Auto merge of #143036 - compiler-errors:no-dyn-star, r=oli-obk

Remove support for `dyn*` from the compiler

This PR removes support for `dyn*` (#102425), which are a currently un-RFC'd experiment that was opened a few years ago to explore a component that we thought was necessary for AFIDT (async fn in dyn trait).

It doesn't seem like we are going to need `dyn*` types -- even in an not-exposed-to-the-user way[^1] -- for us to implement AFIDT. Given that AFIDT was the original motivating purpose of `dyn*` types, I don't really see a compelling reason to have to maintain their implementation in the compiler.

[^1]: Compared to, e.g., generators whih are an unstable building block we use to implement stable syntax like `async {}`.

We've learned quite a lot from `dyn*`, but I think at this point its current behavior leads to more questions than answers. For example, `dyn*` support today remains somewhat fragile; it ICEs in many cases where the current "normal" `dyn Trait` types rely on their unsizedness for their vtable-based implementation to be sound I wouldn't be surprised if it's unsound in other ways, though I didn't play around with it too much. See the examples below.

```rust
#![feature(dyn_star)]

trait Foo {
    fn hello(self);
}

impl Foo for usize {
    fn hello(self) {
        println!("hello, world");
    }
}

fn main() {
    let x: dyn* Foo = 1usize;
    x.hello();
}
```

And:

```rust
#![feature(dyn_star)]

trait Trait {
    type Out where Self: Sized;
}

fn main() {
    let x: <dyn* Trait as Trait>::Out;
}
```

...and probably many more problems having to do with the intersection of dyn-compatibility and `Self: Sized` bounds that I was too lazy to look into like:
* GATs
* Methods with invalid signatures
* Associated consts

Generally, `dyn*` types also end up getting in the way of working with [normal `dyn` types](#102425 (comment)) to an extent that IMO outweighs the benefit of experimentation.

I recognize that there are probably other, more creative usages of `dyn*` that are orthogonal to AFIDT. However, I think any work along those lines should first have to think through some of the more fundamental interactions between `dyn*` and dyn-compatibility before we think about reimplementing them in the type system.

---

I'm planning on removing the `DynKind` enum and the `PointerLike` built-in trait from the compiler after this PR lands.

Closes #102425.

cc `@eholk` `@rust-lang/lang` `@rust-lang/types`

Closes #116979.
Closes #119694.
Closes #134591.
Closes #104800.

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2585,8 +2585,7 @@ pub enum TyPatKind {
 pub enum TraitObjectSyntax {
     // SAFETY: When adding new variants make sure to update the `Tag` impl.
     Dyn = 0,
-    DynStar = 1,
-    None = 2,
+    None = 1,
 }
 
 /// SAFETY: `TraitObjectSyntax` only has 3 data-less variants which means
@@ -2602,8 +2601,7 @@ unsafe impl Tag for TraitObjectSyntax {
     unsafe fn from_usize(tag: usize) -> Self {
         match tag {
             0 => TraitObjectSyntax::Dyn,
-            1 => TraitObjectSyntax::DynStar,
-            2 => TraitObjectSyntax::None,
+            1 => TraitObjectSyntax::None,
             _ => unreachable!(),
         }
     }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -505,7 +505,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     );
     gate_all!(associated_const_equality, "associated const equality is incomplete");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
-    gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(ergonomic_clones, "ergonomic clones are experimental");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1303,7 +1303,6 @@ impl<'a> State<'a> {
             ast::TyKind::TraitObject(bounds, syntax) => {
                 match syntax {
                     ast::TraitObjectSyntax::Dyn => self.word_nbsp("dyn"),
-                    ast::TraitObjectSyntax::DynStar => self.word_nbsp("dyn*"),
                     ast::TraitObjectSyntax::None => {}
                 }
                 self.print_type_bounds(bounds);

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -25,9 +25,9 @@ use rustc_middle::traits::query::NoSolution;
 use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::{
-    self, Binder, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, CoroutineArgsExt,
-    Dynamic, GenericArgsRef, OpaqueHiddenType, OpaqueTypeKey, RegionVid, Ty, TyCtxt,
-    TypeVisitableExt, UserArgs, UserTypeAnnotationIndex, fold_regions,
+    self, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, CoroutineArgsExt,
+    GenericArgsRef, OpaqueHiddenType, OpaqueTypeKey, RegionVid, Ty, TyCtxt, TypeVisitableExt,
+    UserArgs, UserTypeAnnotationIndex, fold_regions,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_mir_dataflow::move_paths::MoveData;
@@ -1233,38 +1233,6 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         );
                     }
 
