Auto merge of #101212 - eholk:dyn-star, r=compiler-errors

Initial implementation of dyn*

This PR adds extremely basic and incomplete support for [dyn*](https://smallcultfollowing.com/babysteps//blog/2022/03/29/dyn-can-we-make-dyn-sized/). The goal is to get something in tree behind a flag to make collaboration easier, and also to make sure the implementation so far is not unreasonable. This PR does quite a few things:

* Introduce `dyn_star` feature flag
* Adds parsing for `dyn* Trait` types
* Defines `dyn* Trait` as a sized type
* Adds support for explicit casts, like `42usize as dyn* Debug`
  * Including const evaluation of such casts
* Adds codegen for drop glue so things are cleaned up properly when a `dyn* Trait` object goes out of scope
* Adds codegen for method calls, at least for methods that take `&self`

Quite a bit is still missing, but this gives us a starting point. Note that this is never intended to become stable surface syntax for Rust, but rather `dyn*` is planned to be used as an implementation detail for async functions in dyn traits.

Joint work with `@nikomatsakis` and `@compiler-errors.`

r? `@bjorn3`

compiler/rustc_ast/src/ast.rs

@@ -2072,6 +2072,7 @@ impl TyKind {
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum TraitObjectSyntax {
     Dyn,
+    DynStar,
     None,
 }
 

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -554,6 +554,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::TyKind::Never => {
                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
             }
+            ast::TyKind::TraitObject(_, ast::TraitObjectSyntax::DynStar, ..) => {
+                gate_feature_post!(&self, dyn_star, ty.span, "dyn* trait objects are unstable");
+            }
             _ => {}
         }
         visit::walk_ty(self, ty)

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -30,8 +30,9 @@ use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef, UserSubsts};
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::{
-    self, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, OpaqueHiddenType,
-    OpaqueTypeKey, RegionVid, ToPredicate, Ty, TyCtxt, UserType, UserTypeAnnotationIndex,
+    self, Binder, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, Dynamic,
+    OpaqueHiddenType, OpaqueTypeKey, RegionVid, ToPredicate, Ty, TyCtxt, UserType,
+    UserTypeAnnotationIndex,
 };
 use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::{Span, DUMMY_SP};
@@ -2013,6 +2014,36 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                         );
                     }
 
+                    CastKind::DynStar => {
+                        // 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(body, tcx);
+
+                        self.prove_predicates(
+                            existential_predicates
+                                .iter()
+                                .map(|predicate| predicate.with_self_ty(tcx, self_ty)),
+                            location.to_locations(),
+                            ConstraintCategory::Cast,
+                        );
+
+                        let outlives_predicate =
+                            tcx.mk_predicate(Binder::dummy(ty::PredicateKind::TypeOutlives(
+                                ty::OutlivesPredicate(self_ty, *region),
+                            )));
+                        self.prove_predicate(
+                            outlives_predicate,
+                            location.to_locations(),
+                            ConstraintCategory::Cast,
+                        );
+                    }
+
                     CastKind::Pointer(PointerCast::MutToConstPointer) => {
                         let ty::RawPtr(ty::TypeAndMut {
                             ty: ty_from,

compiler/rustc_codegen_cranelift/src/base.rs

@@ -701,6 +701,10 @@ fn codegen_stmt<'tcx>(
                     let operand = codegen_operand(fx, operand);
                     operand.unsize_value(fx, lval);
                 }
+                Rvalue::Cast(CastKind::DynStar, _, _) => {
+                    // FIXME(dyn-star)
+                    unimplemented!()
+                }
                 Rvalue::Discriminant(place) => {
                     let place = codegen_place(fx, place);
                     let value = place.to_cvalue(fx);

