Arbitrary self types v2: pointers feature gate.

The main `arbitrary_self_types` feature gate will shortly be reused for
a new version of arbitrary self types which we are amending per [this
RFC](https://github.com/rust-lang/rfcs/blob/master/text/3519-arbitrary-self-types-v2.md).
The main amendments are:

* _do_ support `self` types which can't safely implement `Deref`
* do _not_ support generic `self` types
* do _not_ support raw pointers as `self` types.

This PR relates to the last of those bullet points: this strips pointer
support from the current `arbitrary_self_types` feature.
We expect this to cause some amount of breakage for crates using this
unstable feature to allow raw pointer self types. If that's the case, we
want to know about it, and we want crate authors to know of the upcoming
changes.

For now, this can be resolved by adding the new
`arbitrary_self_types_pointers` feature to such crates. If we determine
that use of raw pointers as self types is common, then we may maintain
that as an unstable feature even if we come to stabilize the rest of the
`arbitrary_self_types` support in future. If we don't hear that this PR
is causing breakage, then perhaps we don't need it at all, even behind
an unstable feature gate.

[Tracking issue](#44874)

This is [step 4 of the plan outlined here](#44874 (comment))

compiler/rustc_feature/src/unstable.rs

@@ -348,8 +348,10 @@ declare_features! (
     (unstable, adt_const_params, "1.56.0", Some(95174)),
     /// Allows defining an `#[alloc_error_handler]`.
     (unstable, alloc_error_handler, "1.29.0", Some(51540)),
-    /// Allows trait methods with arbitrary self types.
+    /// Allows inherent and trait methods with arbitrary self types.
     (unstable, arbitrary_self_types, "1.23.0", Some(44874)),
+    /// Allows inherent and trait methods with arbitrary self types that are raw pointers.
+    (unstable, arbitrary_self_types_pointers, "CURRENT_RUSTC_VERSION", Some(44874)),
     /// Enables experimental inline assembly support for additional architectures.
     (unstable, asm_experimental_arch, "1.58.0", Some(93335)),
     /// Allows using `label` operands in inline assembly.

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1651,6 +1651,14 @@ fn check_fn_or_method<'tcx>(
     }
 }
 
+/// The `arbitrary_self_types_pointers` feature implies `arbitrary_self_types`.
+#[derive(Clone, Copy)]
+enum ArbitrarySelfTypesLevel {
+    None,                       // neither arbitrary_self_types nor arbitrary_self_types_pointers
+    ArbitrarySelfTypes,         // just arbitrary_self_types
+    ArbitrarySelfTypesPointers, // both arbitrary_self_types and arbitrary_self_types_pointers
+}
+
 #[instrument(level = "debug", skip(wfcx))]
 fn check_method_receiver<'tcx>(
     wfcx: &WfCheckingCtxt<'_, 'tcx>,
@@ -1683,40 +1691,78 @@ fn check_method_receiver<'tcx>(
         return Ok(());
     }
 
