Arbitrary self types v2: Weak, NonNull hints

Various types can be used as method receivers, such as Rc<>, Box<> and
Arc<>. The arbitrary self types v2 work allows further types to be made
method receivers by implementing the Receiver trait.

With that in mind, it may come as a surprise to people when certain
common types do not implement Receiver and thus cannot be used as a
method receiver.

The RFC for arbitrary self types v2 therefore proposes emitting specific
lint hints for these cases:
* NonNull
* Weak
* Raw pointers

The code already emits a hint for this third case, in that it advises
folks that the `arbitrary_self_types_pointers` feature may meet their
need. This PR adds diagnostic hints for the Weak and NonNull cases.

compiler/rustc_hir_analysis/messages.ftl

@@ -249,6 +249,12 @@ hir_analysis_invalid_receiver_ty_help =
 hir_analysis_invalid_receiver_ty_help_no_arbitrary_self_types =
     consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
 
+hir_analysis_invalid_receiver_ty_help_nonnull_note =
+    `NonNull` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `NonNull` in a newtype wrapper for which you implement `Receiver`
+
+hir_analysis_invalid_receiver_ty_help_weak_note =
+    `Weak` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `Weak` in a newtype wrapper for which you implement `Receiver`
+
 hir_analysis_invalid_receiver_ty_no_arbitrary_self_types = invalid `self` parameter type: `{$receiver_ty}`
     .note = type of `self` must be `Self` or a type that dereferences to it
 

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -44,6 +44,7 @@ use {rustc_ast as ast, rustc_hir as hir};
 use crate::autoderef::Autoderef;
 use crate::collect::CollectItemTypesVisitor;
 use crate::constrained_generic_params::{Parameter, identify_constrained_generic_params};
+use crate::errors::InvalidReceiverTyHint;
 use crate::{errors, fluent_generated as fluent};
 
 pub(super) struct WfCheckingCtxt<'a, 'tcx> {
@@ -1749,7 +1750,18 @@ fn check_method_receiver<'tcx>(
             {
                 match receiver_validity_err {
                     ReceiverValidityError::DoesNotDeref if arbitrary_self_types_level.is_some() => {
-                        tcx.dcx().emit_err(errors::InvalidReceiverTy { span, receiver_ty })
+                        let hint = match receiver_ty
+                            .builtin_deref(false)
+                            .unwrap_or(receiver_ty)
+                            .ty_adt_def()
+                            .and_then(|adt_def| tcx.get_diagnostic_name(adt_def.did()))
+                        {
+                            Some(sym::RcWeak | sym::ArcWeak) => Some(InvalidReceiverTyHint::Weak),
+                            Some(sym::NonNull) => Some(InvalidReceiverTyHint::NonNull),
+                            _ => None,
+                        };
+
+                        tcx.dcx().emit_err(errors::InvalidReceiverTy { span, receiver_ty, hint })
                     }
                     ReceiverValidityError::DoesNotDeref => {
                         tcx.dcx().emit_err(errors::InvalidReceiverTyNoArbitrarySelfTypes {

compiler/rustc_hir_analysis/src/errors.rs

@@ -1655,6 +1655,14 @@ pub(crate) struct NonConstRange {
     pub span: Span,
 }
 
+#[derive(Subdiagnostic)]
+pub(crate) enum InvalidReceiverTyHint {
+    #[note(hir_analysis_invalid_receiver_ty_help_weak_note)]
+    Weak,
+    #[note(hir_analysis_invalid_receiver_ty_help_nonnull_note)]
+    NonNull,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_invalid_receiver_ty_no_arbitrary_self_types, code = E0307)]
 #[note]
@@ -1673,6 +1681,8 @@ pub(crate) struct InvalidReceiverTy<'tcx> {
     #[primary_span]
     pub span: Span,
     pub receiver_ty: Ty<'tcx>,
+    #[subdiagnostic]
+    pub hint: Option<InvalidReceiverTyHint>,
 }
 
 #[derive(Diagnostic)]

tests/ui/self/arbitrary_self_types_nonnull.rs

@@ -0,0 +1,13 @@
+#![feature(arbitrary_self_types)]
+
+struct A;
+
+impl A {
+    fn m(self: std::ptr::NonNull<Self>) {}
+    //~^ ERROR: invalid `self` parameter type
+    fn n(self: &std::ptr::NonNull<Self>) {}
+    //~^ ERROR: invalid `self` parameter type
+}
+
+fn main() {
+}

tests/ui/self/arbitrary_self_types_nonnull.stderr

@@ -0,0 +1,23 @@
+error[E0307]: invalid `self` parameter type: `NonNull<A>`
+  --> $DIR/arbitrary_self_types_nonnull.rs:6:16
+   |
+LL |     fn m(self: std::ptr::NonNull<Self>) {}
+   |                ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
+   = note: `NonNull` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `NonNull` in a newtype wrapper for which you implement `Receiver`
+
+error[E0307]: invalid `self` parameter type: `&NonNull<A>`
+  --> $DIR/arbitrary_self_types_nonnull.rs:8:16
+   |
+LL |     fn n(self: &std::ptr::NonNull<Self>) {}
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
+   = note: `NonNull` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `NonNull` in a newtype wrapper for which you implement `Receiver`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0307`.

tests/ui/self/arbitrary_self_types_weak.rs

@@ -0,0 +1,13 @@
+#![feature(arbitrary_self_types)]
+
+struct A;
+
+impl A {
+    fn m(self: std::rc::Weak<Self>) {}
+    //~^ ERROR: invalid `self` parameter type
+    fn n(self: std::sync::Weak<Self>) {}
+    //~^ ERROR: invalid `self` parameter type
+}
+
+fn main() {
+}

tests/ui/self/arbitrary_self_types_weak.stderr

@@ -0,0 +1,23 @@
+error[E0307]: invalid `self` parameter type: `std::rc::Weak<A>`
+  --> $DIR/arbitrary_self_types_weak.rs:6:16
+   |
+LL |     fn m(self: std::rc::Weak<Self>) {}
+   |                ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
+   = note: `Weak` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `Weak` in a newtype wrapper for which you implement `Receiver`
+
+error[E0307]: invalid `self` parameter type: `std::sync::Weak<A>`
+  --> $DIR/arbitrary_self_types_weak.rs:8:16
+   |
+LL |     fn n(self: std::sync::Weak<Self>) {}
+   |                ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
+   = note: `Weak` does not implement `Receiver` because it has methods that may shadow the referent; consider wrapping your `Weak` in a newtype wrapper for which you implement `Receiver`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0307`.