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -815,7 +815,8 @@ pub(crate) fn assert_assignable<'tcx>(
             );
             // fn(&T) -> for<'l> fn(&'l T) is allowed
         }
-        (&ty::Dynamic(from_traits, _), &ty::Dynamic(to_traits, _)) => {
+        (&ty::Dynamic(from_traits, _, _from_kind), &ty::Dynamic(to_traits, _, _to_kind)) => {
+            // FIXME(dyn-star): Do the right thing with DynKinds
             for (from, to) in from_traits.iter().zip(to_traits) {
                 let from =
                     fx.tcx.normalize_erasing_late_bound_regions(ParamEnv::reveal_all(), from);

compiler/rustc_codegen_ssa/src/meth.rs

@@ -69,7 +69,7 @@ impl<'a, 'tcx> VirtualIndex {
 fn expect_dyn_trait_in_self<'tcx>(ty: Ty<'tcx>) -> ty::PolyExistentialTraitRef<'tcx> {
     for arg in ty.peel_refs().walk() {
         if let GenericArgKind::Type(ty) = arg.unpack() {
-            if let ty::Dynamic(data, _) = ty.kind() {
+            if let ty::Dynamic(data, _, _) = ty.kind() {
                 return data.principal().expect("expected principal trait object");
             }
         }
@@ -86,15 +86,14 @@ fn expect_dyn_trait_in_self<'tcx>(ty: Ty<'tcx>) -> ty::PolyExistentialTraitRef<'
 /// The `trait_ref` encodes the erased self type. Hence if we are
 /// making an object `Foo<dyn Trait>` from a value of type `Foo<T>`, then
 /// `trait_ref` would map `T: Trait`.
+#[instrument(level = "debug", skip(cx))]
 pub fn get_vtable<'tcx, Cx: CodegenMethods<'tcx>>(
     cx: &Cx,
     ty: Ty<'tcx>,
     trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
 ) -> Cx::Value {
     let tcx = cx.tcx();
 
-    debug!("get_vtable(ty={:?}, trait_ref={:?})", ty, trait_ref);
-
     // Check the cache.
     if let Some(&val) = cx.vtables().borrow().get(&(ty, trait_ref)) {
         return val;

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -367,6 +367,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         bx.ret(llval);
     }
 
+    #[tracing::instrument(level = "trace", skip(self, helper, bx))]
     fn codegen_drop_terminator(
         &mut self,
         helper: TerminatorCodegenHelper<'tcx>,
@@ -397,14 +398,75 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let (drop_fn, fn_abi) = match ty.kind() {
             // FIXME(eddyb) perhaps move some of this logic into
             // `Instance::resolve_drop_in_place`?
-            ty::Dynamic(..) => {
+            ty::Dynamic(_, _, ty::Dyn) => {
+                // IN THIS ARM, WE HAVE:
+                // ty = *mut (dyn Trait)
+                // which is: exists<T> ( *mut T,    Vtable<T: Trait> )
+                //                       args[0]    args[1]
+                //
+                // args = ( Data, Vtable )
+                //                  |
+                //                  v
+                //                /-------\
+                //                | ...   |
+                //                \-------/
+                //
                 let virtual_drop = Instance {
                     def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0),
                     substs: drop_fn.substs,
                 };
+                debug!("ty = {:?}", ty);
+                debug!("drop_fn = {:?}", drop_fn);
+                debug!("args = {:?}", args);
                 let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
                 let vtable = args[1];
+                // Truncate vtable off of args list
+                args = &args[..1];
+                (
+                    meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
+                        .get_fn(&mut bx, vtable, ty, &fn_abi),
+                    fn_abi,
+                )
+            }
+            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 virtual_drop = Instance {
+                    def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0),
+                    substs: drop_fn.substs,
+                };
+                debug!("ty = {:?}", ty);
+                debug!("drop_fn = {:?}", drop_fn);
+                debug!("args = {:?}", args);
+                let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
+                let data = args[0];
+                let data_ty = bx.cx().backend_type(place.layout);
+                let vtable_ptr =
+                    bx.gep(data_ty, data, &[bx.cx().const_i32(0), bx.cx().const_i32(1)]);
+                let vtable = bx.load(bx.type_i8p(), vtable_ptr, abi::Align::ONE);
+                // Truncate vtable off of args list
                 args = &args[..1];
+                debug!("args' = {:?}", args);
                 (
                     meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
                         .get_fn(&mut bx, vtable, ty, &fn_abi),
@@ -845,7 +907,30 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         llargs.push(data_ptr);
                         continue;
                     }
-                    _ => span_bug!(span, "can't codegen a virtual call on {:?}", op),
+                    Immediate(_) => {
+                        let ty::Ref(_, ty, _) = op.layout.ty.kind() else {
+                            span_bug!(span, "can't codegen a virtual call on {:#?}", op);
+                        };
+                        if !ty.is_dyn_star() {
+                            span_bug!(span, "can't codegen a virtual call on {:#?}", op);
+                        }
+                        // FIXME(dyn-star): Make sure this is done on a &dyn* receiver
+                        let place = op.deref(bx.cx());
+                        let data_ptr = place.project_field(&mut bx, 0);
+                        let meta_ptr = place.project_field(&mut bx, 1);
+                        let meta = bx.load_operand(meta_ptr);
+                        llfn = Some(meth::VirtualIndex::from_index(idx).get_fn(
+                            &mut bx,
+                            meta.immediate(),
+                            op.layout.ty,
+                            &fn_abi,
+                        ));
+                        llargs.push(data_ptr.llval);
+                        continue;
+                    }
+                    _ => {
+                        span_bug!(span, "can't codegen a virtual call on {:#?}", op);
+                    }
                 }
             }
 

