diff options
Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_typeck/src/check/coercion.rs | 1804 |
1 files changed, 1804 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs new file mode 100644 index 000000000..2ed5f569b --- /dev/null +++ b/compiler/rustc_typeck/src/check/coercion.rs @@ -0,0 +1,1804 @@ +//! # Type Coercion +//! +//! Under certain circumstances we will coerce from one type to another, +//! for example by auto-borrowing. This occurs in situations where the +//! compiler has a firm 'expected type' that was supplied from the user, +//! and where the actual type is similar to that expected type in purpose +//! but not in representation (so actual subtyping is inappropriate). +//! +//! ## Reborrowing +//! +//! Note that if we are expecting a reference, we will *reborrow* +//! even if the argument provided was already a reference. This is +//! useful for freezing mut things (that is, when the expected type is &T +//! but you have &mut T) and also for avoiding the linearity +//! of mut things (when the expected is &mut T and you have &mut T). See +//! the various `src/test/ui/coerce/*.rs` tests for +//! examples of where this is useful. +//! +//! ## Subtle note +//! +//! When inferring the generic arguments of functions, the argument +//! order is relevant, which can lead to the following edge case: +//! +//! ```ignore (illustrative) +//! fn foo<T>(a: T, b: T) { +//! // ... +//! } +//! +//! foo(&7i32, &mut 7i32); +//! // This compiles, as we first infer `T` to be `&i32`, +//! // and then coerce `&mut 7i32` to `&7i32`. +//! +//! foo(&mut 7i32, &7i32); +//! // This does not compile, as we first infer `T` to be `&mut i32` +//! // and are then unable to coerce `&7i32` to `&mut i32`. +//! ``` + +use crate::astconv::AstConv; +use crate::check::FnCtxt; +use rustc_errors::{ + struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, +}; +use rustc_hir as hir; +use rustc_hir::def_id::DefId; +use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind}; +use rustc_infer::infer::{Coercion, InferOk, InferResult}; +use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt}; +use rustc_middle::lint::in_external_macro; +use rustc_middle::ty::adjustment::{ + Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast, +}; +use rustc_middle::ty::error::TypeError; +use rustc_middle::ty::relate::RelateResult; +use rustc_middle::ty::subst::SubstsRef; +use rustc_middle::ty::visit::TypeVisitable; +use rustc_middle::ty::{self, ToPredicate, Ty, TypeAndMut}; +use rustc_session::parse::feature_err; +use rustc_span::symbol::sym; +use rustc_span::{self, BytePos, DesugaringKind, Span}; +use rustc_target::spec::abi::Abi; +use rustc_trait_selection::infer::InferCtxtExt as _; +use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _; +use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode}; + +use smallvec::{smallvec, SmallVec}; +use std::ops::Deref; + +struct Coerce<'a, 'tcx> { + fcx: &'a FnCtxt<'a, 'tcx>, + cause: ObligationCause<'tcx>, + use_lub: bool, + /// Determines whether or not allow_two_phase_borrow is set on any + /// autoref adjustments we create while coercing. We don't want to + /// allow deref coercions to create two-phase borrows, at least initially, + /// but we do need two-phase borrows for function argument reborrows. + /// See #47489 and #48598 + /// See docs on the "AllowTwoPhase" type for a more detailed discussion + allow_two_phase: AllowTwoPhase, +} + +impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> { + type Target = FnCtxt<'a, 'tcx>; + fn deref(&self) -> &Self::Target { + &self.fcx + } +} + +type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>; + +/// Coercing a mutable reference to an immutable works, while +/// coercing `&T` to `&mut T` should be forbidden. +fn coerce_mutbls<'tcx>( + from_mutbl: hir::Mutability, + to_mutbl: hir::Mutability, +) -> RelateResult<'tcx, ()> { + match (from_mutbl, to_mutbl) { + (hir::Mutability::Mut, hir::Mutability::Mut | hir::Mutability::Not) + | (hir::Mutability::Not, hir::Mutability::Not) => Ok(()), + (hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability), + } +} + +/// Do not require any adjustments, i.e. coerce `x -> x`. +fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> { + vec![] +} + +fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> { + move |target| vec![Adjustment { kind, target }] +} + +/// This always returns `Ok(...)`. +fn success<'tcx>( + adj: Vec<Adjustment<'tcx>>, + target: Ty<'tcx>, + obligations: traits::PredicateObligations<'tcx>, +) -> CoerceResult<'tcx> { + Ok(InferOk { value: (adj, target), obligations }) +} + +impl<'f, 'tcx> Coerce<'f, 'tcx> { + fn new( + fcx: &'f FnCtxt<'f, 'tcx>, + cause: ObligationCause<'tcx>, + allow_two_phase: AllowTwoPhase, + ) -> Self { + Coerce { fcx, cause, allow_two_phase, use_lub: false } + } + + fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> { + debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub); + self.commit_if_ok(|_| { + if self.use_lub { + self.at(&self.cause, self.fcx.param_env).lub(b, a) + } else { + self.at(&self.cause, self.fcx.param_env) + .sup(b, a) + .