Auto merge of #80014 - jyn514:box-item-kind, r=nnethercote

[rustdoc] Box ItemKind to reduce the size of `Item`

This brings the size of `Item` from

```
[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 536
```

to

```
[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 136
```

This is an alternative to #79967; I don't think it makes sense to make both changes.

Helps with #79103.

Author: bors

src/librustdoc/clean/auto_trait.rs

@@ -123,7 +123,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                     attrs: Default::default(),
                     visibility: Inherited,
                     def_id: self.cx.next_def_id(param_env_def_id.krate),
-                    kind: ImplItem(Impl {
+                    kind: box ImplItem(Impl {
                         unsafety: hir::Unsafety::Normal,
                         generics: new_generics,
                         provided_trait_methods: Default::default(),

src/librustdoc/clean/blanket_impl.rs

@@ -112,7 +112,7 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                     attrs: Default::default(),
                     visibility: Inherited,
                     def_id: self.cx.next_def_id(impl_def_id.krate),
-                    kind: ImplItem(Impl {
+                    kind: box ImplItem(Impl {
                         unsafety: hir::Unsafety::Normal,
                         generics: (
                             self.cx.tcx.generics_of(impl_def_id),

src/librustdoc/clean/inline.rs

@@ -482,7 +482,7 @@ fn build_module(cx: &DocContext<'_>, did: DefId, visited: &mut FxHashSet<DefId>)
                         source: clean::Span::dummy(),
                         def_id: DefId::local(CRATE_DEF_INDEX),
                         visibility: clean::Public,
-                        kind: clean::ImportItem(clean::Import::new_simple(
+                        kind: box clean::ImportItem(clean::Import::new_simple(
                             item.ident.name,
                             clean::ImportSource {
                                 path: clean::Path {

src/librustdoc/clean/mod.rs

@@ -2160,7 +2160,7 @@ fn clean_extern_crate(
         source: krate.span.clean(cx),
         def_id: crate_def_id,
         visibility: krate.vis.clean(cx),
-        kind: ExternCrateItem(name, orig_name),
+        kind: box ExternCrateItem(name, orig_name),
     }]
 }
 
@@ -2228,7 +2228,7 @@ impl Clean<Vec<Item>> for doctree::Import<'_> {
                         source: self.span.clean(cx),
                         def_id: cx.tcx.hir().local_def_id(self.id).to_def_id(),
                         visibility: self.vis.clean(cx),
-                        kind: ImportItem(Import::new_simple(
+                        kind: box ImportItem(Import::new_simple(
                             self.name,
                             resolve_use_source(cx, path),
                             false,
@@ -2246,7 +2246,7 @@ impl Clean<Vec<Item>> for doctree::Import<'_> {
             source: self.span.clean(cx),
             def_id: cx.tcx.hir().local_def_id(self.id).to_def_id(),
             visibility: self.vis.clean(cx),
-            kind: ImportItem(inner),
+            kind: box ImportItem(inner),
         }]
     }
 }

src/librustdoc/clean/types.rs

@@ -84,10 +84,14 @@ crate struct Item {
     crate name: Option<Symbol>,
     crate attrs: Attributes,
     crate visibility: Visibility,
-    crate kind: ItemKind,
+    crate kind: Box<ItemKind>,
     crate def_id: DefId,
 }
 
+// `Item` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(target_arch = "x86_64")]
+rustc_data_structures::static_assert_size!(Item, 136);
+
 impl fmt::Debug for Item {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let def_id: &dyn fmt::Debug = if self.is_fake() { &"**FAKE**" } else { &self.def_id };
@@ -152,7 +156,7 @@ impl Item {
 
         Item {
             def_id,
-            kind,
+            kind: box kind,
             name,
             source: source.clean(cx),
             attrs: cx.tcx.get_attrs(def_id).clean(cx),
@@ -171,7 +175,7 @@ impl Item {
     }
 
     crate fn is_crate(&self) -> bool {
-        match self.kind {
+        match *self.kind {
             StrippedItem(box ModuleItem(Module { is_crate: true, .. }))
             | ModuleItem(Module { is_crate: true, .. }) => true,
             _ => false,
@@ -223,14 +227,14 @@ impl Item {
         self.type_() == ItemType::Keyword
     }
     crate fn is_stripped(&self) -> bool {
-        match self.kind {
+        match *self.kind {
             StrippedItem(..) => true,
             ImportItem(ref i) => !i.should_be_displayed,
             _ => false,
         }
     }
     crate fn has_stripped_fields(&self) -> Option<bool> {
-        match self.kind {
+        match *self.kind {
             StructItem(ref _struct) => Some(_struct.fields_stripped),
             UnionItem(ref union) => Some(union.fields_stripped),
             VariantItem(Variant { kind: VariantKind::Struct(ref vstruct) }) => {
@@ -281,7 +285,7 @@ impl Item {
     }
 
     crate fn is_default(&self) -> bool {
-        match self.kind {
+        match *self.kind {
             ItemKind::MethodItem(_, Some(defaultness)) => {
                 defaultness.has_value() && !defaultness.is_final()
             }

src/librustdoc/clean/utils.rs

@@ -43,7 +43,7 @@ crate fn krate(mut cx: &mut DocContext<'_>) -> Crate {
     let mut module = module.clean(cx);
     let mut masked_crates = FxHashSet::default();
 
