derive(SmartPointer): assume pointee from the single generic and better error messages

compiler/rustc_builtin_macros/src/deriving/smart_ptr.rs

@@ -11,7 +11,6 @@ use rustc_data_structures::flat_map_in_place::FlatMapInPlace;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, Symbol};
-use smallvec::{smallvec, SmallVec};
 use thin_vec::{thin_vec, ThinVec};
 
 macro_rules! path {
@@ -68,43 +67,63 @@ pub fn expand_deriving_smart_ptr(
     };
 
     // Convert generic parameters (from the struct) into generic args.
-    let mut pointee_param = None;
-    let mut multiple_pointee_diag: SmallVec<[_; 2]> = smallvec![];
-    let self_params = generics
+    let self_params: Vec<_> = generics
         .params
         .iter()
-        .enumerate()
-        .map(|(idx, p)| match p.kind {
+        .map(|p| match p.kind {
             GenericParamKind::Lifetime => GenericArg::Lifetime(cx.lifetime(p.span(), p.ident)),
-            GenericParamKind::Type { .. } => {
-                if p.attrs().iter().any(|attr| attr.has_name(sym::pointee)) {
-                    if pointee_param.is_some() {
-                        multiple_pointee_diag.push(cx.dcx().struct_span_err(
-                            p.span(),
-                            "`SmartPointer` can only admit one type as pointee",
-                        ));
-                    } else {
-                        pointee_param = Some(idx);
-                    }
-                }
-                GenericArg::Type(cx.ty_ident(p.span(), p.ident))
-            }
+            GenericParamKind::Type { .. } => GenericArg::Type(cx.ty_ident(p.span(), p.ident)),
             GenericParamKind::Const { .. } => GenericArg::Const(cx.const_ident(p.span(), p.ident)),
         })
-        .collect::<Vec<_>>();
-    let Some(pointee_param_idx) = pointee_param else {
+        .collect();
+    let type_params: Vec<_> = generics
+        .params
+        .iter()
+        .enumerate()
+        .filter_map(|(idx, p)| {
+            if let GenericParamKind::Type { .. } = p.kind {
+                Some((idx, p.span(), p.attrs().iter().any(|attr| attr.has_name(sym::pointee))))
+            } else {
+                None
+            }
+        })
+        .collect();
+
+    let pointee_param_idx = if type_params.is_empty() {
+        // `#[derive(SmartPointer)]` requires at least one generic type on the target `struct`
         cx.dcx().struct_span_err(
             span,
-            "At least one generic type should be designated as `#[pointee]` in order to derive `SmartPointer` traits",
+            "`SmartPointer` can only be derived on `struct`s that are generic over at least one type",
         ).emit();
         return;
-    };
-    if !multiple_pointee_diag.is_empty() {
-        for diag in multiple_pointee_diag {
-            diag.emit();
+    } else if type_params.len() == 1 {
+        // Regardless of the only type param being designed as `#[pointee]` or not, we can just use it as such
+        type_params[0].0
+    } else {
+        let mut pointees = type_params
+            .iter()
+            .filter_map(|&(idx, span, is_pointee)| is_pointee.then_some((idx, span)))
+            .fuse();
+        match (pointees.next(), pointees.next()) {
+            (Some((idx, _span)), None) => idx,
+            (None, _) => {
+                cx.dcx().struct_span_err(
+                    span,
+                    "exactly one generic type parameter must be marked as #[pointee] to derive SmartPointer traits",
+                ).emit();
+                return;
+            }
+            (Some((_, one)), Some((_, another))) => {
+                cx.dcx()
+                    .struct_span_err(
+                        vec![one, another],
+                        "only one type parameter can be marked as `#[pointee]` when deriving SmartPointer traits",
+                    )
+                    .emit();
+                return;
+            }
         }
-        return;
-    }
+    };
 
     // Create the type of `self`.
     let path = cx.path_all(span, false, vec![name_ident], self_params.clone());

