rustdoc: generate implementors for all auto traits

Previously we would only generate a list of synthetic implementations
for two well known traits – Send and Sync. With this patch all the auto
traits known to rustc are considered. This includes such traits like
Unpin and user’s own traits.

Sadly the implementation still iterates through the list of crate items
and checks them against the traits, which for non-std crates containing
their own auto-traits will still not include types defined in std/core.

It is an improvement nontheless.

src/test/rustdoc/empty-section.rs

@@ -8,3 +8,6 @@ pub struct Foo;
 // @!has - 'Auto Trait Implementations'
 impl !Send for Foo {}
 impl !Sync for Foo {}
+impl !std::marker::Unpin for Foo {}
+impl !std::panic::RefUnwindSafe for Foo {}
+impl !std::panic::UnwindSafe for Foo {}

src/test/rustdoc/issue-50159.rs

@@ -14,7 +14,7 @@ impl<B, C> Signal2 for B where B: Signal<Item = C> {
 // @has - '//code' 'impl<B> Send for Switch<B> where <B as Signal>::Item: Send'
 // @has - '//code' 'impl<B> Sync for Switch<B> where <B as Signal>::Item: Sync'
 // @count - '//*[@id="implementations-list"]/*[@class="impl"]' 0
-// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 2
+// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 5
 pub struct Switch<B: Signal> {
     pub inner: <B as Signal2>::Item2,
 }

src/test/rustdoc/synthetic_auto/basic.rs

@@ -2,7 +2,7 @@
 // @has - '//code' 'impl<T> Send for Foo<T> where T: Send'
 // @has - '//code' 'impl<T> Sync for Foo<T> where T: Sync'
 // @count - '//*[@id="implementations-list"]/*[@class="impl"]' 0
-// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 2
+// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 5
 pub struct Foo<T> {
     field: T,
 }

src/test/rustdoc/synthetic_auto/crate-local.rs

@@ -0,0 +1,9 @@
+#![feature(optin_builtin_traits)]
+
+pub auto trait Banana {}
+
+// @has crate_local/struct.Peach.html
+// @has - '//code' 'impl Banana for Peach'
+// @has - '//code' 'impl Send for Peach'
+// @has - '//code' 'impl Sync for Peach'
+pub struct Peach;

src/test/rustdoc/synthetic_auto/manual.rs

@@ -6,7 +6,7 @@
 // 'impl<T> Send for Foo<T>'
 //
 // @count - '//*[@id="implementations-list"]/*[@class="impl"]' 1
-// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 1
+// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 4
 pub struct Foo<T> {
     field: T,
 }