Rollup merge of #136127 - WaffleLapkin:dyn_ptr_unwrap_cast, r=compiler-errors

Allow `*const W<dyn A> -> *const dyn A` ptr cast

Followup of https://github.com/rust-lang/rust/pull/120248#discussion_r1487936000.

This PR allows casting pointers from something wrapping a trait object, to the trait object, i.e. `*const W<dyn A> -> *const dyn A` where `W` is `struct W<T: ?Sized>(T);`.

r? compiler-errors

Fixes https://github.com/rust-lang/rust/issues/128625

compiler/rustc_hir_typeck/src/coercion.rs

@@ -41,8 +41,8 @@ use rustc_abi::ExternAbi;
 use rustc_attr_parsing::InlineAttr;
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, Diag, struct_span_code_err};
-use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::{self as hir, LangItem};
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::infer::relate::RelateResult;
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
@@ -56,7 +56,7 @@ use rustc_middle::ty::adjustment::{
 };
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::visit::TypeVisitableExt;
-use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
+use rustc_middle::ty::{self, AliasTy, GenericArgsRef, Ty, TyCtxt};
 use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span};
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
@@ -593,6 +593,63 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         // Create an obligation for `Source: CoerceUnsized<Target>`.
         let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
+        let root_obligation = Obligation::new(
+            self.tcx,
+            cause.clone(),
+            self.fcx.param_env,
+            ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target]),
+        );
+
+        // If the root `Source: CoerceUnsized<Target>` obligation can't possibly hold,
+        // we don't have to assume that this is unsizing coercion (it will always lead to an error)
+        //
+        // However, we don't want to bail early all the time, since the unholdable obligations
+        // may be interesting for diagnostics (such as trying to coerce `&T` to `&dyn Id<This = U>`),
+        // so we only bail if there (likely) is another way to convert the types.
+        if !self.infcx.predicate_may_hold(&root_obligation) {
+            if let Some(dyn_metadata_adt_def_id) = self.tcx.lang_items().get(LangItem::DynMetadata)
+                && let Some(metadata_type_def_id) = self.tcx.lang_items().get(LangItem::Metadata)
+            {
+                self.probe(|_| {
+                    let ocx = ObligationCtxt::new(&self.infcx);
+
+                    // returns `true` if `<ty as Pointee>::Metadata` is `DynMetadata<_>`
+                    let has_dyn_trait_metadata = |ty| {
+                        let metadata_ty: Result<_, _> = ocx.structurally_normalize_ty(
+                            &ObligationCause::dummy(),
+                            self.fcx.param_env,
+                            Ty::new_alias(
+                                self.tcx,
+                                ty::AliasTyKind::Projection,
+                                AliasTy::new(self.tcx, metadata_type_def_id, [ty]),
+                            ),
+                        );
+
+                        metadata_ty.is_ok_and(|metadata_ty| {
+                            metadata_ty
+                                .ty_adt_def()
+                                .is_some_and(|d| d.did() == dyn_metadata_adt_def_id)
+                        })
+                    };
+
+                    // If both types are raw pointers to a (wrapper over a) trait object,
+                    // this might be a cast like `*const W<dyn Trait> -> *const dyn Trait`.
+                    // So it's better to bail and try that. (even if the cast is not possible, for
+                    // example due to vtables not matching, cast diagnostic will likely still be better)
+                    //
+                    // N.B. use `target`, not `coerce_target` (the latter is a var)
+                    if let &ty::RawPtr(source_pointee, _) = coerce_source.kind()
+                        && let &ty::RawPtr(target_pointee, _) = target.kind()
+                        && has_dyn_trait_metadata(source_pointee)
+                        && has_dyn_trait_metadata(target_pointee)
+                    {
+                        return Err(TypeError::Mismatch);
+                    }
+
+                    Ok(())
+                })?;
+            }
+        }
 
         // Use a FIFO queue for this custom fulfillment procedure.
         //
@@ -601,12 +658,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // and almost never more than 3. By using a SmallVec we avoid an
         // allocation, at the (very small) cost of (occasionally) having to
         // shift subsequent elements down when removing the front element.
-        let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![Obligation::new(
-            self.tcx,
-            cause,
-            self.fcx.param_env,
-            ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
-        )];
+        let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![root_obligation];
 
         // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
         // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where

tests/ui/cast/ptr-to-trait-obj-wrap-add-auto.rs

@@ -0,0 +1,36 @@
+// Combination of `ptr-to-trait-obj-wrap.rs` and `ptr-to-trait-obj-add-auto.rs`.
+//
+// Checks that you *can't* add auto traits to trait object in pointer casts involving wrapping said
+// traits structures.
+
+trait A {}
+
+struct W<T: ?Sized>(T);
+struct X<T: ?Sized>(T);
+
+fn unwrap(a: *const W<dyn A>) -> *const (dyn A + Send) {
+    a as _
+    //~^ error: cannot add auto trait `Send` to dyn bound via pointer cast
+}
+
+fn unwrap_nested(a: *const W<W<dyn A>>) -> *const W<dyn A + Send> {
+    a as _
+    //~^ error: cannot add auto trait `Send` to dyn bound via pointer cast
+}
+
+fn rewrap(a: *const W<dyn A>) -> *const X<dyn A + Send> {
+    a as _
+    //~^ error: cannot add auto trait `Send` to dyn bound via pointer cast
+}
+
+fn rewrap_nested(a: *const W<W<dyn A>>) -> *const W<X<dyn A + Send>> {
+    a as _
+    //~^ error: cannot add auto trait `Send` to dyn bound via pointer cast
+}
+
+fn wrap(a: *const dyn A) -> *const W<dyn A + Send> {
+    a as _
+    //~^ error: cannot add auto trait `Send` to dyn bound via pointer cast
+}
+
+fn main() {}

tests/ui/cast/ptr-to-trait-obj-wrap-add-auto.stderr

@@ -0,0 +1,48 @@
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-wrap-add-auto.rs:12:5
+   |
+LL |     a as _
+   |     ^^^^^^ unsupported cast
+   |
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
+
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-wrap-add-auto.rs:17:5
+   |
+LL |     a as _
+   |     ^^^^^^ unsupported cast
+   |
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
+
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-wrap-add-auto.rs:22:5
+   |
+LL |     a as _
+   |     ^^^^^^ unsupported cast
+   |
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
+
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-wrap-add-auto.rs:27:5
+   |
+LL |     a as _
+   |     ^^^^^^ unsupported cast
+   |
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
+
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-wrap-add-auto.rs:32:5
+   |
+LL |     a as _
+   |     ^^^^^^ unsupported cast
+   |
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0804`.

tests/ui/cast/ptr-to-trait-obj-wrap-different-args.rs

@@ -0,0 +1,36 @@
+// Combination of `ptr-to-trait-obj-different-args.rs` and `ptr-to-trait-obj-wrap.rs`.
+//
+// Checks that you *can't* change type arguments of trait objects in pointer casts involving
+// wrapping said traits structures.
+
+trait A<T> {}
+
+struct W<T: ?Sized>(T);
+struct X<T: ?Sized>(T);
+
+fn unwrap<F, G>(a: *const W<dyn A<F>>) -> *const dyn A<G> {
+    a as _
+    //~^ error casting `*const W<(dyn A<F> + 'static)>` as `*const dyn A<G>` is invalid
+}
+
+fn unwrap_nested<F, G>(a: *const W<W<dyn A<F>>>) -> *const W<dyn A<G>> {
+    a as _
+    //~^ error casting `*const W<W<(dyn A<F> + 'static)>>` as `*const W<dyn A<G>>` is invalid
+}
+
+fn rewrap<F, G>(a: *const W<dyn A<F>>) -> *const X<dyn A<G>> {
+    a as _
+    //~^ error: casting `*const W<(dyn A<F> + 'static)>` as `*const X<dyn A<G>>` is invalid
+}
+
+fn rewrap_nested<F, G>(a: *const W<W<dyn A<F>>>) -> *const W<X<dyn A<G>>> {
+    a as _
+    //~^ error: casting `*const W<W<(dyn A<F> + 'static)>>` as `*const W<X<dyn A<G>>>` is invalid
+}
+
+fn wrap<F, G>(a: *const dyn A<F>) -> *const W<dyn A<G>> {
+    a as _
+    //~^ error: casting `*const (dyn A<F> + 'static)` as `*const W<dyn A<G>>` is invalid
+}
+
+fn main() {}

tests/ui/cast/ptr-to-trait-obj-wrap-different-args.stderr

@@ -0,0 +1,43 @@
+error[E0606]: casting `*const W<(dyn A<F> + 'static)>` as `*const dyn A<G>` is invalid
+  --> $DIR/ptr-to-trait-obj-wrap-different-args.rs:12:5
+   |
+LL |     a as _
+   |     ^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error[E0606]: casting `*const W<W<(dyn A<F> + 'static)>>` as `*const W<dyn A<G>>` is invalid
+  --> $DIR/ptr-to-trait-obj-wrap-different-args.rs:17:5
+   |
+LL |     a as _
+   |     ^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error[E0606]: casting `*const W<(dyn A<F> + 'static)>` as `*const X<dyn A<G>>` is invalid
+  --> $DIR/ptr-to-trait-obj-wrap-different-args.rs:22:5
+   |
+LL |     a as _
+   |     ^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error[E0606]: casting `*const W<W<(dyn A<F> + 'static)>>` as `*const W<X<dyn A<G>>>` is invalid
+  --> $DIR/ptr-to-trait-obj-wrap-different-args.rs:27:5
+   |
+LL |     a as _
+   |     ^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error[E0606]: casting `*const (dyn A<F> + 'static)` as `*const W<dyn A<G>>` is invalid
+  --> $DIR/ptr-to-trait-obj-wrap-different-args.rs:32:5
+   |
+LL |     a as _
+   |     ^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0606`.