-                    CastKind::PointerCoercion(PointerCoercion::DynStar, coercion_source) => {
-                        // get the constraints from the target type (`dyn* Clone`)
-                        //
-                        // apply them to prove that the source type `Foo` implements `Clone` etc
-                        let (existential_predicates, region) = match ty.kind() {
-                            Dynamic(predicates, region, ty::DynStar) => (predicates, region),
-                            _ => panic!("Invalid dyn* cast_ty"),
-                        };
-
-                        let self_ty = op.ty(self.body, tcx);
-
-                        let is_implicit_coercion = coercion_source == CoercionSource::Implicit;
-                        self.prove_predicates(
-                            existential_predicates
-                                .iter()
-                                .map(|predicate| predicate.with_self_ty(tcx, self_ty)),
-                            location.to_locations(),
-                            ConstraintCategory::Cast { is_implicit_coercion, unsize_to: None },
-                        );
-
-                        let outlives_predicate = tcx.mk_predicate(Binder::dummy(
-                            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(
-                                ty::OutlivesPredicate(self_ty, *region),
-                            )),
-                        ));
-                        self.prove_predicate(
-                            outlives_predicate,
-                            location.to_locations(),
-                            ConstraintCategory::Cast { is_implicit_coercion, unsize_to: None },
-                        );
-                    }
-
                     CastKind::PointerCoercion(
                         PointerCoercion::MutToConstPointer,
                         coercion_source,

compiler/rustc_codegen_cranelift/src/abi/mod.rs

@@ -753,51 +753,6 @@ pub(crate) fn codegen_drop<'tcx>(
                 fx.bcx.ins().call_indirect(sig, drop_fn, &[ptr]);
                 fx.bcx.ins().jump(ret_block, &[]);
             }
-            ty::Dynamic(_, _, ty::DynStar) => {
-                // IN THIS ARM, WE HAVE:
-                // ty = *mut (dyn* Trait)
-                // which is: *mut exists<T: sizeof(T) == sizeof(usize)> (T, Vtable<T: Trait>)
-                //
-                // args = [ * ]
-                //          |
-                //          v
-                //      ( Data, Vtable )
-                //                |
-                //                v
-                //              /-------\
-                //              | ...   |
-                //              \-------/
-                //
-                //
-                // WE CAN CONVERT THIS INTO THE ABOVE LOGIC BY DOING
-                //
-                // data = &(*args[0]).0    // gives a pointer to Data above (really the same pointer)
-                // vtable = (*args[0]).1   // loads the vtable out
-                // (data, vtable)          // an equivalent Rust `*mut dyn Trait`
-                //
-                // SO THEN WE CAN USE THE ABOVE CODE.
-                let (data, vtable) = drop_place.to_cvalue(fx).dyn_star_force_data_on_stack(fx);
-                let drop_fn = crate::vtable::drop_fn_of_obj(fx, vtable);
-
-                let is_null = fx.bcx.ins().icmp_imm(IntCC::Equal, drop_fn, 0);
-                let target_block = fx.get_block(target);
-                let continued = fx.bcx.create_block();
-                fx.bcx.ins().brif(is_null, target_block, &[], continued, &[]);
-                fx.bcx.switch_to_block(continued);
-
-                let virtual_drop = Instance {
-                    def: ty::InstanceKind::Virtual(drop_instance.def_id(), 0),
-                    args: drop_instance.args,
-                };
-                let fn_abi = FullyMonomorphizedLayoutCx(fx.tcx)
-                    .fn_abi_of_instance(virtual_drop, ty::List::empty());
-
-                let sig = clif_sig_from_fn_abi(fx.tcx, fx.target_config.default_call_conv, &fn_abi);
-                let sig = fx.bcx.import_signature(sig);
-                fx.bcx.ins().call_indirect(sig, drop_fn, &[data]);
-                // FIXME implement cleanup on exceptions
-                fx.bcx.ins().jump(ret_block, &[]);
-            }
             _ => {
                 assert!(!matches!(drop_instance.def, InstanceKind::Virtual(_, _)));
 

