Auto merge of #88552 - nbdd0121:vtable, r=nagisa

Stop allocating vtable entries for non-object-safe methods

Current a vtable entry is allocated for all associated fns, even if the method is not object-safe: https://godbolt.org/z/h7vx6f35T

As a result, each vtable for `Iterator`' currently consumes 74 `usize`s. This PR stops allocating vtable entries for those methods, reducing vtable size of each `Iterator` vtable to 7 `usize`s.

Note that this PR introduces will cause more invocations of `is_vtable_safe_method`. So a perf run might be needed. If result isn't favorable then we might need to query-ify `is_vtable_safe_method`.

4 files changed, 41 insertions, 4 deletions

diff --git a/src/test/ui/traits/vtable/vtable-non-object-safe.rs b/src/test/ui/traits/vtable/vtable-non-object-safe.rs
new file mode 100644
index 00000000000..45b6a8a98a7
--- /dev/null
+++ b/src/test/ui/traits/vtable/vtable-non-object-safe.rs
@@ -0,0 +1,18 @@
+// build-fail
+#![feature(rustc_attrs)]
+
+// Ensure that non-object-safe methods in Iterator does not generate
+// vtable entries.
+
+#[rustc_dump_vtable]
+trait A: Iterator {}
+//~^ error Vtable
+
+impl<T> A for T where T: Iterator {}
+
+fn foo(_a: &mut dyn A<Item=u8>) {
+}
+
+fn main() {
+    foo(&mut vec![0, 1, 2, 3].into_iter());
+}
diff --git a/src/test/ui/traits/vtable/vtable-non-object-safe.stderr b/src/test/ui/traits/vtable/vtable-non-object-safe.stderr
new file mode 100644
index 00000000000..f3175b805d1
--- /dev/null
+++ b/src/test/ui/traits/vtable/vtable-non-object-safe.stderr
@@ -0,0 +1,16 @@
+error: Vtable entries for `<std::vec::IntoIter<u8> as A>`: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(<std::vec::IntoIter<u8> as Iterator>::next),
+    Method(<std::vec::IntoIter<u8> as Iterator>::size_hint),
+    Method(<std::vec::IntoIter<u8> as Iterator>::advance_by),
+    Method(<std::vec::IntoIter<u8> as Iterator>::nth),
+]
+  --> $DIR/vtable-non-object-safe.rs:8:1
+   |
+LL | trait A: Iterator {}
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/traits/vtable/vtable-vacant.rs b/src/test/ui/traits/vtable/vtable-vacant.rs
index ebea94171f2..429ce523799 100644
--- a/src/test/ui/traits/vtable/vtable-vacant.rs
+++ b/src/test/ui/traits/vtable/vtable-vacant.rs
@@ -1,22 +1,25 @@
 // build-fail
 #![feature(rustc_attrs)]
+#![feature(negative_impls)]
+#![allow(where_clauses_object_safety)]
 
 // B --> A
 
 #[rustc_dump_vtable]
 trait A {
     fn foo_a1(&self) {}
-    fn foo_a2(&self) where Self: Sized {}
+    fn foo_a2(&self) where Self: Send {}
 }
 
 #[rustc_dump_vtable]
 trait B: A {
     //~^ error Vtable
     fn foo_b1(&self) {}
-    fn foo_b2() where Self: Sized {}
+    fn foo_b2(&self) where Self: Send {}
 }
 
 struct S;
+impl !Send for S {}
 
 impl A for S {}
 impl B for S {}
diff --git a/src/test/ui/traits/vtable/vtable-vacant.stderr b/src/test/ui/traits/vtable/vtable-vacant.stderr
index 768cca52689..f5cd36264fc 100644
--- a/src/test/ui/traits/vtable/vtable-vacant.stderr
+++ b/src/test/ui/traits/vtable/vtable-vacant.stderr
@@ -7,12 +7,12 @@ error: Vtable entries for `<S as B>`: [
     Method(<S as B>::foo_b1),
     Vacant,
 ]
-  --> $DIR/vtable-vacant.rs:13:1
+  --> $DIR/vtable-vacant.rs:15:1
    |
 LL | / trait B: A {
 LL | |
 LL | |     fn foo_b1(&self) {}
-LL | |     fn foo_b2() where Self: Sized {}
+LL | |     fn foo_b2(&self) where Self: Send {}
 LL | | }
    | |_^