compiler suggests "&mut mut"

[matthiaskrgr]

I was messing around with this function

    fn build_sidebar_items(&self, m: &clean::Module) -> BTreeMap<String, Vec<NameDoc>> {
        // BTreeMap instead of HashMap to get a sorted output
        let mut map: BTreeMap<_, Vec<_>> = BTreeMap::new();
        for item in &m.items {
            if item.is_stripped() {
                continue;
            }

            let short = item.type_();
            let myname = match item.name {
                None => continue,
                Some(ref s) => s.to_string(),
            };
            let short = short.to_string();
            map.entry(short)
                .or_default()
                .push((myname, Some(plain_summary_line(item.doc_value()))));
        }

        if self.shared.sort_modules_alphabetically {
            for (_, items) in &mut map {
                items.sort();
            }
        }
        map
    }

After I changed the last block to

        if self.shared.sort_modules_alphabetically {
            for items in &mut map.values() {
                items.sort();
            }
        }

--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -1542,7 +1542,7 @@ impl Context {
         }
 
         if self.shared.sort_modules_alphabetically {
-            for (_, items) in &mut map {
+            for items in &mut map.values() {
                 items.sort();
             }
         }

The compiler gave this interesting but wrong suggestion: &mut mut map.values()

error[E0596]: cannot borrow `*items` as mutable, as it is behind a `&` reference
    --> src/librustdoc/html/render.rs:1546:17
     |
1545 |             for items in &mut map.values() {
     |                          ----------------- help: consider changing this to be a mutable reference: `&mut mut map.values()`
1546 |                 items.sort();
     |                 ^^^^^ `items` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.

which does not compiler obviously 😄

The correct solution would be to use &mut map.values_mut():

    if self.shared.sort_modules_alphabetically {
            for items in &mut map.values_mut() {
                items.sort();
            }
        }
```

[CAD97]

Just a minor note: typically these iterators are iterated by-value (e.g. for items in map.values()), so this explains why the diagnostics issue was missed. This gives a less bad error message:

error[E0596]: cannot borrow `*items` as mutable, as it is behind a `&` reference
  --> src/main.rs:10:9
   |
9  |     for items in map.values() {
   |         ----- help: consider changing this to be a mutable reference: `&mut std::vec::Vec<u8>`
10 |         items.sort();
   |         ^^^^^ `items` is a `&` reference, so the data it refers to cannot be borrowed as mutable