compiler/rustc_codegen_cranelift/src/base.rs

@@ -790,14 +790,6 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                     let operand = codegen_operand(fx, operand);
                     crate::unsize::coerce_unsized_into(fx, operand, lval);
                 }
-                Rvalue::Cast(
-                    CastKind::PointerCoercion(PointerCoercion::DynStar, _),
-                    ref operand,
-                    _,
-                ) => {
-                    let operand = codegen_operand(fx, operand);
-                    crate::unsize::coerce_dyn_star(fx, operand, lval);
-                }
                 Rvalue::Cast(CastKind::Transmute, ref operand, _to_ty) => {
                     let operand = codegen_operand(fx, operand);
                     lval.write_cvalue_transmute(fx, operand);

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -112,21 +112,6 @@ fn unsize_ptr<'tcx>(
     }
 }
 
-/// Coerces `src` to `dst_ty` which is guaranteed to be a `dyn*` type.
-pub(crate) fn cast_to_dyn_star<'tcx>(
-    fx: &mut FunctionCx<'_, '_, 'tcx>,
-    src: Value,
-    src_ty_and_layout: TyAndLayout<'tcx>,
-    dst_ty: Ty<'tcx>,
-    old_info: Option<Value>,
-) -> (Value, Value) {
-    assert!(
-        matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
-        "destination type must be a dyn*"
-    );
-    (src, unsized_info(fx, src_ty_and_layout.ty, dst_ty, old_info))
-}
-
 /// Coerce `src`, which is a reference to a value of type `src_ty`,
 /// to a value of type `dst_ty` and store the result in `dst`
 pub(crate) fn coerce_unsized_into<'tcx>(
@@ -174,24 +159,6 @@ pub(crate) fn coerce_unsized_into<'tcx>(
     }
 }
 
-pub(crate) fn coerce_dyn_star<'tcx>(
-    fx: &mut FunctionCx<'_, '_, 'tcx>,
-    src: CValue<'tcx>,
-    dst: CPlace<'tcx>,
-) {
-    let (data, extra) = if let ty::Dynamic(_, _, ty::DynStar) = src.layout().ty.kind() {
-        let (data, vtable) = src.load_scalar_pair(fx);
-        (data, Some(vtable))
-    } else {
-        let data = src.load_scalar(fx);
-        (data, None)
-    };
-
-    let (data, vtable) = cast_to_dyn_star(fx, data, src.layout(), dst.layout().ty, extra);
-
-    dst.write_cvalue(fx, CValue::by_val_pair(data, vtable, dst.layout()));
-}
-
 // Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/glue.rs
 
 pub(crate) fn size_and_align_of<'tcx>(

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -121,43 +121,6 @@ impl<'tcx> CValue<'tcx> {
         }
     }
 
-    // FIXME remove
-    /// Forces the data value of a dyn* value to the stack and returns a pointer to it as well as the
-    /// vtable pointer.
-    pub(crate) fn dyn_star_force_data_on_stack(
-        self,
-        fx: &mut FunctionCx<'_, '_, 'tcx>,
-    ) -> (Value, Value) {
-        assert!(self.1.ty.is_dyn_star());
-
-        match self.0 {
-            CValueInner::ByRef(ptr, None) => {
-                let (a_scalar, b_scalar) = match self.1.backend_repr {
-                    BackendRepr::ScalarPair(a, b) => (a, b),
-                    _ => unreachable!("dyn_star_force_data_on_stack({:?})", self),
-                };
-                let b_offset = scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
-                let clif_ty2 = scalar_to_clif_type(fx.tcx, b_scalar);
-                let mut flags = MemFlags::new();
-                flags.set_notrap();
-                let vtable = ptr.offset(fx, b_offset).load(fx, clif_ty2, flags);
-                (ptr.get_addr(fx), vtable)
-            }
-            CValueInner::ByValPair(data, vtable) => {
-                let data_ptr = fx.create_stack_slot(
-                    u32::try_from(fx.target_config.pointer_type().bytes()).unwrap(),
-                    u32::try_from(fx.target_config.pointer_type().bytes()).unwrap(),
-                );
-                data_ptr.store(fx, data, MemFlags::trusted());
-
-                (data_ptr.get_addr(fx), vtable)
-            }
-            CValueInner::ByRef(_, Some(_)) | CValueInner::ByVal(_) => {
-                unreachable!("dyn_star_force_data_on_stack({:?})", self)
-            }
-        }
-    }
-
     pub(crate) fn try_to_ptr(self) -> Option<(Pointer, Option<Value>)> {
         match self.0 {
             CValueInner::ByRef(ptr, meta) => Some((ptr, meta)),