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations }) + } + }) + } + + /// Unify two types (using sub or lub) and produce a specific coercion. + fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx> + where + F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>, + { + self.unify(a, b) + .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations)) + } + + #[instrument(skip(self))] + fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> { + // First, remove any resolved type variables (at the top level, at least): + let a = self.shallow_resolve(a); + let b = self.shallow_resolve(b); + debug!("Coerce.tys({:?} => {:?})", a, b); + + // Just ignore error types. + if a.references_error() || b.references_error() { + return success(vec![], self.fcx.tcx.ty_error(), vec![]); + } + + // Coercing from `!` to any type is allowed: + if a.is_never() { + return success(simple(Adjust::NeverToAny)(b), b, vec![]); + } + + // Coercing *from* an unresolved inference variable means that + // we have no information about the source type. This will always + // ultimately fall back to some form of subtyping. + if a.is_ty_var() { + return self.coerce_from_inference_variable(a, b, identity); + } + + // Consider coercing the subtype to a DST + // + // NOTE: this is wrapped in a `commit_if_ok` because it creates + // a "spurious" type variable, and we don't want to have that + // type variable in memory if the coercion fails. + let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b)); + match unsize { + Ok(_) => { + debug!("coerce: unsize successful"); + return unsize; + } + Err(TypeError::ObjectUnsafeCoercion(did)) => { + debug!("coerce: unsize not object safe"); + return Err(TypeError::ObjectUnsafeCoercion(did)); + } + Err(error) => { + debug!(?error, "coerce: unsize failed"); + } + } + + // Examine the supertype and consider auto-borrowing. + match *b.kind() { + ty::RawPtr(mt_b) => { + return self.coerce_unsafe_ptr(a, b, mt_b.mutbl); + } + ty::Ref(r_b, _, mutbl_b) => { + return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b); + } + _ => {} + } + + match *a.kind() { + ty::FnDef(..) => { + // Function items are coercible to any closure + // type; function pointers are not (that would + // require double indirection). + // Additionally, we permit coercion of function + // items to drop the unsafe qualifier. + self.coerce_from_fn_item(a, b) + } + ty::FnPtr(a_f) => { + // We permit coercion of fn pointers to drop the + // unsafe qualifier. + self.coerce_from_fn_pointer(a, a_f, b) + } + ty::Closure(closure_def_id_a, substs_a) => { + // Non-capturing closures are coercible to + // function pointers or unsafe function pointers. + // It cannot convert closures that require unsafe. + self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b) + } + _ => { + // Otherwise, just use unification rules. + self.unify_and(a, b, identity) + } + } + } + + /// Coercing *from* an inference variable. In this case, we have no information + /// about the source type, so we can't really do a true coercion and we always + /// fall back to subtyping (`unify_and`). + fn coerce_from_inference_variable( + &self, + a: Ty<'tcx>, + b: Ty<'tcx>, + make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>, + ) -> CoerceResult<'tcx> { + debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b); + assert!(a.is_ty_var() && self.shallow_resolve(a) == a); + assert!(self.shallow_resolve(b) == b); + + if b.is_ty_var() { + // Two unresolved type variables: create a `Coerce` predicate. + let target_ty = if self.use_lub { + self.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::LatticeVariable, + span: self.cause.span, + }) + } else { + b + }; + + let mut obligations = Vec::with_capacity(2); + for &source_ty in &[a, b] { + if source_ty != target_ty { + obligations.push(Obligation::new( + self.cause.clone(), + self.param_env, + ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate { + a: source_ty, + b: target_ty, + })) + .to_predicate(self.tcx()), + )); + } + } + + debug!