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

@@ -20,3 +20,9 @@ where
     data: &'a mut T,
     x: core::marker::PhantomData<X>,
 }
+
+#[derive(SmartPointer)]
+#[repr(transparent)]
+struct MyPointerWithoutPointee<'a, T: ?Sized> {
+    ptr: &'a T,
+}

tests/ui/deriving/deriving-smart-pointer-expanded.stdout

@@ -42,3 +42,18 @@ impl<'a, Y, Z: MyTrait<T> + MyTrait<__S>, T: ?Sized + MyTrait<T> +
     MyTrait<__S>> ::core::ops::CoerceUnsized<MyPointer2<'a, Y, Z, __S, X>> for
     MyPointer2<'a, Y, Z, T, X> where Y: MyTrait<T>, Y: MyTrait<__S> {
 }
+
+#[repr(transparent)]
+struct MyPointerWithoutPointee<'a, T: ?Sized> {
+    ptr: &'a T,
+}
+#[automatically_derived]
+impl<'a, T: ?Sized + ::core::marker::Unsize<__S>, __S: ?Sized>
+    ::core::ops::DispatchFromDyn<MyPointerWithoutPointee<'a, __S>> for
+    MyPointerWithoutPointee<'a, T> {
+}
+#[automatically_derived]
+impl<'a, T: ?Sized + ::core::marker::Unsize<__S>, __S: ?Sized>
+    ::core::ops::CoerceUnsized<MyPointerWithoutPointee<'a, __S>> for
+    MyPointerWithoutPointee<'a, T> {
+}

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

@@ -9,13 +9,6 @@ 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)]
@@ -30,6 +23,23 @@ 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 that are generic over at least one type
+#[repr(transparent)]
+struct NoGeneric<'a>(&'a u8);
+
+#[derive(SmartPointer)]
+//~^ ERROR: exactly one generic type parameter must be marked as #[pointee] to derive SmartPointer traits
+#[repr(transparent)]
+struct AmbiguousPointee<'a, T1: ?Sized, T2: ?Sized> {
+    a: (&'a T1, &'a T2),
+}
+
+#[derive(SmartPointer)]
+#[repr(transparent)]
+struct TooManyPointees<'a, #[pointee] A: ?Sized, #[pointee] B: ?Sized>((&'a A, &'a B));
+//~^ ERROR: only one type parameter can be marked as `#[pointee]` when deriving SmartPointer traits
+
 #[derive(SmartPointer)]
 //~^ ERROR: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
 struct NotTransparent<'a, #[pointee] T: ?Sized> {

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

@@ -6,7 +6,7 @@ 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
+error: `SmartPointer` can only be derived on `struct`s with at least one field
   --> $DIR/deriving-smart-pointer-neg.rs:12:10
    |
 LL | #[derive(SmartPointer)]
@@ -22,60 +22,74 @@ 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
+error: `SmartPointer` can only be derived on `struct`s that are generic over at least one type
   --> $DIR/deriving-smart-pointer-neg.rs:26:10
    |
 LL | #[derive(SmartPointer)]
    |          ^^^^^^^^^^^^
    |
    = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error: exactly one generic type parameter must be marked as #[pointee] to derive SmartPointer traits
+  --> $DIR/deriving-smart-pointer-neg.rs:31:10
+   |
+LL | #[derive(SmartPointer)]
+   |          ^^^^^^^^^^^^
+   |
+   = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: only one type parameter can be marked as `#[pointee]` when deriving SmartPointer traits
+  --> $DIR/deriving-smart-pointer-neg.rs:40:39
+   |
+LL | struct TooManyPointees<'a, #[pointee] A: ?Sized, #[pointee] B: ?Sized>((&'a A, &'a B));
+   |                                       ^                     ^
+
 error: `SmartPointer` can only be derived on `struct`s with `#[repr(transparent)]`
-  --> $DIR/deriving-smart-pointer-neg.rs:33:10
+  --> $DIR/deriving-smart-pointer-neg.rs:43:10
    |
 LL | #[derive(SmartPointer)]
    |          ^^^^^^^^^^^^
    |
    = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: `derive(SmartPointer)` requires T to be marked `?Sized`
-  --> $DIR/deriving-smart-pointer-neg.rs:41:36
+  --> $DIR/deriving-smart-pointer-neg.rs:51:36
    |
 LL | struct NoMaybeSized<'a, #[pointee] T> {
    |                                    ^
 
 error[E0392]: lifetime parameter `'a` is never used
-  --> $DIR/deriving-smart-pointer-neg.rs:22:16
+  --> $DIR/deriving-smart-pointer-neg.rs:15:16
    |
 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:22:31
+  --> $DIR/deriving-smart-pointer-neg.rs:15:31
    |
 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:29:20
+  --> $DIR/deriving-smart-pointer-neg.rs:22:20
    |
 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:29:35
+  --> $DIR/deriving-smart-pointer-neg.rs:22:35
    |
 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 10 previous errors
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0392`.