borrowck ptr-to-ptr casts via traits

Author: Lukas Markeffsky

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2296,7 +2296,27 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                         let cast_ty_from = CastTy::from_ty(ty_from);
                         let cast_ty_to = CastTy::from_ty(*ty);
                         match (cast_ty_from, cast_ty_to) {
-                            (Some(CastTy::Ptr(_)), Some(CastTy::Ptr(_))) => (),
+                            (Some(CastTy::Ptr(pointee_from)), Some(CastTy::Ptr(pointee_to))) => {
+                                let metadata_def_id =
+                                    tcx.require_lang_item(LangItem::Metadata, Some(span));
+                                let metadata_from =
+                                    Ty::new_projection(tcx, metadata_def_id, [pointee_from.ty]);
+                                let metadata_to =
+                                    Ty::new_projection(tcx, metadata_def_id, [pointee_to.ty]);
+
+                                let predicate = ty::TraitRef::from_lang_item(
+                                    tcx,
+                                    LangItem::MetadataCast,
+                                    span,
+                                    [metadata_from, metadata_to],
+                                );
+
+                                self.prove_predicate(
+                                    predicate,
+                                    location.to_locations(),
+                                    ConstraintCategory::Cast { unsize_to: None },
+                                );
+                            }
                             _ => {
                                 span_mirbug!(
                                     self,

tests/ui/traits/metadata-cast-fail-lifetimes.rs

@@ -37,3 +37,13 @@ where
     cast(ptr)
     //~^ ERROR may not live long enough
 }
+
+fn borrowck_cast<'short, 'long: 'short, T, U>(ptr: *mut T) -> *mut U
+where
+    T: ?Sized + Pointee<Metadata = &'short ()>,
+    U: ?Sized + Pointee<Metadata = &'long ()>,
+{
+    // test that pointer-to-pointer casts are borrow-checked
+    ptr as _
+    //~^ ERROR may not live long enough
+}

tests/ui/traits/metadata-cast-fail-lifetimes.stderr

@@ -36,5 +36,18 @@ LL |     cast(ptr)
    |
    = help: consider adding the following bound: `'short: 'long`
 
-error: aborting due to 3 previous errors
+error: lifetime may not live long enough
+  --> $DIR/metadata-cast-fail-lifetimes.rs:47:5
+   |
+LL | fn borrowck_cast<'short, 'long: 'short, T, U>(ptr: *mut T) -> *mut U
+   |                  ------  ----- lifetime `'long` defined here
+   |                  |
+   |                  lifetime `'short` defined here
+...
+LL |     ptr as _
+   |     ^^^^^^^^ cast requires that `'short` must outlive `'long`
+   |
+   = help: consider adding the following bound: `'short: 'long`
+
+error: aborting due to 4 previous errors