trait_sel: {Meta,Pointee}Sized on Sized types

Introduce the `MetaSized` and `PointeeSized` traits as supertraits of
`Sized` and initially implement it on everything that currently
implements `Sized` to isolate any changes that simply adding the
traits introduces.

Author: davidtwco

core/src/marker.rs

@@ -151,11 +151,48 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 #[rustc_specialization_trait]
 #[rustc_deny_explicit_impl]
 #[rustc_do_not_implement_via_object]
+// `Sized` being coinductive, despite having supertraits, is okay as there are no user-written impls,
+// and we know that the supertraits are always implemented if the subtrait is just by looking at
+// the builtin impls.
 #[rustc_coinductive]
-pub trait Sized {
+pub trait Sized: MetaSized {
     // Empty.
 }
 
+/// Types with a size that can be determined from pointer metadata.
+#[unstable(feature = "sized_hierarchy", issue = "none")]
+#[lang = "meta_sized"]
+#[diagnostic::on_unimplemented(
+    message = "the size for values of type `{Self}` cannot be known",
+    label = "doesn't have a known size"
+)]
+#[fundamental]
+#[rustc_specialization_trait]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
+// `MetaSized` being coinductive, despite having supertraits, is okay for the same reasons as
+// `Sized` above.
+#[rustc_coinductive]
+pub trait MetaSized: PointeeSized {
+    // Empty
+}
+
+/// Types that may or may not have a size.
+#[unstable(feature = "sized_hierarchy", issue = "none")]
+#[lang = "pointee_sized"]
+#[diagnostic::on_unimplemented(
+    message = "values of type `{Self}` may or may not have a size",
+    label = "may or may not have a known size"
+)]
+#[fundamental]
+#[rustc_specialization_trait]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
+#[rustc_coinductive]
+pub trait PointeeSized {
+    // Empty
+}
+
 /// Types that can be "unsized" to a dynamically-sized type.
 ///
 /// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and