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

@@ -0,0 +1,41 @@
+// Combination of `ptr-to-trait-obj-different-regions-misc.rs` and `ptr-to-trait-obj-wrap.rs`.
+//
+// Checks that you *can't* change lifetime arguments of trait objects in pointer casts involving
+// wrapping said traits structures.
+
+trait A<'a> {}
+
+struct W<T: ?Sized>(T);
+struct X<T: ?Sized>(T);
+
+fn unwrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const dyn A<'b> {
+    a as _
+    //~^ error
+    //~| error
+}
+
+fn unwrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<dyn A<'b>> {
+    a as _
+    //~^ error
+    //~| error
+}
+
+fn rewrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const X<dyn A<'b>> {
+    a as _
+    //~^ error: lifetime may not live long enough
+    //~| error: lifetime may not live long enough
+}
+
+fn rewrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<X<dyn A<'b>>> {
+    a as _
+    //~^ error: lifetime may not live long enough
+    //~| error: lifetime may not live long enough
+}
+
+fn wrap<'a, 'b>(a: *const dyn A<'a>) -> *const W<dyn A<'b>> {
+    a as _
+    //~^ error: lifetime may not live long enough
+    //~| error: lifetime may not live long enough
+}
+
+fn main() {}

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

@@ -0,0 +1,140 @@
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:12:5
+   |
+LL | fn unwrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const dyn A<'b> {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:12:5
+   |
+LL | fn unwrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const dyn A<'b> {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+help: `'b` and `'a` must be the same: replace one with the other
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:18:5
+   |
+LL | fn unwrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<dyn A<'b>> {
+   |                  --  -- lifetime `'b` defined here
+   |                  |
+   |                  lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:18:5
+   |
+LL | fn unwrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<dyn A<'b>> {
+   |                  --  -- lifetime `'b` defined here
+   |                  |
+   |                  lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+help: `'b` and `'a` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:24:5
+   |
+LL | fn rewrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const X<dyn A<'b>> {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:24:5
+   |
+LL | fn rewrap<'a, 'b>(a: *const W<dyn A<'a>>) -> *const X<dyn A<'b>> {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+help: `'b` and `'a` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:30:5
+   |
+LL | fn rewrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<X<dyn A<'b>>> {
+   |                  --  -- lifetime `'b` defined here
+   |                  |
+   |                  lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:30:5
+   |
+LL | fn rewrap_nested<'a, 'b>(a: *const W<W<dyn A<'a>>>) -> *const W<X<dyn A<'b>>> {
+   |                  --  -- lifetime `'b` defined here
+   |                  |
+   |                  lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+help: `'b` and `'a` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:36:5
+   |
+LL | fn wrap<'a, 'b>(a: *const dyn A<'a>) -> *const W<dyn A<'b>> {
+   |         --  -- lifetime `'b` defined here
+   |         |
+   |         lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-wrap-different-regions.rs:36:5
+   |
+LL | fn wrap<'a, 'b>(a: *const dyn A<'a>) -> *const W<dyn A<'b>> {
+   |         --  -- lifetime `'b` defined here
+   |         |
+   |         lifetime `'a` defined here
+LL |     a as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+help: `'b` and `'a` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 10 previous errors
+

tests/ui/cast/ptr-to-trait-obj-wrap.rs

@@ -0,0 +1,33 @@
+// Checks that various casts of pointers to trait objects wrapped in structures
+// work. Note that the metadata doesn't change when a DST is wrapped in a
+// structure, so these casts *are* fine.
+//
+// `unwrap` and `unwrap_nested` currently don't work due to a compiler limitation.
+//@ check-pass
+
+trait A {}
+
+struct W<T: ?Sized>(T);
+struct X<T: ?Sized>(T);
+
+fn unwrap(a: *const W<dyn A>) -> *const dyn A {
+    a as _
+}
+
+fn unwrap_nested(a: *const W<W<dyn A>>) -> *const W<dyn A> {
+    a as _
+}
+
+fn rewrap(a: *const W<dyn A>) -> *const X<dyn A> {
+    a as _
+}
+
+fn rewrap_nested(a: *const W<W<dyn A>>) -> *const W<X<dyn A>> {
+    a as _
+}
+
+fn wrap(a: *const dyn A) -> *const W<dyn A> {
+    a as _
+}
+
+fn main() {}