Normalize struct tail properly in borrowck

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -7,6 +7,7 @@ use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, Upcast};
 use rustc_span::def_id::DefId;
 use rustc_span::Span;
+use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
 use rustc_trait_selection::traits::query::type_op::{self, TypeOpOutput};
 use rustc_trait_selection::traits::ObligationCause;
 
@@ -165,6 +166,52 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         result.unwrap_or(value)
     }
 
+    #[instrument(skip(self), level = "debug")]
+    pub(super) fn struct_tail(
+        &mut self,
+        ty: Ty<'tcx>,
+        location: impl NormalizeLocation,
+    ) -> Ty<'tcx> {
+        let tcx = self.tcx();
+        if self.infcx.next_trait_solver() {
+            let body = self.body;
+            let param_env = self.param_env;
+            self.fully_perform_op(
+                location.to_locations(),
+                ConstraintCategory::Boring,
+                CustomTypeOp::new(
+                    |ocx| {
+                        let normalize = |ty| {
+                            ocx.structurally_normalize(
+                                &ObligationCause::misc(
+                                    location.to_locations().span(body),
+                                    body.source.def_id().expect_local(),
+                                ),
+                                param_env,
+                                ty,
+                            )
+                            .unwrap_or_else(|_| bug!("struct tail should have been computable, since we computed it in HIR"))
+                        };
+
+                        let tail = tcx.struct_tail_with_normalize(
+                            ty,
+                            normalize,
+                            || {},
+                        );
+
+                        Ok(normalize(tail))
+                    },
+                    "s",
+                ),
+            )
+            .unwrap_or_else(|guar| Ty::new_error(tcx, guar))
+        } else {
+            let mut normalize = |ty| self.normalize(ty, location);
+            let tail = tcx.struct_tail_with_normalize(ty, &mut normalize, || {});
+            normalize(tail)
+        }
+    }
+
     #[instrument(skip(self), level = "debug")]
     pub(super) fn ascribe_user_type(
         &mut self,

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2329,17 +2329,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                         let cast_ty_to = CastTy::from_ty(*ty);
                         match (cast_ty_from, cast_ty_to) {
                             (Some(CastTy::Ptr(src)), Some(CastTy::Ptr(dst))) => {
-                                let mut normalize = |t| self.normalize(t, location);
-
-                                // N.B. `struct_tail_with_normalize` only "structurally resolves"
-                                // the type. It is not fully normalized, so we have to normalize it
-                                // afterwards.
-                                let src_tail =
-                                    tcx.struct_tail_with_normalize(src.ty, &mut normalize, || ());
-                                let src_tail = normalize(src_tail);
-                                let dst_tail =
-                                    tcx.struct_tail_with_normalize(dst.ty, &mut normalize, || ());
-                                let dst_tail = normalize(dst_tail);
+                                let src_tail = self.struct_tail(src.ty, location);
+                                let dst_tail = self.struct_tail(dst.ty, location);
 
                                 // This checks (lifetime part of) vtable validity for pointer casts,
                                 // which is irrelevant when there are aren't principal traits on both sides (aka only auto traits).

compiler/rustc_hir_typeck/src/cast.rs

@@ -97,6 +97,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return Ok(Some(PointerKind::Thin));
         }
 
+        let t = self.try_structurally_resolve_type(span, t);
+
         Ok(match *t.kind() {
             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
             ty::Dynamic(tty, _, ty::Dyn) => Some(PointerKind::VTable(tty)),

tests/ui/cast/ptr-to-trait-obj-different-regions-id-trait.stderr → tests/ui/cast/ptr-to-trait-obj-different-regions-id-trait.current.stderr

@@ -1,5 +1,5 @@
 error: lifetime may not live long enough
-  --> $DIR/ptr-to-trait-obj-different-regions-id-trait.rs:21:17
+  --> $DIR/ptr-to-trait-obj-different-regions-id-trait.rs:24:17
    |
 LL | fn m<'a>() {
    |      -- lifetime `'a` defined here

tests/ui/cast/ptr-to-trait-obj-different-regions-id-trait.next.stderr

@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-different-regions-id-trait.rs:24:17
+   |
+LL | fn m<'a>() {
+   |      -- lifetime `'a` defined here
+LL |     let unsend: *const dyn Cat<'a> = &();
+LL |     let _send = unsend as *const S<dyn Cat<'static>>;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type `S<dyn Cat<'_>>`, which makes the generic argument `dyn Cat<'_>` invariant
+   = note: the struct `S<T>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to 1 previous error
+

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

@@ -1,3 +1,6 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
 //@ check-fail
 //
 // Make sure we can't trick the compiler by using a projection.