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_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_convert", issue = "none")]
+impl<T> const From<T> for T {
     fn from(t: T) -> T {
         t
     }

library/core/src/lib.rs

@@ -82,6 +82,7 @@
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
 #![feature(const_heap)]
+#![feature(const_convert)]
 #![feature(const_inherent_unchecked_arith)]
 #![feature(const_int_unchecked_arith)]
 #![feature(const_intrinsic_copy)]