Implement macro-based deref!() syntax for deref patterns

Stop using `box PAT` syntax for deref patterns, as it's misleading and
also causes their semantics being tangled up.

Author: compiler-errors

core/src/macros/mod.rs

@@ -1714,6 +1714,18 @@ pub(crate) mod builtin {
         builtin # type_ascribe($expr, $ty)
     }
 
+    #[cfg(not(bootstrap))]
+    /// Unstable placeholder for deref patterns.
+    #[allow_internal_unstable(builtin_syntax)]
+    #[unstable(
+        feature = "deref_patterns",
+        issue = "87121",
+        reason = "placeholder syntax for deref patterns"
+    )]
+    pub macro deref($pat:pat) {
+        builtin # deref($pat)
+    }
+
     /// 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

@@ -103,3 +103,11 @@ pub use crate::macros::builtin::cfg_eval;
     reason = "placeholder syntax for type ascription"
 )]
 pub use crate::macros::builtin::type_ascribe;
+
+#[cfg(not(bootstrap))]
+#[unstable(
+    feature = "deref_patterns",
+    issue = "87121",
+    reason = "placeholder syntax for deref patterns"
+)]
+pub use crate::macros::builtin::deref;

std/src/prelude/v1.rs

@@ -91,6 +91,15 @@ pub use core::prelude::v1::cfg_eval;
 )]
 pub use core::prelude::v1::type_ascribe;
 
+#[cfg(not(bootstrap))]
+// Do not `doc(no_inline)` either.
+#[unstable(
+    feature = "deref_patterns",
+    issue = "87121",
+    reason = "placeholder syntax for deref patterns"
+)]
+pub use core::prelude::v1::deref;
+
 // The file so far is equivalent to core/src/prelude/v1.rs. It is duplicated
 // rather than glob imported because we want docs to show these re-exports as
 // pointing to within `std`.