line is true, any preceding blank lines are removed from the reformatted
comment. A source/target "system test" was added to prevent regression.

Signed-off-by: Ross Williams <ross@ross-williams.net>

Author: overhacked

src/comment.rs

@@ -811,7 +811,13 @@ impl<'a> CommentRewrite<'a> {
         } else if self.is_prev_line_multi_line && !line.is_empty() {
             self.result.push(' ')
         } else if is_last && line.is_empty() {
-            // trailing blank lines are unwanted
+            // Trailing blank lines are unwanted; if the last line is blank, look for additional,
+            // preceding blank lines to remove, also.
+            let trailing_line = self.comment_line_separator.trim_end_matches(' ');
+            while self.result.ends_with(trailing_line) {
+                self.result
+                    .truncate(self.result.len() - trailing_line.len());
+            }
             if !self.closer.is_empty() {
                 self.result.push_str(&self.indent_str);
             }

tests/source/comment_trailing_lines.rs

@@ -0,0 +1,21 @@
+// rustfmt-wrap_comments: true
+
+//! Module comment with multiple trailing lines
+//!
+//!
+
+/// Doc comment with multiple trailing lines
+///
+///
+pub mod foo {}
+
+/// Doc comment with one trailing line
+///
+pub mod bar {}
+
+/*
+ * Block comment with multiple trailing lines
+ *
+ *
+ */
+pub mod block {}

tests/target/comment_trailing_lines.rs

@@ -0,0 +1,14 @@
+// rustfmt-wrap_comments: true
+
+//! Module comment with multiple trailing lines
+
+/// Doc comment with multiple trailing lines
+pub mod foo {}
+
+/// Doc comment with one trailing line
+pub mod bar {}
+
+/*
+ * Block comment with multiple trailing lines
+ */
+pub mod block {}