Don't align comments on extern crates

Closes rust-lang#3128

Author: nrc

Diff for: src/comment.rs

@@ -163,6 +163,14 @@ pub fn combine_strs_with_missing_comments(
     shape: Shape,
     allow_extend: bool,
 ) -> Option<String> {
+    trace!(
+        "combine_strs_with_missing_comments `{}` `{}` {:?} {:?}",
+        prev_str,
+        next_str,
+        span,
+        shape
+    );
+
     let mut result =
         String::with_capacity(prev_str.len() + next_str.len() + shape.indent.width() + 128);
     result.push_str(prev_str);

Diff for: src/lists.rs

@@ -36,6 +36,8 @@ pub struct ListFormatting<'a> {
     preserve_newline: bool,
     // Nested import lists get some special handling for the "Mixed" list type
     nested: bool,
+    // Whether comments should be visually aligned.
+    align_comments: bool,
     config: &'a Config,
 }
 
@@ -50,6 +52,7 @@ impl<'a> ListFormatting<'a> {
             ends_with_newline: true,
             preserve_newline: false,
             nested: false,
+            align_comments: true,
             config,
         }
     }
@@ -89,6 +92,11 @@ impl<'a> ListFormatting<'a> {
         self
     }
 
+    pub fn align_comments(mut self, align_comments: bool) -> Self {
+        self.align_comments = align_comments;
+        self
+    }
+
     pub fn needs_trailing_separator(&self) -> bool {
         match self.trailing_separator {
             // We always put separator in front.
@@ -465,23 +473,31 @@ where
             let mut formatted_comment = rewrite_post_comment(&mut item_max_width)?;
 
             if !starts_with_newline(comment) {
-                let mut comment_alignment =
-                    post_comment_alignment(item_max_width, inner_item.len());
-                if first_line_width(&formatted_comment)
-                    + last_line_width(&result)
-                    + comment_alignment
-                    + 1
-                    > formatting.config.max_width()
-                {
-                    item_max_width = None;
-                    formatted_comment = rewrite_post_comment(&mut item_max_width)?;
-                    comment_alignment = post_comment_alignment(item_max_width, inner_item.len());
-                }
-                for _ in 0..=comment_alignment {
-                    result.push(' ');
+                if formatting.align_comments {
+                    let mut comment_alignment =
+                        post_comment_alignment(item_max_width, inner_item.len());
+                    if first_line_width(&formatted_comment)
+                        + last_line_width(&result)
+                        + comment_alignment
+                        + 1
+                        > formatting.config.max_width()
+                    {
+                        item_max_width = None;
+                        formatted_comment = rewrite_post_comment(&mut item_max_width)?;
+                        comment_alignment =
+                            post_comment_alignment(item_max_width, inner_item.len());
+                    }
+                    for _ in 0..=comment_alignment {
+                        result.push(' ');
+                    }
                 }
-                // An additional space for the missing trailing separator.
-                if last && item_max_width.is_some() && !separate && !formatting.separator.is_empty()
+                // An additional space for the missing trailing separator (or
+                // if we skipped alignment above).
+                if !formatting.align_comments
+                    || (last
+                        && item_max_width.is_some()
+                        && !separate
+                        && !formatting.separator.is_empty())
                 {
                     result.push(' ');
                 }
@@ -902,6 +918,7 @@ pub fn struct_lit_formatting<'a>(
         ends_with_newline,
         preserve_newline: true,
         nested: false,
+        align_comments: true,
         config: context.config,
     }
 }

Diff for: src/reorder.rs

@@ -69,7 +69,9 @@ fn wrap_reorderable_items(
     list_items: &[ListItem],
     shape: Shape,
 ) -> Option<String> {
-    let fmt = ListFormatting::new(shape, context.config).separator("");
+    let fmt = ListFormatting::new(shape, context.config)
+        .separator("")
+        .align_comments(false);
     write_list(list_items, &fmt)
 }