Treat pointee as a macro attribute

compiler/rustc_builtin_macros/messages.ftl

@@ -235,6 +235,8 @@ builtin_macros_non_exhaustive_default = default variant must be exhaustive
     .label = declared `#[non_exhaustive]` here
     .help = consider a manual implementation of `Default`
 
+builtin_macros_non_generic_pointee = the `#[pointee]` attribute may only be used on generic params
+
 builtin_macros_non_unit_default = the `#[default]` attribute may only be used on unit enum variants
     .help = consider a manual implementation of `Default`
 

compiler/rustc_builtin_macros/src/deriving/smart_ptr.rs

@@ -14,6 +14,8 @@ use rustc_span::{Span, Symbol};
 use smallvec::{smallvec, SmallVec};
 use thin_vec::{thin_vec, ThinVec};
 
+use crate::errors;
+
 macro_rules! path {
     ($span:expr, $($part:ident)::*) => { vec![$(Ident::new(sym::$part, $span),)*] }
 }
@@ -26,6 +28,8 @@ pub fn expand_deriving_smart_ptr(
     push: &mut dyn FnMut(Annotatable),
     _is_const: bool,
 ) {
+    item.visit_with(&mut DetectNonGenericPointeeAttr { cx });
+
     let (name_ident, generics) = if let Annotatable::Item(aitem) = item
         && let ItemKind::Struct(struct_data, g) = &aitem.kind
     {
@@ -377,3 +381,24 @@ impl<'a> ast::mut_visit::MutVisitor for TypeSubstitution<'a> {
         }
     }
 }
+
+struct DetectNonGenericPointeeAttr<'a, 'b> {
+    cx: &'a ExtCtxt<'b>,
+}
+
+impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonGenericPointeeAttr<'a, 'b> {
+    fn visit_attribute(&mut self, attr: &'a rustc_ast::Attribute) -> Self::Result {
+        if attr.has_name(sym::pointee) {
+            self.cx.dcx().emit_err(errors::NonGenericPointee { span: attr.span });
+        }
+    }
+
+    fn visit_generic_param(&mut self, param: &'a rustc_ast::GenericParam) -> Self::Result {
+        match param.kind {
+            GenericParamKind::Type { .. } => return,
+            GenericParamKind::Lifetime | GenericParamKind::Const { .. } => {
+                rustc_ast::visit::walk_generic_param(self, param)
+            }
+        }
+    }
+}

compiler/rustc_builtin_macros/src/errors.rs

@@ -937,3 +937,10 @@ pub struct NakedFunctionTestingAttribute {
     #[label]
     pub testing_span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_non_generic_pointee)]
+pub struct NonGenericPointee {
+    #[primary_span]
+    pub span: Span,
+}

library/core/src/marker.rs

@@ -1060,7 +1060,7 @@ pub trait FnPtr: Copy + Clone {
 }
 
 /// Derive macro generating impls of traits related to smart pointers.
-#[rustc_builtin_macro]
+#[rustc_builtin_macro(SmartPointer, attributes(pointee))]
 #[allow_internal_unstable(dispatch_from_dyn, coerce_unsized, unsize)]
 #[unstable(feature = "derive_smart_pointer", issue = "123430")]
 pub macro SmartPointer($item:item) {