Constify identify conversions

Author: usbalbin

library/core/src/convert/mod.rs

@@ -532,7 +532,8 @@ where
 
 // From implies Into
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, U> Into<U> for T
+#[rustc_const_unstable(feature = "const_identity_convert", issue = "none")]
+impl<T, U> const Into<U> for T
 where
     U: From<T>,
 {
@@ -543,7 +544,8 @@ where
 
 // From (and thus Into) is reflexive
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> From<T> for T {
+#[rustc_const_unstable(feature = "const_identity_convert", issue = "none")]
+impl<T> const From<T> for T {
     fn from(t: T) -> T {
         t
     }

library/core/src/lib.rs

@@ -105,6 +105,9 @@
 #![feature(const_size_of_val)]
 #![feature(const_swap)]
 #![feature(const_align_of_val)]
+#![allow(incomplete_features)] // FIXME: We very much probably want to remove this line whenever possible
+#![feature(const_trait_impl)]
+#![feature(const_identity_convert)]
 #![feature(const_type_id)]
 #![feature(const_type_name)]
 #![feature(const_likely)]