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -4,6 +4,7 @@ use super::{FunctionCx, LocalRef};
 
 use crate::base;
 use crate::common::{self, IntPredicate};
+use crate::meth::get_vtable;
 use crate::traits::*;
 use crate::MemFlags;
 
@@ -271,6 +272,21 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             bug!("unexpected non-pair operand");
                         }
                     }
+                    mir::CastKind::DynStar => {
+                        let data = match operand.val {
+                            OperandValue::Ref(_, _, _) => todo!(),
+                            OperandValue::Immediate(v) => v,
+                            OperandValue::Pair(_, _) => todo!(),
+                        };
+                        let trait_ref =
+                            if let ty::Dynamic(data, _, ty::DynStar) = cast.ty.kind() {
+                                data.principal()
+                            } else {
+                                bug!("Only valid to do a DynStar cast into a DynStar type")
+                            };
+                        let vtable = get_vtable(bx.cx(), source.ty(self.mir, bx.tcx()), trait_ref);
+                        OperandValue::Pair(data, vtable)
+                    }
                     mir::CastKind::Pointer(
                         PointerCast::MutToConstPointer | PointerCast::ArrayToPointer,
                     )

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -108,6 +108,20 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     _ => span_bug!(self.cur_span(), "closure fn pointer on {:?}", src.layout.ty),
                 }
             }
+
+            DynStar => {
+                if let ty::Dynamic(data, _, ty::DynStar) = cast_ty.kind() {
+                    // Initial cast from sized to dyn trait
+                    let vtable = self.get_vtable_ptr(src.layout.ty, data.principal())?;
+                    let vtable = Scalar::from_maybe_pointer(vtable, self);
+                    let data = self.read_immediate(src)?.to_scalar();
+                    let _assert_pointer_sized = data.to_pointer(self)?;
+                    let val = Immediate::ScalarPair(data, vtable);
+                    self.write_immediate(val, dest)?;
+                } else {
+                    bug!()
+                }
+            }
         }
         Ok(())
     }
@@ -312,7 +326,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let new_vptr = self.get_vtable_ptr(ty, data_b.principal())?;
                 self.write_immediate(Immediate::new_dyn_trait(old_data, new_vptr, self), dest)
             }
-            (_, &ty::Dynamic(ref data, _)) => {
+            (_, &ty::Dynamic(ref data, _, ty::Dyn)) => {
                 // Initial cast from sized to dyn trait
                 let vtable = self.get_vtable_ptr(src_pointee_ty, data.principal())?;
                 let ptr = self.read_scalar(src)?;

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -95,7 +95,7 @@ pub(crate) fn eval_nullary_intrinsic<'tcx>(
             | ty::Ref(_, _, _)
             | ty::FnDef(_, _)
             | ty::FnPtr(_)
-            | ty::Dynamic(_, _)
+            | ty::Dynamic(_, _, _)
             | ty::Closure(_, _)
             | ty::Generator(_, _, _)
             | ty::GeneratorWitness(_)

compiler/rustc_const_eval/src/interpret/intrinsics/type_name.rs

@@ -48,7 +48,7 @@ impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
             | ty::FnPtr(_)
             | ty::Never
             | ty::Tuple(_)
-            | ty::Dynamic(_, _) => self.pretty_print_type(ty),
+            | ty::Dynamic(_, _, _) => self.pretty_print_type(ty),
 
