Create unstable From builtin macro and register it

Author: Kobzol

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -0,0 +1,13 @@
+use rustc_ast as ast;
+use rustc_expand::base::{Annotatable, ExtCtxt};
+use rustc_span::{Span, sym};
+
+pub(crate) fn expand_deriving_from(
+ cx: &ExtCtxt<'_>,
+ span: Span,
+ mitem: &ast::MetaItem,
+ item: &Annotatable,
+ push: &mut dyn FnMut(Annotatable),
+ is_const: bool,
+) {
+}

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -23,6 +23,7 @@ pub(crate) mod clone;
 pub(crate) mod coerce_pointee;
 pub(crate) mod debug;
 pub(crate) mod default;
+pub(crate) mod from;
 pub(crate) mod hash;
 
 #[path = "cmp/eq.rs"]

compiler/rustc_builtin_macros/src/lib.rs

@@ -139,6 +139,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
  PartialEq: partial_eq::expand_deriving_partial_eq,
  PartialOrd: partial_ord::expand_deriving_partial_ord,
  CoercePointee: coerce_pointee::expand_deriving_coerce_pointee,
+ From: from::expand_deriving_from,
  }
 
  let client = rustc_proc_macro::bridge::client::Client::expand1(rustc_proc_macro::quote);

library/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 */
+ }
 }

library/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;