Box ItemKind::Impl to shrink the size of Item

Author: jyn514

src/librustdoc/clean/auto_trait.rs

@@ -126,7 +126,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                     stability: None,
                     const_stability: None,
                     deprecation: None,
-                    kind: ImplItem(Impl {
+                    kind: ImplItem(box Impl {
                         unsafety: hir::Unsafety::Normal,
                         generics: new_generics,
                         provided_trait_methods: Default::default(),

src/librustdoc/clean/blanket_impl.rs

@@ -115,7 +115,7 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                     stability: None,
                     const_stability: None,
                     deprecation: None,
-                    kind: ImplItem(Impl {
+                    kind: ImplItem(box Impl {
                         unsafety: hir::Unsafety::Normal,
                         generics: (
                             self.cx.tcx.generics_of(impl_def_id),

src/librustdoc/clean/inline.rs

@@ -438,7 +438,7 @@ crate fn build_impl(
     ret.push(clean::Item::from_def_id_and_parts(
         did,
         None,
-        clean::ImplItem(clean::Impl {
+        clean::ImplItem(box clean::Impl {
             unsafety: hir::Unsafety::Normal,
             generics,
             provided_trait_methods: provided,

src/librustdoc/clean/mod.rs

@@ -2088,7 +2088,7 @@ fn clean_impl(impl_: &hir::Item<'_>, cx: &DocContext<'_>) -> Vec<Item> {
         _ => None,
     });
     let make_item = |trait_: Option<Type>, for_: Type, items: Vec<Item>| {
-        let kind = ImplItem(Impl {
+        let kind = ImplItem(box Impl {
             unsafety,
             generics: generics.clean(cx),
             provided_trait_methods: provided.clone(),

src/librustdoc/clean/types.rs

@@ -313,7 +313,9 @@ crate enum ItemKind {
     ConstantItem(Constant),
     TraitItem(Trait),
     TraitAliasItem(TraitAlias),
-    ImplItem(Impl),
+    // Impl is 400 bytes (!!), so box it to avoid penalizing other items
+    // FIXME(jyn514): calculate this information on demand instead
+    ImplItem(Box<Impl>),
     /// A method signature only. Used for required methods in traits (ie,
     /// non-default-methods).
     TyMethodItem(Function),

src/librustdoc/json/conversions.rs

@@ -175,7 +175,7 @@ impl From<clean::ItemKind> for ItemEnum {
             TraitAliasItem(t) => ItemEnum::TraitAliasItem(t.into()),
             MethodItem(m, _) => ItemEnum::MethodItem(m.into()),
             TyMethodItem(m) => ItemEnum::MethodItem(m.into()),
-            ImplItem(i) => ItemEnum::ImplItem(i.into()),
+            ImplItem(box i) => ItemEnum::ImplItem(i.into()),
             StaticItem(s) => ItemEnum::StaticItem(s.into()),
             ForeignStaticItem(s) => ItemEnum::StaticItem(s.into()),
             ForeignTypeItem => ItemEnum::ForeignTypeItem,

src/librustdoc/passes/collect_trait_impls.rs

@@ -55,7 +55,7 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
 
     // scan through included items ahead of time to splice in Deref targets to the "valid" sets
     for it in &new_items {
-        if let ImplItem(Impl { ref for_, ref trait_, ref items, .. }) = it.kind {
+        if let ImplItem(box Impl { ref for_, ref trait_, ref items, .. }) = it.kind {
             if cleaner.keep_item(for_) && trait_.def_id() == cx.tcx.lang_items().deref_trait() {
                 let target = items
                     .iter()
@@ -75,7 +75,7 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
     }
 
     new_items.retain(|it| {
-        if let ImplItem(Impl { ref for_, ref trait_, ref blanket_impl, .. }) = it.kind {
+        if let ImplItem(box Impl { ref for_, ref trait_, ref blanket_impl, .. }) = it.kind {
             cleaner.keep_item(for_)
                 || trait_.as_ref().map_or(false, |t| cleaner.keep_item(t))
                 || blanket_impl.is_some()