reject SmartPointer constructions not serving the purpose

compiler/rustc_builtin_macros/src/deriving/smart_ptr.rs

@@ -3,8 +3,9 @@ use std::mem::swap;
 use ast::HasAttrs;
 use rustc_ast::{
  self as ast, GenericArg, GenericBound, GenericParamKind, ItemKind, MetaItem,
- TraitBoundModifiers,
+ TraitBoundModifiers, VariantData,
 };
+use rustc_attr as attr;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -24,11 +25,43 @@ pub fn expand_deriving_smart_ptr(
  _is_const: bool,
 ) {
  let (name_ident, generics) = if let Annotatable::Item(aitem) = item
- && let ItemKind::Struct(_, g) = &aitem.kind
+ && let ItemKind::Struct(struct_data, g) = &aitem.kind
  {
+ let is_transparent = aitem.attrs.iter().any(|attr| {
+ attr::find_repr_attrs(cx.sess, attr)
+ .into_iter()
+ .any(|r| matches!(r, attr::ReprTransparent))
+ });
+ if !is_transparent {
+ cx.dcx()
+ .struct_span_err(
+ span,
+ "`SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`",
+ )
+ .emit();
+ return;
+ }
+ if !matches!(
+ struct_data,
+ VariantData::Struct { fields, recovered: _ } | VariantData::Tuple(fields, _)
+ if !fields.is_empty())
+ {
+ cx.dcx()
+ .struct_span_err(
+ span,
+ "`SmartPointer` can only be derived on `struct`s with at least one field",
+ )
+ .emit();
+ return;
+ }
  (aitem.ident, g)
  } else {
- cx.dcx().struct_span_err(span, "`SmartPointer` can only be derived on `struct`s").emit();
+ cx.dcx()
+ .struct_span_err(
+ span,
+ "`SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`",
+ )
+ .emit();
  return;
  };
 

tests/ui/deriving/deriving-smart-pointer-neg.rs

@@ -0,0 +1,38 @@
+#![feature(derive_smart_pointer, arbitrary_self_types)]
+
+use std::marker::SmartPointer;
+
+#[derive(SmartPointer)]
+//~^ ERROR: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
+enum NotStruct<'a, T: ?Sized> {
+ Variant(&'a T),
+}
+
+#[derive(SmartPointer)]
+//~^ ERROR: At least one generic type should be designated as `#[pointee]` in order to derive `SmartPointer` traits
+#[repr(transparent)]
+struct NoPointee<'a, T: ?Sized> {
+ ptr: &'a T,
+}
+
+#[derive(SmartPointer)]
+//~^ ERROR: `SmartPointer` can only be derived on `struct`s with at least one field
+#[repr(transparent)]
+struct NoField<'a, #[pointee] T: ?Sized> {}
+//~^ ERROR: lifetime parameter `'a` is never used
+//~| ERROR: type parameter `T` is never used
+
+#[derive(SmartPointer)]
+//~^ ERROR: `SmartPointer` can only be derived on `struct`s with at least one field
+#[repr(transparent)]
+struct NoFieldUnit<'a, #[pointee] T: ?Sized>();
+//~^ ERROR: lifetime parameter `'a` is never used
+//~| ERROR: type parameter `T` is never used
+
+#[derive(SmartPointer)]
+//~^ ERROR: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
+struct NotTransparent<'a, #[pointee] T: ?Sized> {
+ ptr: &'a T,
+}
+
+fn main() {}

tests/ui/deriving/deriving-smart-pointer-neg.stderr

@@ -0,0 +1,75 @@
+error: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
+ --> $DIR/deriving-smart-pointer-neg.rs:5:10
+ |
+LL | #[derive(SmartPointer)]
+ | ^^^^^^^^^^^^
+ |
+ = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: At least one generic type should be designated as `#[pointee]` in order to derive `SmartPointer` traits
+ --> $DIR/deriving-smart-pointer-neg.rs:11:10
+ |
+LL | #[derive(SmartPointer)]
+ | ^^^^^^^^^^^^
+ |
+ = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `SmartPointer` can only be derived on `struct`s with at least one field
+ --> $DIR/deriving-smart-pointer-neg.rs:18:10
+ |
+LL | #[derive(SmartPointer)]
+ | ^^^^^^^^^^^^
+ |
+ = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `SmartPointer` can only be derived on `struct`s with at least one field
+ --> $DIR/deriving-smart-pointer-neg.rs:25:10
+ |
+LL | #[derive(SmartPointer)]
+ | ^^^^^^^^^^^^
+ |
+ = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
+ --> $DIR/deriving-smart-pointer-neg.rs:32:10
+ |
+LL | #[derive(SmartPointer)]
+ | ^^^^^^^^^^^^
+ |
+ = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0392]: lifetime parameter `'a` is never used
+ --> $DIR/deriving-smart-pointer-neg.rs:21:16
+ |
+LL | struct NoField<'a, #[pointee] T: ?Sized> {}
+ | ^^ unused lifetime parameter
+ |
+ = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0392]: type parameter `T` is never used
+ --> $DIR/deriving-smart-pointer-neg.rs:21:31
+ |
+LL | struct NoField<'a, #[pointee] T: ?Sized> {}
+ | ^ unused type parameter
+ |
+ = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0392]: lifetime parameter `'a` is never used
+ --> $DIR/deriving-smart-pointer-neg.rs:28:20
+ |
+LL | struct NoFieldUnit<'a, #[pointee] T: ?Sized>();
+ | ^^ unused lifetime parameter
+ |
+ = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0392]: type parameter `T` is never used
+ --> $DIR/deriving-smart-pointer-neg.rs:28:35
+ |
+LL | struct NoFieldUnit<'a, #[pointee] T: ?Sized>();
+ | ^ unused type parameter
+ |
+ = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0392`.

tests/ui/deriving/deriving-smart-pointer.rs

@@ -4,6 +4,7 @@
 use std::marker::SmartPointer;
 
 #[derive(SmartPointer)]
+#[repr(transparent)]
 struct MyPointer<'a, #[pointee] T: ?Sized> {
  ptr: &'a T,
 }

tests/ui/feature-gates/feature-gate-derive-smart-pointer.rs

@@ -1,6 +1,7 @@
 use std::marker::SmartPointer; //~ ERROR use of unstable library feature 'derive_smart_pointer'
 
 #[derive(SmartPointer)] //~ ERROR use of unstable library feature 'derive_smart_pointer'
+#[repr(transparent)]
 struct MyPointer<'a, #[pointee] T: ?Sized> {
  //~^ ERROR the `#[pointee]` attribute is an experimental feature
  ptr: &'a T,

tests/ui/feature-gates/feature-gate-derive-smart-pointer.stderr

@@ -9,7 +9,7 @@ LL | #[derive(SmartPointer)]
  = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: the `#[pointee]` attribute is an experimental feature
- --> $DIR/feature-gate-derive-smart-pointer.rs:4:22
+ --> $DIR/feature-gate-derive-smart-pointer.rs:5:22
  |
 LL | struct MyPointer<'a, #[pointee] T: ?Sized> {
  | ^^^^^^^^^^