implement Waffle's cast semantics

compiler/rustc_middle/src/traits/select.rs

@@ -180,11 +180,13 @@ pub enum SelectionCandidate<'tcx> {
 }
 
 #[derive(PartialEq, Eq, Debug, Clone, TypeVisitable)]
-pub enum MetadataCastKind {
+pub enum MetadataCastKind<'tcx> {
     /// No further obligations.
     Unconditional,
     /// `T: MetadataCast<U>` if `T <: U`.
     Subtype,
+    /// Require principals of dyn traits to be equal.
+    Dyn(ty::PolyExistentialTraitRef<'tcx>, ty::PolyExistentialTraitRef<'tcx>),
 }
 
 /// The result of trait evaluation. The order is important

compiler/rustc_middle/src/ty/context.rs

@@ -1760,7 +1760,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// does not compute the full elaborated super-predicates but just the set of def-ids. It is used
     /// to identify which traits may define a given associated type to help avoid cycle errors.
     /// Returns a `DefId` iterator.
-    fn super_traits_of(self, trait_def_id: DefId) -> impl Iterator<Item = DefId> + 'tcx {
+    pub fn super_traits_of(self, trait_def_id: DefId) -> impl Iterator<Item = DefId> + 'tcx {
         let mut set = FxHashSet::default();
         let mut stack = vec![trait_def_id];
 

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -1183,21 +1183,47 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
+        let tcx = self.tcx();
         if let ty::Adt(src_def, src_args) = source.kind()
             && let ty::Adt(tgt_def, tgt_args) = target.kind()
             && src_def == tgt_def
-            && Some(src_def.did()) == self.tcx().lang_items().dyn_metadata()
+            && Some(src_def.did()) == tcx.lang_items().dyn_metadata()
         {
             let src_dyn = src_args.type_at(0);
             let tgt_dyn = tgt_args.type_at(0);
 
-            // We could theoretically allow casting the principal away, but `as` casts
-            // don't allow that, so neither does `MetadataCast` for now.
-            if let ty::Dynamic(src_pred, _, ty::Dyn) = src_dyn.kind()
-                && let ty::Dynamic(tgt_pred, _, ty::Dyn) = tgt_dyn.kind()
-                && src_pred.principal_def_id() == tgt_pred.principal_def_id()
+            if let ty::Dynamic(src_preds, _, ty::Dyn) = src_dyn.kind()
+                && let ty::Dynamic(tgt_preds, _, ty::Dyn) = tgt_dyn.kind()
             {
-                candidates.vec.push(MetadataCastCandidate(MetadataCastKind::Unconditional));
+                let mut src_traits = src_preds.auto_traits().collect::<Vec<_>>();
+
+                if let Some(src_principal_did) = src_preds.principal_def_id() {
+                    src_traits.extend(tcx.super_traits_of(src_principal_did));
+                }
+
+                for tgt_auto_trait in tgt_preds.auto_traits() {
+                    if !src_traits.contains(&tgt_auto_trait) {
+                        // Adding auto traits that aren't on the source or implied by
+                        // the source principal is not allowed.
+                        return;
+                    }
+                }
+
+                if let Some(src_principal) = src_preds.principal() {
+                    if let Some(tgt_principal) = tgt_preds.principal() {
+                        candidates.vec.push(MetadataCastCandidate(MetadataCastKind::Dyn(
+                            src_principal,
+                            tgt_principal,
+                        )));
+                    } else {
+                        // We could theoretically allow casting the principal away, but `as` casts
+                        // don't allow that, so neither does `MetadataCast` for now.
+                    }
+                } else if tgt_preds.principal().is_none() {
+                    // Casting between auto-trait-only trait objects is allowed if the target
+                    // traits are a subset of the source traits, which we checked above.
+                    candidates.vec.push(MetadataCastCandidate(MetadataCastKind::Unconditional));
+                }
                 return;
             }
         }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -281,7 +281,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn confirm_metadata_cast_candidate(
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
-        kind: MetadataCastKind,
+        kind: MetadataCastKind<'tcx>,
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         match kind {
             MetadataCastKind::Unconditional => Ok(Vec::new()),
@@ -297,6 +297,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
                 Ok(obligations)
             }
+            MetadataCastKind::Dyn(source, target) => {
+                let InferOk { obligations, .. } = self
+                    .infcx
+                    .at(&obligation.cause, obligation.param_env)
+                    .eq(DefineOpaqueTypes::No, source, target)
+                    .map_err(|_| Unimplemented)?;
+
+                Ok(obligations)
+            }
         }
     }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1910,7 +1910,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | MetadataCastCandidate(MetadataCastKind::Subtype)
+                | MetadataCastCandidate(MetadataCastKind::Subtype | MetadataCastKind::Dyn(..))
                 | TraitAliasCandidate,
                 ParamCandidate(ref victim_cand),
             ) => {
@@ -2073,7 +2073,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | MetadataCastCandidate(MetadataCastKind::Subtype)
+                | MetadataCastCandidate(MetadataCastKind::Subtype | MetadataCastKind::Dyn(..))
                 | TraitAliasCandidate,
                 ImplCandidate(_)
                 | ClosureCandidate { .. }
@@ -2086,7 +2086,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | MetadataCastCandidate(MetadataCastKind::Subtype)
+                | MetadataCastCandidate(MetadataCastKind::Subtype | MetadataCastKind::Dyn(..))
                 | TraitAliasCandidate,
             ) => DropVictim::No,
         }

