Fixes #1184 where reordering import items could cause a panic.

Also extends an existing test to avoid regression.

src/imports.rs

@@ -158,7 +158,10 @@ pub fn format_imports(&mut self, use_items: &[ptr::P<ast::Item>]) {
         // Find the location immediately before the first use item in the run. This must not lie
         // before the current `self.last_pos`
         let pos_before_first_use_item = use_items.first()
-            .map(|p_i| cmp::max(self.last_pos, p_i.span.lo))
+            .map(|p_i| {
+                cmp::max(self.last_pos,
+                         p_i.attrs.iter().map(|attr| attr.span.lo).min().unwrap_or(p_i.span.lo))
+            })
             .unwrap_or(self.last_pos);
         // Construct a list of pairs, each containing a `use` item and the start of span before
         // that `use` item.

tests/source/imports-reorder-lines-and-items.rs

@@ -1,6 +1,7 @@
 // rustfmt-reorder_imports: true
 // rustfmt-reorder_imported_names: true
 
+/// This comment should stay with `use std::str;`
 use std::str;
 use std::cmp::{d, c, b, a};
 use std::ddd::aaa;

tests/target/imports-reorder-lines-and-items.rs

@@ -6,4 +6,5 @@
 use std::ddd::aaa;
 // This comment should stay with `use std::ddd:bbb;`
 use std::ddd::bbb;
+/// This comment should stay with `use std::str;`
 use std::str;