rustdoc: Refactor Impl.{synthetic,blanket_impl} into enum

This change has two advantages:

1. It makes the possible states clearer, and it makes it impossible to
   construct invalid states, such as a blanket impl that is also an auto
   trait impl.

2. It shrinks the size of `Impl` a bit, since now there is only one
   field, rather than two.

src/librustdoc/html/format.rs

@@ -997,7 +997,7 @@ impl clean::Impl {
                 write!(f, " for ")?;
             }
 
-            if let Some(ref ty) = self.blanket_impl {
+            if let Some(ref ty) = self.blanket_impl_ty() {
                 fmt_type(ty, f, use_absolute, cx)?;
             } else {
                 fmt_type(&self.for_, f, use_absolute, cx)?;

src/librustdoc/html/render/mod.rs

@@ -1147,9 +1147,9 @@ fn render_assoc_items_inner(
         }
 
         let (synthetic, concrete): (Vec<&&Impl>, Vec<&&Impl>) =
-            traits.iter().partition(|t| t.inner_impl().synthetic);
+            traits.iter().partition(|t| t.inner_impl().is_auto_impl());
         let (blanket_impl, concrete): (Vec<&&Impl>, _) =
-            concrete.into_iter().partition(|t| t.inner_impl().blanket_impl.is_some());
+            concrete.into_iter().partition(|t| t.inner_impl().is_blanket_impl());
 
         let mut impls = Buffer::empty_from(w);
         render_impls(cx, &mut impls, &concrete, containing_item);
@@ -2058,10 +2058,9 @@ fn sidebar_assoc_items(cx: &Context<'_>, out: &mut Buffer, it: &clean::Item) {
             };
 
             let (synthetic, concrete): (Vec<&Impl>, Vec<&Impl>) =
-                v.iter().partition::<Vec<_>, _>(|i| i.inner_impl().synthetic);
-            let (blanket_impl, concrete): (Vec<&Impl>, Vec<&Impl>) = concrete
-                .into_iter()
-                .partition::<Vec<_>, _>(|i| i.inner_impl().blanket_impl.is_some());
+                v.iter().partition::<Vec<_>, _>(|i| i.inner_impl().is_auto_impl());
+            let (blanket_impl, concrete): (Vec<&Impl>, Vec<&Impl>) =
+                concrete.into_iter().partition::<Vec<_>, _>(|i| i.inner_impl().is_blanket_impl());
 
             let concrete_format = format_impls(concrete);
             let synthetic_format = format_impls(synthetic);

src/librustdoc/html/render/print_item.rs

@@ -746,7 +746,7 @@ fn item_trait(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Tra
         });
 
         let (mut synthetic, mut concrete): (Vec<&&Impl>, Vec<&&Impl>) =
-            local.iter().partition(|i| i.inner_impl().synthetic);
+            local.iter().partition(|i| i.inner_impl().is_auto_impl());
 
         synthetic.sort_by(|a, b| compare_impl(a, b, cx));
         concrete.sort_by(|a, b| compare_impl(a, b, cx));

src/librustdoc/html/render/write_shared.rs

@@ -585,7 +585,7 @@ pub(super) fn write_shared(
                 } else {
                     Some(Implementor {
                         text: imp.inner_impl().print(false, cx).to_string(),
-                        synthetic: imp.inner_impl().synthetic,
+                        synthetic: imp.inner_impl().is_auto_impl(),
                         types: collect_paths_for_type(imp.inner_impl().for_.clone(), cache),
                     })
                 }