Rollup merge of #125575 - dingxiangfei2009:derive-smart-ptr, r=davidtwco

SmartPointer derive-macro

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Possibly replacing #123472 for continued upkeep of the proposal rust-lang/rfcs#3621 and implementation of the tracking issue #123430.

cc `@Darksonn` `@wedsonaf`

compiler/rustc_feature/src/builtin_attrs.rs

@@ -575,6 +575,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         EncodeCrossCrate::No, coroutines, experimental!(coroutines)
     ),
 
+    // `#[pointee]` attribute to designate the pointee type in SmartPointer derive-macro
+    gated!(
+        pointee, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::No, derive_smart_pointer, experimental!(pointee)
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================

compiler/rustc_feature/src/unstable.rs

@@ -436,6 +436,8 @@ declare_features! (
     (unstable, deprecated_suggestion, "1.61.0", Some(94785)),
     /// Allows deref patterns.
     (incomplete, deref_patterns, "1.79.0", Some(87121)),
+    /// Allows deriving `SmartPointer` traits
+    (unstable, derive_smart_pointer, "1.79.0", Some(123430)),
     /// Controls errors in trait implementations.
     (unstable, do_not_recommend, "1.67.0", Some(51992)),
     /// Tells rustdoc to automatically generate `#[doc(cfg(...))]`.