-    if tcx.features().arbitrary_self_types {
-        if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
-            // Report error; `arbitrary_self_types` was enabled.
-            return Err(tcx.dcx().emit_err(errors::InvalidReceiverTy { span, receiver_ty }));
-        }
+    let arbitrary_self_types_level = if tcx.features().arbitrary_self_types_pointers {
+        ArbitrarySelfTypesLevel::ArbitrarySelfTypesPointers
+    } else if tcx.features().arbitrary_self_types {
+        ArbitrarySelfTypesLevel::ArbitrarySelfTypes
     } else {
-        if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, false) {
-            return Err(if receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
+        ArbitrarySelfTypesLevel::None
+    };
+
+    if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, arbitrary_self_types_level) {
+        return Err(match arbitrary_self_types_level {
+            // Wherever possible, emit a message advising folks that the features
+            // `arbitrary_self_types` or `arbitrary_self_types_pointers` might
+            // have helped.
+            ArbitrarySelfTypesLevel::None
+                if receiver_is_valid(
+                    wfcx,
+                    span,
+                    receiver_ty,
+                    self_ty,
+                    ArbitrarySelfTypesLevel::ArbitrarySelfTypes,
+                ) =>
+            {
                 // Report error; would have worked with `arbitrary_self_types`.
                 feature_err(
                     &tcx.sess,
                     sym::arbitrary_self_types,
                     span,
                     format!(
                         "`{receiver_ty}` cannot be used as the type of `self` without \
-                         the `arbitrary_self_types` feature",
+                            the `arbitrary_self_types` feature",
                     ),
                 )
                 .with_help(fluent::hir_analysis_invalid_receiver_ty_help)
                 .emit()
-            } else {
-                // Report error; would not have worked with `arbitrary_self_types`.
+            }
+            ArbitrarySelfTypesLevel::ArbitrarySelfTypes | ArbitrarySelfTypesLevel::None
+                if receiver_is_valid(
+                    wfcx,
+                    span,
+                    receiver_ty,
+                    self_ty,
+                    ArbitrarySelfTypesLevel::ArbitrarySelfTypesPointers,
+                ) =>
+            {
+                // Report error; would have worked with `arbitrary_self_types_pointers`.
+                feature_err(
+                    &tcx.sess,
+                    sym::arbitrary_self_types_pointers,
+                    span,
+                    format!(
+                        "`{receiver_ty}` cannot be used as the type of `self` without \
+                            the `arbitrary_self_types_pointers` feature",
+                    ),
+                )
+                .with_help(fluent::hir_analysis_invalid_receiver_ty_help)
+                .emit()
+            }
+            _ =>
+            // Report error; would not have worked with `arbitrary_self_types[_pointers]`.
+            {
                 tcx.dcx().emit_err(errors::InvalidReceiverTy { span, receiver_ty })
-            });
-        }
+            }
+        });
     }
     Ok(())
 }
 
 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
-/// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
-/// strict: `receiver_ty` must implement `Receiver` and directly implement
-/// `Deref<Target = self_ty>`.
+/// through a `*const/mut T` raw pointer if  `arbitrary_self_types_pointers` is also enabled.
+/// If neither feature is enabled, the requirements are more strict: `receiver_ty` must implement
+/// `Receiver` and directly implement `Deref<Target = self_ty>`.
 ///
 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
@@ -1726,7 +1772,7 @@ fn receiver_is_valid<'tcx>(
     span: Span,
     receiver_ty: Ty<'tcx>,
     self_ty: Ty<'tcx>,
-    arbitrary_self_types_enabled: bool,
+    arbitrary_self_types_enabled: ArbitrarySelfTypesLevel,
 ) -> bool {
     let infcx = wfcx.infcx;
     let tcx = wfcx.tcx();
@@ -1744,8 +1790,8 @@ fn receiver_is_valid<'tcx>(
 
     let mut autoderef = Autoderef::new(infcx, wfcx.param_env, wfcx.body_def_id, span, receiver_ty);
 
-    // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
-    if arbitrary_self_types_enabled {
+    // The `arbitrary_self_types_pointers` feature allows raw pointer receivers like `self: *const Self`.
+    if matches!(arbitrary_self_types_enabled, ArbitrarySelfTypesLevel::ArbitrarySelfTypesPointers) {
         autoderef = autoderef.include_raw_pointers();
     }
 
@@ -1771,7 +1817,7 @@ fn receiver_is_valid<'tcx>(
 
         // Without `feature(arbitrary_self_types)`, we require that each step in the
         // deref chain implement `receiver`.
-        if !arbitrary_self_types_enabled {
+        if matches!(arbitrary_self_types_enabled, ArbitrarySelfTypesLevel::None) {
             if !receiver_is_implemented(
                 wfcx,
                 receiver_trait_def_id,

