Introduce CoerceShared lang item and trait

Author: aapoalas

core/src/marker.rs

@@ -1372,3 +1372,12 @@ pub trait CoercePointeeValidated {
 pub trait Reborrow {
     // Empty.
 }
+
+/// Allows reborrowable value to be reborrowed as shared, creating a copy of
+/// that disables the source for writes for the lifetime of the copy.
+#[lang = "coerce_shared"]
+#[unstable(feature = "reborrow", issue = "145612")]
+pub trait CoerceShared: Reborrow {
+    /// The type of this value when reborrowed as shared.
+    type Target: Copy;
+}