library/alloc/src/boxed.rs

@@ -2192,7 +2192,7 @@ impl dyn Error + Send {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
             // Reapply the `Send` marker.
-            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
         })
     }
 }
@@ -2205,8 +2205,8 @@ impl dyn Error + Send + Sync {
     pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
-            // Reapply the `Send + Sync` marker.
-            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
+            // Reapply the `Send + Sync` markers.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
         })
     }
 }

tests/ui/cast/cast-rfc0401-vtable-kinds.rs

@@ -4,25 +4,6 @@
 
 #![feature(unsized_tuple_coercion)]
 
-trait Foo<T> {
-    fn foo(&self, _: T) -> u32 { 42 }
-}
-
-trait Bar {
-    fn bar(&self) { println!("Bar!"); }
-}
-
-impl<T> Foo<T> for () {}
-impl Foo<u32> for u32 { fn foo(&self, _: u32) -> u32 { self+43 } }
-impl Bar for () {}
-
-unsafe fn round_trip_and_call<'a>(t: *const (dyn Foo<u32>+'a)) -> u32 {
-    let foo_e : *const dyn Foo<u16> = t as *const _;
-    let r_1 = foo_e as *mut dyn Foo<u32>;
-
-    (&*r_1).foo(0)
-}
-
 #[repr(C)]
 struct FooS<T:?Sized>(T);
 #[repr(C)]
@@ -38,11 +19,6 @@ fn tuple_i32_to_u32<T:?Sized>(u: *const (i32, T)) -> *const (u32, T) {
 
 
 fn main() {
-    let x = 4u32;
-    let y : &dyn Foo<u32> = &x;
-    let fl = unsafe { round_trip_and_call(y as *const dyn Foo<u32>) };
-    assert_eq!(fl, (43+4));
-
     let s = FooS([0,1,2]);
     let u: &FooS<[u32]> = &s;
     let u: *const FooS<[u32]> = u;

tests/ui/cast/fat-ptr-cast-borrowck.rs

@@ -0,0 +1,8 @@
+trait LifetimeParam<'a> {}
+fn lifetime_param<'a, 'b>(ptr: *const dyn LifetimeParam<'a>) -> *const dyn LifetimeParam<'b> {
+    ptr as _
+    //~^ ERROR lifetime may not live long enough
+    //~| ERROR lifetime may not live long enough
+}
+
+fn main() {}