             // Placeholders (all printed as `_` to uniformize them).
             ty::Param(_) | ty::Bound(..) | ty::Placeholder(_) | ty::Infer(_) | ty::Error(_) => {

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -546,6 +546,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 // Since no pointer can ever get exposed (rejected above), this is easy to support.
             }
 
+            Rvalue::Cast(CastKind::DynStar, _, _) => {
+                unimplemented!()
+            }
+
             Rvalue::Cast(CastKind::Misc, _, _) => {}
 
             Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => {}

compiler/rustc_const_eval/src/transform/validate.rs

@@ -569,6 +569,9 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             );
                         }
                     }
+                    CastKind::DynStar => {
+                        // FIXME(dyn-star): make sure nothing needs to be done here.
+                    }
                     // Nothing to check here
                     CastKind::PointerFromExposedAddress
                     | CastKind::PointerExposeAddress

compiler/rustc_feature/src/active.rs

@@ -384,6 +384,8 @@ declare_features! (
     (active, doc_cfg_hide, "1.57.0", Some(43781), None),
     /// Allows `#[doc(masked)]`.
     (active, doc_masked, "1.21.0", Some(44027), None),
+    /// Allows `dyn* Trait` objects.
+    (incomplete, dyn_star, "CURRENT_RUSTC_VERSION", Some(91611), None),
     /// Allows `X..Y` patterns.
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -544,7 +544,7 @@ pub struct TraitObjectVisitor(pub FxHashSet<DefId>);
 impl<'tcx> TypeVisitor<'tcx> for TraitObjectVisitor {
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         match t.kind() {
-            ty::Dynamic(preds, re) if re.is_static() => {
+            ty::Dynamic(preds, re, _) if re.is_static() => {
                 if let Some(def_id) = preds.principal_def_id() {
                     self.0.insert(def_id);
                 }

compiler/rustc_lint/src/unused.rs

@@ -222,7 +222,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     }
                     has_emitted
                 }
-                ty::Dynamic(binder, _) => {
+                ty::Dynamic(binder, _, _) => {
                     let mut has_emitted = false;
                     for predicate in binder.iter() {
                         if let ty::ExistentialPredicate::Trait(ref trait_ref) =

compiler/rustc_middle/src/mir/mod.rs

@@ -1818,6 +1818,7 @@ impl<'tcx> Rvalue<'tcx> {
             // While the model is undecided, we should be conservative. See
             // <https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html>
             Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => false,
+            Rvalue::Cast(CastKind::DynStar, _, _) => false,
 
             Rvalue::Use(_)
             | Rvalue::CopyForDeref(_)

compiler/rustc_middle/src/mir/syntax.rs

@@ -1139,6 +1139,8 @@ pub enum CastKind {
     /// All sorts of pointer-to-pointer casts. Note that reference-to-raw-ptr casts are
     /// translated into `&raw mut/const *r`, i.e., they are not actually casts.
     Pointer(PointerCast),
+    /// Cast into a dyn* object.
+    DynStar,
     /// Remaining unclassified casts.
     Misc,
 }

compiler/rustc_middle/src/ty/cast.rs

@@ -33,6 +33,8 @@ pub enum CastTy<'tcx> {
     FnPtr,
     /// Raw pointers.
     Ptr(ty::TypeAndMut<'tcx>),
+    /// Casting into a `dyn*` value.
+    DynStar,
 }
 
 /// Cast Kind. See [RFC 401](https://rust-lang.github.io/rfcs/0401-coercions.html)
@@ -50,6 +52,7 @@ pub enum CastKind {
     ArrayPtrCast,
     FnPtrPtrCast,
     FnPtrAddrCast,
+    DynStarCast,
 }
 
 impl<'tcx> CastTy<'tcx> {
@@ -67,6 +70,7 @@ impl<'tcx> CastTy<'tcx> {
             ty::Adt(d, _) if d.is_enum() && d.is_payloadfree() => Some(CastTy::Int(IntTy::CEnum)),
             ty::RawPtr(mt) => Some(CastTy::Ptr(mt)),
             ty::FnPtr(..) => Some(CastTy::FnPtr),
+            ty::Dynamic(_, _, ty::DynStar) => Some(CastTy::DynStar),
             _ => None,
         }
     }