-    match module.kind {
+    match *module.kind {
         ItemKind::ModuleItem(ref module) => {
             for it in &module.items {
                 // `compiler_builtins` should be masked too, but we can't apply
@@ -61,7 +61,7 @@ crate fn krate(mut cx: &mut DocContext<'_>) -> Crate {
 
     let ExternalCrate { name, src, primitives, keywords, .. } = LOCAL_CRATE.clean(cx);
     {
-        let m = match module.kind {
+        let m = match *module.kind {
             ItemKind::ModuleItem(ref mut m) => m,
             _ => unreachable!(),
         };
@@ -338,7 +338,7 @@ crate fn build_deref_target_impls(cx: &DocContext<'_>, items: &[Item], ret: &mut
     let tcx = cx.tcx;
 
     for item in items {
-        let target = match item.kind {
+        let target = match *item.kind {
             ItemKind::TypedefItem(ref t, true) => &t.type_,
             _ => continue,
         };

src/librustdoc/fold.rs

@@ -5,9 +5,9 @@ crate struct StripItem(pub Item);
 impl StripItem {
     crate fn strip(self) -> Option<Item> {
         match self.0 {
-            Item { kind: StrippedItem(..), .. } => Some(self.0),
+            Item { kind: box StrippedItem(..), .. } => Some(self.0),
             mut i => {
-                i.kind = StrippedItem(box i.kind);
+                i.kind = box StrippedItem(i.kind);
                 Some(i)
             }
         }
@@ -72,9 +72,9 @@ crate trait DocFolder: Sized {
 
     /// don't override!
     fn fold_item_recur(&mut self, mut item: Item) -> Item {
-        item.kind = match item.kind {
+        item.kind = box match *item.kind {
             StrippedItem(box i) => StrippedItem(box self.fold_inner_recur(i)),
-            _ => self.fold_inner_recur(item.kind),
+            _ => self.fold_inner_recur(*item.kind),
         };
         item
     }

src/librustdoc/formats/cache.rs

@@ -219,7 +219,7 @@ impl DocFolder for Cache {
 
         // If this is a stripped module,
         // we don't want it or its children in the search index.
-        let orig_stripped_mod = match item.kind {
+        let orig_stripped_mod = match *item.kind {
             clean::StrippedItem(box clean::ModuleItem(..)) => {
                 mem::replace(&mut self.stripped_mod, true)
             }
@@ -228,7 +228,7 @@ impl DocFolder for Cache {
 
         // If the impl is from a masked crate or references something from a
         // masked crate then remove it completely.
-        if let clean::ImplItem(ref i) = item.kind {
+        if let clean::ImplItem(ref i) = *item.kind {
             if self.masked_crates.contains(&item.def_id.krate)
                 || i.trait_.def_id().map_or(false, |d| self.masked_crates.contains(&d.krate))
                 || i.for_.def_id().map_or(false, |d| self.masked_crates.contains(&d.krate))
@@ -239,12 +239,12 @@ impl DocFolder for Cache {
 
         // Propagate a trait method's documentation to all implementors of the
         // trait.
-        if let clean::TraitItem(ref t) = item.kind {
+        if let clean::TraitItem(ref t) = *item.kind {
             self.traits.entry(item.def_id).or_insert_with(|| t.clone());
         }
 
         // Collect all the implementors of traits.
-        if let clean::ImplItem(ref i) = item.kind {
+        if let clean::ImplItem(ref i) = *item.kind {
             if let Some(did) = i.trait_.def_id() {
                 if i.blanket_impl.is_none() {
                     self.implementors
@@ -257,7 +257,7 @@ impl DocFolder for Cache {
 
         // Index this method for searching later on.
         if let Some(ref s) = item.name {
-            let (parent, is_inherent_impl_item) = match item.kind {
+            let (parent, is_inherent_impl_item) = match *item.kind {
                 clean::StrippedItem(..) => ((None, None), false),
                 clean::AssocConstItem(..) | clean::TypedefItem(_, true)
                     if self.parent_is_trait_impl =>
@@ -348,7 +348,7 @@ impl DocFolder for Cache {
             _ => false,
         };
 
