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.

compiler/rustc_builtin_macros/src/lib.rs

@@ -45,6 +45,7 @@ mod log_syntax;
 mod source_util;
 mod test;
 mod trace_macros;
+mod type_ascribe;
 mod util;
 
 pub mod asm;
@@ -92,6 +93,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         unreachable: edition_panic::expand_unreachable,
         stringify: source_util::expand_stringify,
         trace_macros: trace_macros::expand_trace_macros,
+        type_ascribe: type_ascribe::expand_type_ascribe,
     }
 
     register_attr! {

compiler/rustc_builtin_macros/src/type_ascribe.rs

@@ -0,0 +1,35 @@
+use rustc_ast::ptr::P;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_ast::{token, Expr, ExprKind, Ty};
+use rustc_errors::PResult;
+use rustc_expand::base::{self, DummyResult, ExtCtxt, MacEager};
+use rustc_span::Span;
+
+pub fn expand_type_ascribe(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    tts: TokenStream,
+) -> Box<dyn base::MacResult + 'static> {
+    let (expr, ty) = match parse_ascribe(cx, tts) {
+        Ok(parsed) => parsed,
+        Err(mut err) => {
+            err.emit();
+            return DummyResult::any(span);
+        }
+    };
+
+    let asc_expr = cx.expr(span, ExprKind::Type(expr, ty));
+
+    return MacEager::expr(asc_expr);
+}
+
+fn parse_ascribe<'a>(cx: &mut ExtCtxt<'a>, stream: TokenStream) -> PResult<'a, (P<Expr>, P<Ty>)> {
+    let mut parser = cx.new_parser_from_tts(stream);
+
+    let expr = parser.parse_expr()?;
+    parser.expect(&token::Comma)?;
+
+    let ty = parser.parse_ty()?;
+
+    Ok((expr, ty))
+}

compiler/rustc_span/src/symbol.rs

@@ -1485,6 +1485,7 @@ symbols! {
         ty,
         type_alias_enum_variants,
         type_alias_impl_trait,
+        type_ascribe,
         type_ascription,
         type_changing_struct_update,
         type_id,

library/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")]

library/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;

library/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