compiler/rustc_codegen_cranelift/src/vtable.rs

@@ -46,34 +46,22 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
     mut arg: CValue<'tcx>,
     idx: usize,
 ) -> (Pointer, Value) {
-    let (ptr, vtable) = 'block: {
-        if let BackendRepr::Scalar(_) = arg.layout().backend_repr {
-            while !arg.layout().ty.is_raw_ptr() && !arg.layout().ty.is_ref() {
-                let (idx, _) = arg
-                    .layout()
-                    .non_1zst_field(fx)
-                    .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type");
-                arg = arg.value_field(fx, idx);
-            }
-        }
-
-        if let ty::Ref(_, ty, _) = arg.layout().ty.kind() {
-            if ty.is_dyn_star() {
-                let inner_layout = fx.layout_of(arg.layout().ty.builtin_deref(true).unwrap());
-                let dyn_star = CPlace::for_ptr(Pointer::new(arg.load_scalar(fx)), inner_layout);
-                let ptr = dyn_star.place_field(fx, FieldIdx::ZERO).to_ptr();
-                let vtable = dyn_star.place_field(fx, FieldIdx::ONE).to_cvalue(fx).load_scalar(fx);
-                break 'block (ptr, vtable);
-            }
+    if let BackendRepr::Scalar(_) = arg.layout().backend_repr {
+        while !arg.layout().ty.is_raw_ptr() && !arg.layout().ty.is_ref() {
+            let (idx, _) = arg
+                .layout()
+                .non_1zst_field(fx)
+                .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type");
+            arg = arg.value_field(fx, idx);
         }
+    }
 
-        if let BackendRepr::ScalarPair(_, _) = arg.layout().backend_repr {
-            let (ptr, vtable) = arg.load_scalar_pair(fx);
-            (Pointer::new(ptr), vtable)
-        } else {
-            let (ptr, vtable) = arg.try_to_ptr().unwrap();
-            (ptr, vtable.unwrap())
-        }
+    let (ptr, vtable) = if let BackendRepr::ScalarPair(_, _) = arg.layout().backend_repr {
+        let (ptr, vtable) = arg.load_scalar_pair(fx);
+        (Pointer::new(ptr), vtable)
+    } else {
+        let (ptr, vtable) = arg.try_to_ptr().unwrap();
+        (ptr, vtable.unwrap())
     };
 
     let usize_size = fx.layout_of(fx.tcx.types.usize).size.bytes();

compiler/rustc_codegen_ssa/src/base.rs

@@ -262,28 +262,6 @@ pub(crate) fn unsize_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
-/// Coerces `src` to `dst_ty` which is guaranteed to be a `dyn*` type.
-pub(crate) fn cast_to_dyn_star<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
-    bx: &mut Bx,
-    src: Bx::Value,
-    src_ty_and_layout: TyAndLayout<'tcx>,
-    dst_ty: Ty<'tcx>,
-    old_info: Option<Bx::Value>,
-) -> (Bx::Value, Bx::Value) {
-    debug!("cast_to_dyn_star: {:?} => {:?}", src_ty_and_layout.ty, dst_ty);
-    assert!(
-        matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
-        "destination type must be a dyn*"
-    );
-    let src = match bx.cx().type_kind(bx.cx().backend_type(src_ty_and_layout)) {
-        TypeKind::Pointer => src,
-        TypeKind::Integer => bx.inttoptr(src, bx.type_ptr()),
-        // FIXME(dyn-star): We probably have to do a bitcast first, then inttoptr.
-        kind => bug!("unexpected TypeKind for left-hand side of `dyn*` cast: {kind:?}"),
-    };
-    (src, unsized_info(bx, src_ty_and_layout.ty, dst_ty, old_info))
-}
-
 /// Coerces `src`, which is a reference to a value of type `src_ty`,
 /// to a value of type `dst_ty`, and stores the result in `dst`.
 pub(crate) fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(