Include adjustments to allow unsizing coercions for raw slice pointers in receiver position

compiler/rustc_typeck/src/check/method/confirm.rs

@@ -13,6 +13,7 @@ use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{self, Subst, SubstsRef};
 use rustc_middle::ty::{self, GenericParamDefKind, Ty};
 use rustc_span::Span;
+use rustc_trait_selection::autoderef::Autoderef;
 use rustc_trait_selection::traits;
 
 use std::ops::Deref;
@@ -137,8 +138,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
     ) -> Ty<'tcx> {
         // Commit the autoderefs by calling `autoderef` again, but this
         // time writing the results into the various typeck results.
-        let mut autoderef =
-            self.autoderef_overloaded_span(self.span, unadjusted_self_ty, self.call_expr.span);
+        let mut autoderef = self.autoderef_self_ty(unadjusted_self_ty, &pick);
         let (_, n) = match autoderef.nth(pick.autoderefs) {
             Some(n) => n,
             None => {
@@ -191,6 +191,25 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         target
     }
 
+    fn autoderef_self_ty(
+        &'a self,
+        unadjusted_self_ty: Ty<'tcx>,
+        pick: &probe::Pick<'tcx>,
+    ) -> Autoderef<'a, 'tcx> {
+        let autoderef =
+            self.autoderef_overloaded_span(self.span, unadjusted_self_ty, self.call_expr.span);
+
+        // We allow unsize coercions of raw pointers in method callee position
+        if let &ty::RawPtr(ty::TypeAndMut { ty: t, .. }) = unadjusted_self_ty.kind() {
+            if let ty::Array(_, _) = t.kind() {
+                if pick.unsize_raw_ptr() {
+                    return autoderef.include_raw_pointers();
+                }
+            }
+        }
+        autoderef
+    }
+
     /// Returns a set of substitutions for the method *receiver* where all type and region
     /// parameters are instantiated with fresh variables. This substitution does not include any
     /// parameters declared on the method itself.

compiler/rustc_typeck/src/check/method/probe.rs

@@ -177,6 +177,17 @@ pub struct Pick<'tcx> {
     pub unsize: Option<Ty<'tcx>>,
 }
 
+impl Pick<'tcx> {
+    pub(crate) fn unsize_raw_ptr(&self) -> bool {
+        if let Some(t) = &self.unsize {
+            if let &ty::Slice(_) = t.kind() {
+                return self.autoderefs == 1 && self.autoref.is_some();
+            }
+        }
+        false
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum PickKind<'tcx> {
     InherentImplPick,
@@ -1062,14 +1073,23 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
         let steps = self.steps.clone();
 
+        let allow_deref_raw_ptr = |step: &CandidateStep<'tcx>| -> bool {
+            // allow dereferencing of raw pointers to slices
+            if let &ty::Slice(_) = step.self_ty.value.value.kind() {
+                true
+            } else {
+                !step.from_unsafe_deref
+            }
+        };
+
         // find the first step that works
         steps
             .iter()
             .filter(|step| {
                 debug!("pick_core: step={:?}", step);
                 // skip types that are from a type error or that would require dereferencing
                 // a raw pointer
-                !step.self_ty.references_error() && !step.from_unsafe_deref
+                !step.self_ty.references_error() && allow_deref_raw_ptr(&step)
             })
             .flat_map(|step| {
                 let InferOk { value: self_ty, obligations: _ } = self
@@ -1085,6 +1105,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 self.pick_by_value_method(step, self_ty).or_else(|| {
                     self.pick_autorefd_method(step, self_ty, hir::Mutability::Not)
                         .or_else(|| self.pick_autorefd_method(step, self_ty, hir::Mutability::Mut))
+                        .or_else(|| {
+                            // allow unsizing of raw pointers
+                            self.pick_raw_autorefd_slice_method(step, self_ty)
+                        })
                 })
             })
             .next()
@@ -1701,6 +1725,43 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             self.tcx.associated_items(def_id).in_definition_order().copied().collect()
         }
     }
+    fn pick_raw_autorefd_slice_method(
+        &mut self,
+        step: &CandidateStep<'tcx>,
+        self_ty: Ty<'tcx>,
+    ) -> Option<PickResult<'tcx>> {
+        // To enable raw-ptr unsizing we try to autoref slices to raw pointers
+        debug!("pick_raw_ptr_autorefd_slice_method(step: {:?}, self_ty: {:?}", step, self_ty);
+        if let &ty::Slice(_) = self_ty.kind() {
+            let tcx = self.tcx;
+
+            let autoref_ptr_ty =
+                tcx.mk_ptr(ty::TypeAndMut { ty: self_ty, mutbl: hir::Mutability::Not });
+            return self
+                .pick_method(autoref_ptr_ty)
+                .map(|r| {
+                    r.map(|mut pick| {
+                        pick.autoderefs = step.autoderefs;
+                        pick.autoref = Some(hir::Mutability::Not);
+                        pick.unsize = step.unsize.then_some(self_ty);
+                        pick
+                    })
+                })
+                .or_else(|| {
+                    let autoref_mut_ptr_ty =
+                        tcx.mk_ptr(ty::TypeAndMut { ty: self_ty, mutbl: hir::Mutability::Mut });
+                    self.pick_method(autoref_mut_ptr_ty).map(|r| {
+                        r.map(|mut pick| {
+                            pick.autoderefs = step.autoderefs;
+                            pick.autoref = Some(hir::Mutability::Mut);
+                            pick.unsize = step.unsize.then_some(self_ty);
+                            pick
+                        })
+                    })
+                });
+        }
+        None
+    }
 }
 
 impl<'tcx> Candidate<'tcx> {

src/test/ui/raw-ref-op/raw-ref-unsize-coercion-receiver.rs

@@ -0,0 +1,20 @@
+#![feature(raw_ref_op)]
+
+fn main() {
+  let a1 = [4,5,6];
+  let p1 = &raw const a1;
+  let _ = unsafe { p1.get_unchecked(1) };
+
+  let mut a2 = [4,5,6];
+  let p2 = &raw mut a2;
+  let _ = unsafe { p2.get_unchecked(1) };
+
+  let mut a3 = [4,5,6];
+  let p3 = &raw mut a3;
+  let _ = unsafe { p3.get_unchecked_mut(1) };
+
+  let a4 = [4,5,6];
+  let p4 = &raw const a4;
+  let _ = unsafe { p4.get_unchecked_mut(1) };
+  //~^ ERROR cannot borrow
+}

src/test/ui/raw-ref-op/raw-ref-unsize-coercion-receiver.stderr

@@ -0,0 +1,11 @@
+error[E0596]: cannot borrow `*p4` as mutable, as it is behind a `*const` pointer
+  --> $DIR/raw-ref-unsize-coercion-receiver.rs:18:20
+   |
+LL |   let p4 = &raw const a4;
+   |            ------------- help: consider changing this to be a mutable pointer: `&mut raw const a4`
+LL |   let _ = unsafe { p4.get_unchecked_mut(1) };
+   |                    ^^ `p4` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.