Retain trailing separator when extracting the last inline post comment

Fixes 5042

Previously, trailing commas were removed from the last inline comment.
This lead to rustfmt refusing to format code snippets because
the original comment did not match the rewritten comment.

Now, when rustfmt extracts the last inline comment it leaves trailing
separators alone. Rustfmt does not need to remove these separators
because they are commented out.

src/lists.rs

@@ -611,15 +611,30 @@ pub(crate) fn extract_post_comment(
     post_snippet: &str,
     comment_end: usize,
     separator: &str,
+    is_last: bool,
 ) -> Option<String> {
     let white_space: &[_] = &[' ', '\t'];
 
     // Cleanup post-comment: strip separators and whitespace.
     let post_snippet = post_snippet[..comment_end].trim();
+
+    let last_inline_comment_ends_with_separator = if is_last {
+        if let Some(line) = post_snippet.lines().last() {
+            line.ends_with(separator) && line.trim().starts_with("//")
+        } else {
+            false
+        }
+    } else {
+        false
+    };
+
     let post_snippet_trimmed = if post_snippet.starts_with(|c| c == ',' || c == ':') {
         post_snippet[1..].trim_matches(white_space)
     } else if let Some(stripped) = post_snippet.strip_prefix(separator) {
         stripped.trim_matches(white_space)
+    } else if last_inline_comment_ends_with_separator {
+        // since we're on the last item it's fine to keep any trailing separators in comments
+        post_snippet.trim_matches(white_space)
     }
     // not comment or over two lines
     else if post_snippet.ends_with(',')
@@ -748,14 +763,12 @@ where
                 .snippet_provider
                 .span_to_snippet(mk_sp((self.get_hi)(&item), next_start))
                 .unwrap_or("");
-            let comment_end = get_comment_end(
-                post_snippet,
-                self.separator,
-                self.terminator,
-                self.inner.peek().is_none(),
-            );
+            let is_last = self.inner.peek().is_none();
+            let comment_end =
+                get_comment_end(post_snippet, self.separator, self.terminator, is_last);
             let new_lines = has_extra_newline(post_snippet, comment_end);
-            let post_comment = extract_post_comment(post_snippet, comment_end, self.separator);
+            let post_comment =
+                extract_post_comment(post_snippet, comment_end, self.separator, is_last);
 
             self.prev_span_end = (self.get_hi)(&item) + BytePos(comment_end as u32);
 

tests/source/issue-5042/multi-line_comment_with_trailing_comma.rs

@@ -0,0 +1,24 @@
+fn main() {
+    // 5042 deals with trailing commas, not the indentation issue of these comments
+    // When a future PR fixes the inentation issues these test can be updated
+    let _ = std::ops::Add::add(10, 20
+        // ...
+        // ...,
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        /* ... */
+        // ...,
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        // ...,
+        // ...,
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        // ...,
+        /* ...
+        */,
+        );
+}

tests/source/issue-5042/multi-line_comment_without_trailing_comma.rs

@@ -0,0 +1,24 @@
+fn main() {
+    // 5042 deals with trailing commas, not the indentation issue of these comments
+    // When a future PR fixes the inentation issues these test can be updated
+    let _ = std::ops::Add::add(10, 20
+        // ...
+        // ...
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        /* ... */
+        // ...
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        // ...
+        // ...
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        // ...
+        /* ...
+        */
+        );
+}

tests/source/issue-5042/single-line_comment_with_trailing_comma.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let _ = std::ops::Add::add(10, 20
+        // ...,
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        /* ... */,
+        );
+}

tests/source/issue-5042/single-line_comment_without_trailing_comma.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let _ = std::ops::Add::add(10, 20
+        // ...
+        );
+
+    let _ = std::ops::Add::add(10, 20
+        /* ... */
+        );
+}
+

tests/target/issue-5042/multi-line_comment_with_trailing_comma.rs

@@ -0,0 +1,24 @@
+fn main() {
+    // 5042 deals with trailing commas, not the indentation issue of these comments
+    // When a future PR fixes the inentation issues these test can be updated
+    let _ = std::ops::Add::add(
+        10, 20, // ...
+           // ...,
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, /* ... */
+           // ...,
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, // ...,
+           // ...,
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, // ...,
+           /* ...
+            */
+    );
+}

tests/target/issue-5042/multi-line_comment_without_trailing_comma.rs

@@ -0,0 +1,24 @@
+fn main() {
+    // 5042 deals with trailing commas, not the indentation issue of these comments
+    // When a future PR fixes the inentation issues these test can be updated
+    let _ = std::ops::Add::add(
+        10, 20, // ...
+           // ...
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, /* ... */
+           // ...
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, // ...
+           // ...
+    );
+
+    let _ = std::ops::Add::add(
+        10, 20, // ...
+           /* ...
+            */
+    );
+}

tests/target/issue-5042/single-line_comment_with_trailing_comma.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let _ = std::ops::Add::add(
+        10, 20, // ...,
+    );
+
+    let _ = std::ops::Add::add(10, 20 /* ... */);
+}

tests/target/issue-5042/single-line_comment_without_trailing_comma.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let _ = std::ops::Add::add(
+        10, 20, // ...
+    );
+
+    let _ = std::ops::Add::add(10, 20 /* ... */);
+}