rustdoc: Box ty field of GenericParamDefKind::Const

This cuts the size of `GenericParamDef` in half, from 104 bytes to 56
bytes. I think the extra indirection should be worth the size savings.

Author: camelid

Diff for: src/librustdoc/clean/mod.rs

@@ -430,7 +430,7 @@ impl Clean<GenericParamDef> for ty::GenericParamDef {
                 self.name,
                 GenericParamDefKind::Const {
                     did: self.def_id,
-                    ty: cx.tcx.type_of(self.def_id).clean(cx),
+                    ty: Box::new(cx.tcx.type_of(self.def_id).clean(cx)),
                     default: match has_default {
                         true => Some(Box::new(cx.tcx.const_param_default(self.def_id).to_string())),
                         false => None,
@@ -470,7 +470,7 @@ impl Clean<GenericParamDef> for hir::GenericParam<'_> {
                 self.name.ident().name,
                 GenericParamDefKind::Const {
                     did: cx.tcx.hir().local_def_id(self.hir_id).to_def_id(),
-                    ty: ty.clean(cx),
+                    ty: Box::new(ty.clean(cx)),
                     default: default.map(|ct| {
                         let def_id = cx.tcx.hir().local_def_id(ct.hir_id);
                         Box::new(ty::Const::from_anon_const(cx.tcx, def_id).to_string())

Diff for: src/librustdoc/clean/types.rs

@@ -1224,7 +1224,7 @@ crate enum GenericParamDefKind {
     },
     Const {
         did: DefId,
-        ty: Type,
+        ty: Box<Type>,
         default: Option<Box<String>>,
     },
 }
@@ -1240,7 +1240,7 @@ impl GenericParamDefKind {
     crate fn get_type(&self) -> Option<Type> {
         match self {
             GenericParamDefKind::Type { default, .. } => default.as_deref().cloned(),
-            GenericParamDefKind::Const { ty, .. } => Some(ty.clone()),
+            GenericParamDefKind::Const { ty, .. } => Some((&**ty).clone()),
             GenericParamDefKind::Lifetime { .. } => None,
         }
     }
@@ -1254,7 +1254,7 @@ crate struct GenericParamDef {
 
 // `GenericParamDef` is used in many places. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(GenericParamDef, 104);
+rustc_data_structures::static_assert_size!(GenericParamDef, 56);
 
 impl GenericParamDef {
     crate fn is_synthetic_type_param(&self) -> bool {

Diff for: src/librustdoc/json/conversions.rs

@@ -333,7 +333,7 @@ impl FromWithTcx<clean::GenericParamDefKind> for GenericParamDefKind {
                 default: default.map(|x| (*x).into_tcx(tcx)),
             },
             Const { did: _, ty, default } => {
-                GenericParamDefKind::Const { ty: ty.into_tcx(tcx), default: default.map(|x| *x) }
+                GenericParamDefKind::Const { ty: (*ty).into_tcx(tcx), default: default.map(|x| *x) }
             }
         }
     }