Rollup merge of rust-lang#104614 - Nilstrieb:type-ascribe!, r=TaKO8Ki

Add `type_ascribe!` macro as placeholder syntax for type ascription

This makes it still possible to test the internal semantics of type ascription even once the `:`-syntax is removed from the parser. The macro now gets used in a bunch of UI tests that test the semantics and not syntax of type ascription.

I might have forgotten a few tests but this should hopefully be most of them. The remaining ones will certainly be found once type ascription is removed from the parser altogether.

Part of rust-lang#101728

Author: matthiaskrgr

core/src/macros/mod.rs

@@ -1546,6 +1546,18 @@ 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 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 +98,11 @@ 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"
+)]
+#[cfg(not(bootstrap))]
+pub use crate::macros::builtin::type_ascribe;

std/src/prelude/v1.rs

@@ -85,6 +85,15 @@ 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"
+)]
+#[cfg(not(bootstrap))]
+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