Add unstable type_ascribe macro

This macro serves as a placeholder for future type ascription syntax to
make sure that the semantic implementation keeps working.

Author: Noratrieb

core/src/macros/mod.rs

@@ -1546,6 +1546,29 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Unstable placeholder for type ascription.
+    #[rustc_builtin_macro]
+    #[unstable(
+        feature = "type_ascription",
+        issue = "23416",
+        reason = "placeholder syntax for type ascription"
+    )]
+    #[cfg(not(bootstrap))]
+    pub macro type_ascribe($expr:expr, $ty:ty) {
+        /* compiler built-in */
+    }
+
+    /// Unstable placeholder for type ascription.
+    #[unstable(
+        feature = "type_ascription",
+        issue = "23416",
+        reason = "placeholder syntax for type ascription"
+    )]
+    #[cfg(bootstrap)]
+    pub macro type_ascribe($expr:expr, $ty:ty) {
+        $expr: $ty
+    }
+
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]

core/src/prelude/v1.rs

@@ -98,3 +98,10 @@ pub use crate::macros::builtin::cfg_accessible;
     reason = "`cfg_eval` is a recently implemented feature"
 )]
 pub use crate::macros::builtin::cfg_eval;
+
+#[unstable(
+    feature = "type_ascription",
+    issue = "23416",
+    reason = "placeholder syntax for type ascription"
+)]
+pub use crate::macros::builtin::type_ascribe;

std/src/prelude/v1.rs

@@ -85,6 +85,14 @@ pub use core::prelude::v1::cfg_accessible;
 )]
 pub use core::prelude::v1::cfg_eval;
 
+// Do not `doc(no_inline)` either.
+#[unstable(
+    feature = "type_ascription",
+    issue = "23416",
+    reason = "placeholder syntax for type ascription"
+)]
+pub use core::prelude::v1::type_ascribe;
+
 // The file so far is equivalent to src/libcore/prelude/v1.rs,
 // and below to src/liballoc/prelude.rs.
 // Those files are duplicated rather than using glob imports