Auto merge of #138927 - nnethercote:rearrange-Item-ItemInner, r=GuillaumeGomez

rustdoc: Rearrange `Item`/`ItemInner`.

The `Item` struct is 48 bytes and contains a `Box<ItemInner>`;
`ItemInner` is 104 bytes. This is an odd arrangement. Normally you'd
have one of the following.

- A single large struct, which avoids the allocation for the `Box`, but
  can result in lots of wasted space in unused parts of a container like
  `Vec<Item>`, `HashSet<Item>`, etc.

- Or, something like `struct Item(Box<ItemInner>)`, which requires the
  `Box` allocation but gives a very small Item size, which is good for
  containers like `Vec<Item>`.

`Item`/`ItemInner` currently gets the worst of both worlds: it always
requires a `Box`, but `Item` is also pretty big and so wastes space in
containers. It would make sense to push it in one direction or the
other. #138916 showed that the first option is a regression for rustdoc,
so this commit does the second option, which improves speed and reduces
memory usage.

r? `@GuillaumeGomez`

1 files changed, 0 insertions, 1 deletions

diff --git a/src/librustdoc/formats/cache.rs b/src/librustdoc/formats/cache.rs
index e74fd67fbda..19402004ed5 100644
--- a/src/librustdoc/formats/cache.rs
+++ b/src/librustdoc/formats/cache.rs
@@ -385,7 +385,6 @@ impl DocFolder for CacheBuilder<'_, '_> {
         // implementations elsewhere.
         let ret = if let clean::Item {
             inner: box clean::ItemInner { kind: clean::ImplItem(ref i), .. },
-            ..
         } = item
         {
             // Figure out the id of this impl. This may map to a