Create unstable From builtin macro and register it

Author: Kobzol

core/src/macros/mod.rs

@@ -1768,4 +1768,15 @@ pub(crate) mod builtin {
     pub macro deref($pat:pat) {
         builtin # deref($pat)
     }
+
+    /// Derive macro generating an impl of the trait `From`.
+    /// Currently, it can only be used on single-field structs.
+    // Note that the macro is in a different module than the `From` trait,
+    // to avoid triggering an unstable feature being used if someone imports
+    // `std::convert::From`.
+    #[rustc_builtin_macro]
+    #[unstable(feature = "derive_from", issue = "144889")]
+    pub macro From($item: item) {
+        /* compiler built-in */
+    }
 }

core/src/prelude/v1.rs

@@ -117,3 +117,10 @@ pub use crate::macros::builtin::deref;
     reason = "`type_alias_impl_trait` has open design concerns"
 )]
 pub use crate::macros::builtin::define_opaque;
+
+#[unstable(
+    feature = "derive_from",
+    issue = "144889",
+    reason = "`derive(From)` is unstable"
+)]
+pub use crate::macros::builtin::From;