tests/ui/cast/fat-ptr-cast-borrowck.stderr

@@ -0,0 +1,28 @@
+error: lifetime may not live long enough
+  --> $DIR/fat-ptr-cast-borrowck.rs:3:5
+   |
+LL | fn lifetime_param<'a, 'b>(ptr: *const dyn LifetimeParam<'a>) -> *const dyn LifetimeParam<'b> {
+   |                   --  -- lifetime `'b` defined here
+   |                   |
+   |                   lifetime `'a` defined here
+LL |     ptr 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/fat-ptr-cast-borrowck.rs:3:5
+   |
+LL | fn lifetime_param<'a, 'b>(ptr: *const dyn LifetimeParam<'a>) -> *const dyn LifetimeParam<'b> {
+   |                   --  -- lifetime `'b` defined here
+   |                   |
+   |                   lifetime `'a` defined here
+LL |     ptr 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: aborting due to 2 previous errors
+

tests/ui/cast/fat-ptr-cast-rpass.rs

@@ -2,7 +2,7 @@
 
 #![feature(ptr_metadata)]
 
-trait Foo {
+trait Foo: Send {
     fn foo(&self) {}
 }
 
@@ -54,10 +54,10 @@ fn main() {
     assert_eq!(b, d);
     assert_eq!(c, d);
 
-    // Adding auto traits is OK.
+    // Adding auto traits is OK if they're implied by the principal.
     let _ = a as *mut (dyn Foo + Send);
 
-    // Casting between auto-trait-only trait objects is OK.
-    let unprincipled: *mut dyn Send = &mut Bar;
+    // Removing traits from auto-trait-only trait objects is OK.
+    let unprincipled: *mut (dyn Send + Sync) = &mut Bar;
     let _ = unprincipled as *mut dyn Sync;
 }

tests/ui/cast/fat-ptr-cast.rs

@@ -81,9 +81,17 @@ fn main() {
     let _ = cf as *const [u16]; //~ ERROR is invalid
     let _ = cf as *const dyn Bar; //~ ERROR is invalid
 
+    // Adding auto traits is not allowed.
+    let _ = cf as *const (dyn Foo + Send); //~ ERROR is invalid
+
     // casting principal away is not allowed for now
     let _ = cf as *const dyn Send; //~ ERROR is invalid
 
+    // Casting between auto-trait-only trait objects requires the target traits
+    // to be a subset of the source traits.
+    let unprincipled: *mut dyn Send = &mut ();
+    let _ = unprincipled as *mut dyn Sync; //~ ERROR is invalid
+
     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>(); //~ ERROR is invalid
 }
 
@@ -101,3 +109,8 @@ fn illegal_cast_2<U:?Sized>(u: *const U) -> *const str
 {
     u as *const str //~ ERROR is invalid
 }
+
+trait TypeParam<T> {}
+fn type_param<T, U>(ptr: *const dyn TypeParam<T>) -> *const dyn TypeParam<U> {
+    ptr as _ //~ ERROR is invalid
+}

tests/ui/cast/fat-ptr-cast.stderr

@@ -278,14 +278,30 @@ LL |     let _ = cf as *const dyn Bar;
    |
    = note: vtable kinds may not match
 
-error[E0606]: casting `*const dyn Foo` as `*const dyn Send` is invalid
+error[E0606]: casting `*const dyn Foo` as `*const dyn Foo + Send` is invalid
   --> $DIR/fat-ptr-cast.rs:85:13
    |
+LL |     let _ = cf as *const (dyn Foo + Send);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: vtable kinds may not match
+
+error[E0606]: casting `*const dyn Foo` as `*const dyn Send` is invalid
+  --> $DIR/fat-ptr-cast.rs:88:13
+   |
 LL |     let _ = cf as *const dyn Send;
    |             ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: vtable kinds may not match
 
