Create unstable `From` builtin macro and register it

5 files changed, 33 insertions, 0 deletions

diff --git a/compiler/rustc_builtin_macros/src/deriving/from.rs b/compiler/rustc_builtin_macros/src/deriving/from.rs
new file mode 100644
index 00000000000..79fd4caa440
--- /dev/null
+++ b/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,
+) {
+}
diff --git a/compiler/rustc_builtin_macros/src/deriving/mod.rs b/compiler/rustc_builtin_macros/src/deriving/mod.rs
index 1edc2965def..cee6952fa34 100644
--- a/compiler/rustc_builtin_macros/src/deriving/mod.rs
+++ b/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"]
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 7bc448a9acb..86a4927f390 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/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);
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index db8b527d593..dea459e83bc 100644
--- a/library/core/src/macros/mod.rs
+++ b/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 */
+    }
 }
diff --git a/library/core/src/prelude/v1.rs b/library/core/src/prelude/v1.rs
index a4be66b90ca..d8d82afb0e6 100644
--- a/library/core/src/prelude/v1.rs
+++ b/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;