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -402,7 +402,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     mode,
                 }));
             } else if bad_ty.reached_raw_pointer
-                && !self.tcx.features().arbitrary_self_types
+                && !self.tcx.features().arbitrary_self_types_pointers
                 && !self.tcx.sess.at_least_rust_2018()
             {
                 // this case used to be allowed by the compiler,

compiler/rustc_span/src/symbol.rs

@@ -406,6 +406,7 @@ symbols! {
         append_const_msg,
         arbitrary_enum_discriminant,
         arbitrary_self_types,
+        arbitrary_self_types_pointers,
         args,
         arith_offset,
         arm,

tests/mir-opt/building/receiver_ptr_mutability.rs

@@ -1,7 +1,7 @@
 // skip-filecheck
 // EMIT_MIR receiver_ptr_mutability.main.built.after.mir
 
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 struct Test {}
 

tests/ui/cast/ptr-to-trait-obj-different-regions-lt-ext.rs

@@ -2,7 +2,7 @@
 //
 // issue: <https://github.com/rust-lang/rust/issues/120217>
 
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 trait Static<'a> {
     fn proof(self: *const Self, s: &'a str) -> &'static str;

tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.rs

@@ -0,0 +1,15 @@
+trait Foo {
+    fn foo(self: *const Self); //~ ERROR `*const Self` cannot be used as the type of `self`
+}
+
+struct Bar;
+
+impl Foo for Bar {
+    fn foo(self: *const Self) {} //~ ERROR `*const Bar` cannot be used as the type of `self`
+}
+
+impl Bar {
+    fn bar(self: *mut Self) {} //~ ERROR `*mut Bar` cannot be used as the type of `self`
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.stderr

@@ -0,0 +1,36 @@
+error[E0658]: `*const Bar` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
+  --> $DIR/feature-gate-arbitrary-self-types-pointers.rs:8:18
+   |
+LL |     fn foo(self: *const Self) {}
+   |                  ^^^^^^^^^^^
+   |
+   = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: 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`)
+
+error[E0658]: `*mut Bar` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
+  --> $DIR/feature-gate-arbitrary-self-types-pointers.rs:12:18
+   |
+LL |     fn bar(self: *mut Self) {}
+   |                  ^^^^^^^^^
+   |
+   = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: 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`)
+
+error[E0658]: `*const Self` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
+  --> $DIR/feature-gate-arbitrary-self-types-pointers.rs:2:18
+   |
+LL |     fn foo(self: *const Self);
+   |                  ^^^^^^^^^^^
+   |
+   = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: 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`)
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-arbitrary_self_types-raw-pointer.stderr

@@ -1,33 +1,33 @@
-error[E0658]: `*const Foo` cannot be used as the type of `self` without the `arbitrary_self_types` feature
+error[E0658]: `*const Foo` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary_self_types-raw-pointer.rs:4:18
    |
 LL |     fn foo(self: *const Self) {}
    |                  ^^^^^^^^^^^
    |
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
-   = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = help: 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`)
 
-error[E0658]: `*const ()` cannot be used as the type of `self` without the `arbitrary_self_types` feature
+error[E0658]: `*const ()` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary_self_types-raw-pointer.rs:14:18
    |
 LL |     fn bar(self: *const Self) {}
    |                  ^^^^^^^^^^^
    |
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
-   = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = help: 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`)
 
-error[E0658]: `*const Self` cannot be used as the type of `self` without the `arbitrary_self_types` feature
+error[E0658]: `*const Self` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary_self_types-raw-pointer.rs:9:18
    |
 LL |     fn bar(self: *const Self);
    |                  ^^^^^^^^^^^
    |
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
-   = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
+   = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = help: 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`)
 

tests/ui/inference/auxiliary/inference_unstable_iterator.rs

@@ -1,5 +1,5 @@
 #![feature(staged_api)]
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 #![stable(feature = "ipu_iterator", since = "1.0.0")]
 

tests/ui/inference/auxiliary/inference_unstable_itertools.rs

@@ -1,4 +1,4 @@
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 pub trait IpuItertools {
     fn ipu_flatten(&self) -> u32 {

tests/ui/self/arbitrary_self_types_raw_pointer_struct.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 use std::rc::Rc;
 

tests/ui/self/arbitrary_self_types_raw_pointer_trait.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
 
 use std::ptr;