Preserve comments when flattening use statements

Issue rust-lang#4991

src/imports.rs

@@ -610,13 +610,13 @@ impl UseTree {
                 let prefix = &self.path[..self.path.len() - 1];
                 let mut result = vec![];
                 for nested_use_tree in list {
-                    for flattend in &mut nested_use_tree.clone().flatten(import_granularity) {
+                    for mut flattend in nested_use_tree.clone().flatten(import_granularity) {
                         let mut new_path = prefix.to_vec();
                         new_path.append(&mut flattend.path);
                         result.push(UseTree {
                             path: new_path,
                             span: self.span,
-                            list_item: None,
+                            list_item: flattend.list_item,
                             visibility: self.visibility.clone(),
                             // only retain attributes for `ImportGranularity::Item`
                             attrs: match import_granularity {

tests/source/imports_granularity_item.rs

@@ -1,6 +1,14 @@
 // rustfmt-imports_granularity: Item
 
-use a::{b, c, d};
+use a::{
+    b,
+    c,
+    // Comment before a::d
+    d,
+    e, // Comment after a::e
+};
 use a::{f::g, h::{i, j}};
 use a::{l::{self, m, n::o, p::*}};
 use a::q::{self};
+// Comment before a::r
+use a::r;

tests/target/imports_granularity_item.rs

@@ -2,7 +2,9 @@
 
 use a::b;
 use a::c;
+// Comment before a::d
 use a::d;
+use a::e; // Comment after a::e
 use a::f::g;
 use a::h::i;
 use a::h::j;
@@ -11,3 +13,5 @@ use a::l::n::o;
 use a::l::p::*;
 use a::l::{self};
 use a::q::{self};
+// Comment before a::r
+use a::r;

tests/target/imports_granularity_module.rs

@@ -17,6 +17,8 @@ use foo::e;
 #[cfg(test)]
 use foo::{a::b, c::d};
 
+// comment
 use bar::a::b;
+// more comment
 use bar::c::d;
 use bar::e::f;