+error[E0606]: casting `*mut dyn Send` as `*mut dyn Sync` is invalid
+  --> $DIR/fat-ptr-cast.rs:93:13
+   |
+LL |     let _ = unprincipled as *mut dyn Sync;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: vtable kinds may not match
+
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
   --> $DIR/fat-ptr-cast.rs:66:13
    |
@@ -305,7 +321,7 @@ LL |     let _ = a as *const dyn Foo;
    = note: required for the cast from `*const str` to `*const dyn Foo`
 
 error[E0606]: casting `&{float}` as `f32` is invalid
-  --> $DIR/fat-ptr-cast.rs:87:30
+  --> $DIR/fat-ptr-cast.rs:95:30
    |
 LL |     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
    |                              ^^^^^^^^
@@ -316,30 +332,38 @@ LL |     vec![0.0].iter().map(|s| *s as f32).collect::<Vec<f32>>();
    |                              +
 
 error[E0606]: cannot cast `usize` to a pointer that may be wide
-  --> $DIR/fat-ptr-cast.rs:91:18
+  --> $DIR/fat-ptr-cast.rs:99:18
    |
 LL |     let s = 0 as *const T;
    |             -    ^^^^^^^^ creating a `*const T` requires both an address and type-specific metadata
    |             |
    |             consider casting this expression to `*const ()`, then using `core::ptr::from_raw_parts`
 
 error[E0606]: casting `*const U` as `*const V` is invalid
-  --> $DIR/fat-ptr-cast.rs:97:5
+  --> $DIR/fat-ptr-cast.rs:105:5
    |
 LL |     u as *const V
    |     ^^^^^^^^^^^^^
    |
    = note: vtable kinds may not match
 
 error[E0606]: casting `*const U` as `*const str` is invalid
-  --> $DIR/fat-ptr-cast.rs:102:5
+  --> $DIR/fat-ptr-cast.rs:110:5
    |
 LL |     u as *const str
    |     ^^^^^^^^^^^^^^^
    |
    = note: vtable kinds may not match
 
-error: aborting due to 46 previous errors
+error[E0606]: casting `*const (dyn TypeParam<T> + 'static)` as `*const dyn TypeParam<U>` is invalid
+  --> $DIR/fat-ptr-cast.rs:115:5
+   |
+LL |     ptr as _
+   |     ^^^^^^^^
+   |
+   = note: vtable kinds may not match
+
+error: aborting due to 49 previous errors
 
 Some errors have detailed explanations: E0054, E0277, E0604, E0605, E0606, E0607, E0609.
 For more information about an error, try `rustc --explain E0054`.

tests/ui/suggestions/issue-114797-bad-parentheses-dyn-trait.fixed

@@ -5,7 +5,7 @@ trait Trait {}
 
 fn assert_send(ptr: *mut dyn Trait) -> *mut (dyn Trait + Send) {
     //~^ ERROR incorrect parentheses around trait bounds
-    ptr as _
+    unsafe { std::mem::transmute(ptr) }
 }
 
 fn foo2(_: &(dyn Trait + Send)) {}

tests/ui/suggestions/issue-114797-bad-parentheses-dyn-trait.rs

@@ -5,7 +5,7 @@ trait Trait {}
 
 fn assert_send(ptr: *mut dyn Trait) -> *mut dyn (Trait + Send) {
     //~^ ERROR incorrect parentheses around trait bounds
-    ptr as _
+    unsafe { std::mem::transmute(ptr) }
 }
 
 fn foo2(_: &dyn (Trait + Send)) {}

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

@@ -23,6 +23,7 @@ fn main() {
     check::<(), DynMetadata<dyn Trait>>(); //~ ERROR not satisfied
     check::<usize, DynMetadata<dyn Trait>>(); //~ ERROR not satisfied
     check::<DynMetadata<dyn Trait>, usize>(); //~ ERROR not satisfied
+    check::<DynMetadata<dyn Trait>, dyn Trait + Send>(); //~ ERROR not satisfied
     check::<DynMetadata<dyn Trait>, DynMetadata<dyn Trait2>>(); //~ ERROR not satisfied
     check::<dyn Trait + Send, dyn Trait + Sync>(); //~ ERROR not satisfied