-        match item.kind {
+        match *item.kind {
             clean::StructItem(..)
             | clean::EnumItem(..)
             | clean::TypedefItem(..)
@@ -387,7 +387,7 @@ impl DocFolder for Cache {
 
         // Maintain the parent stack
         let orig_parent_is_trait_impl = self.parent_is_trait_impl;
-        let parent_pushed = match item.kind {
+        let parent_pushed = match *item.kind {
             clean::TraitItem(..)
             | clean::EnumItem(..)
             | clean::ForeignTypeItem
@@ -425,38 +425,33 @@ impl DocFolder for Cache {
         // Once we've recursively found all the generics, hoard off all the
         // implementations elsewhere.
         let item = self.fold_item_recur(item);
-        let ret = if let clean::Item { kind: clean::ImplItem(_), .. } = item {
+        let ret = if let clean::Item { kind: box clean::ImplItem(ref i), .. } = item {
             // Figure out the id of this impl. This may map to a
             // primitive rather than always to a struct/enum.
             // Note: matching twice to restrict the lifetime of the `i` borrow.
             let mut dids = FxHashSet::default();
-            if let clean::Item { kind: clean::ImplItem(ref i), .. } = item {
-                match i.for_ {
-                    clean::ResolvedPath { did, .. }
-                    | clean::BorrowedRef { type_: box clean::ResolvedPath { did, .. }, .. } => {
-                        dids.insert(did);
-                    }
-                    ref t => {
-                        let did = t
-                            .primitive_type()
-                            .and_then(|t| self.primitive_locations.get(&t).cloned());
+            match i.for_ {
+                clean::ResolvedPath { did, .. }
+                | clean::BorrowedRef { type_: box clean::ResolvedPath { did, .. }, .. } => {
+                    dids.insert(did);
+                }
+                ref t => {
+                    let did =
+                        t.primitive_type().and_then(|t| self.primitive_locations.get(&t).cloned());
 
-                        if let Some(did) = did {
-                            dids.insert(did);
-                        }
+                    if let Some(did) = did {
+                        dids.insert(did);
                     }
                 }
+            }
 
-                if let Some(generics) = i.trait_.as_ref().and_then(|t| t.generics()) {
-                    for bound in generics {
-                        if let Some(did) = bound.def_id() {
-                            dids.insert(did);
-                        }
+            if let Some(generics) = i.trait_.as_ref().and_then(|t| t.generics()) {
+                for bound in generics {
+                    if let Some(did) = bound.def_id() {
+                        dids.insert(did);
                     }
                 }
-            } else {
-                unreachable!()
-            };
+            }
             let impl_item = Impl { impl_item: item };
             if impl_item.trait_did().map_or(true, |d| self.traits.contains_key(&d)) {
                 for did in dids {

src/librustdoc/formats/item_type.rs

@@ -60,7 +60,7 @@ impl Serialize for ItemType {
 
 impl<'a> From<&'a clean::Item> for ItemType {
     fn from(item: &'a clean::Item) -> ItemType {
-        let kind = match item.kind {
+        let kind = match *item.kind {
             clean::StrippedItem(box ref item) => item,
             ref kind => kind,
         };

src/librustdoc/formats/mod.rs

@@ -32,7 +32,7 @@ crate struct Impl {
 
 impl Impl {
     crate fn inner_impl(&self) -> &clean::Impl {
-        match self.impl_item.kind {
+        match *self.impl_item.kind {
             clean::ImplItem(ref impl_) => impl_,
             _ => panic!("non-impl item found in impl"),
         }

src/librustdoc/formats/renderer.rs

@@ -89,7 +89,7 @@ crate fn run_format<'tcx, T: FormatRenderer<'tcx>>(
             }
 
             cx.mod_item_in(&item, &name, &cache)?;
-            let module = match item.kind {
+            let module = match *item.kind {
                 clean::StrippedItem(box clean::ModuleItem(m)) | clean::ModuleItem(m) => m,
                 _ => unreachable!(),
             };

src/librustdoc/html/render/cache.rs

@@ -165,7 +165,7 @@ crate fn build_index(krate: &clean::Crate, cache: &mut Cache) -> String {
 }
 
 crate fn get_index_search_type(item: &clean::Item) -> Option<IndexItemFunctionType> {
-    let (all_types, ret_types) = match item.kind {
+    let (all_types, ret_types) = match *item.kind {
         clean::FunctionItem(ref f) => (&f.all_types, &f.ret_types),
         clean::MethodItem(ref m, _) => (&m.all_types, &m.ret_types),
         clean::TyMethodItem(ref m) => (&m.all_types, &m.ret_types),