( + "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}", + target_ty, obligations + ); + let adjustments = make_adjustments(target_ty); + InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations }) + } else { + // One unresolved type variable: just apply subtyping, we may be able + // to do something useful. + self.unify_and(a, b, make_adjustments) + } + } + + /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`. + /// To match `A` with `B`, autoderef will be performed, + /// calling `deref`/`deref_mut` where necessary. + fn coerce_borrowed_pointer( + &self, + a: Ty<'tcx>, + b: Ty<'tcx>, + r_b: ty::Region<'tcx>, + mutbl_b: hir::Mutability, + ) -> CoerceResult<'tcx> { + debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b); + + // If we have a parameter of type `&M T_a` and the value + // provided is `expr`, we will be adding an implicit borrow, + // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore, + // to type check, we will construct the type that `&M*expr` would + // yield. + + let (r_a, mt_a) = match *a.kind() { + ty::Ref(r_a, ty, mutbl) => { + let mt_a = ty::TypeAndMut { ty, mutbl }; + coerce_mutbls(mt_a.mutbl, mutbl_b)?; + (r_a, mt_a) + } + _ => return self.unify_and(a, b, identity), + }; + + let span = self.cause.span; + + let mut first_error = None; + let mut r_borrow_var = None; + let mut autoderef = self.autoderef(span, a); + let mut found = None; + + for (referent_ty, autoderefs) in autoderef.by_ref() { + if autoderefs == 0 { + // Don't let this pass, otherwise it would cause + // &T to autoref to &&T. + continue; + } + + // At this point, we have deref'd `a` to `referent_ty`. So + // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`. + // In the autoderef loop for `&'a mut Vec<T>`, we would get + // three callbacks: + // + // - `&'a mut Vec<T>` -- 0 derefs, just ignore it + // - `Vec<T>` -- 1 deref + // - `[T]` -- 2 deref + // + // At each point after the first callback, we want to + // check to see whether this would match out target type + // (`&'b mut [T]`) if we autoref'd it. We can't just + // compare the referent types, though, because we still + // have to consider the mutability. E.g., in the case + // we've been considering, we have an `&mut` reference, so + // the `T` in `[T]` needs to be unified with equality. + // + // Therefore, we construct reference types reflecting what + // the types will be after we do the final auto-ref and + // compare those. Note that this means we use the target + // mutability [1], since it may be that we are coercing + // from `&mut T` to `&U`. + // + // One fine point concerns the region that we use. We + // choose the region such that the region of the final + // type that results from `unify` will be the region we + // want for the autoref: + // + // - if in sub mode, that means we want to use `'b` (the + // region from the target reference) for both + // pointers [2]. This is because sub mode (somewhat + // arbitrarily) returns the subtype region. In the case + // where we are coercing to a target type, we know we + // want to use that target type region (`'b`) because -- + // for the program to type-check -- it must be the + // smaller of the two. + // - One fine point. It may be surprising that we can + // use `'b` without relating `'a` and `'b`. The reason + // that this is ok is that what we produce is + // effectively a `&'b *x` expression (if you could + // annotate the region of a borrow), and regionck has + // code that adds edges from the region of a borrow + // (`'b`, here) into the regions in the borrowed + // expression (`*x`, here). (Search for "link".) + // - if in lub mode, things can get fairly complicated. The + // easiest thing is just to make a fresh + // region variable [4], which effectively means we defer + // the decision to region inference (and regionck, which will add + // some more edges to this variable). However, this can wind up + // creating a crippling number of variables in some cases -- + // e.g., #32278 -- so we optimize one particular case [3]. + // Let me try to explain with some examples: + // - The "running example" above represents the simple case, + // where we have one `&` reference at the outer level and + // ownership all the rest of the way down. In this case, + // we want `LUB('a, 'b)` as the resulting region. + // - However, if there are nested borrows, that region is + // too strong. Consider a coercion from `&'a &'x Rc<T>` to + // `&'b T`. In this case, `'a` is actually irrelevant. + // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)` + // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`). + // (The errors actually show up in borrowck, typically, because + // this extra edge causes the region `'a` to be inferred to something + // too big, which then results in borrowck errors.) + // - We could track the innermost shared reference, but there is already + // code in regionck that has the job of creating links between + // the region of a borrow and the regions in the thing being + // borrowed (here, `'a` and `'x`), and it knows how to handle + // all the various cases. So instead we just make a region variable + // and let regionck figure it out. + let r = if !self.use_lub { + r_b // [2] above + } else if autoderefs == 1 { + r_a // [3] above + } else { + if r_borrow_var.is_none() { + // create var lazily, at most once + let coercion = Coercion(span); + let r = self.next_region_var(coercion); + r_borrow_var = Some(r); // [4] above + } + r_borrow_var.unwrap() + }; + let derefd_ty_a = self.tcx.mk_ref( + r, + TypeAndMut { + ty: referent_ty, + mutbl: mutbl_b, // [1] above + }, + ); + match self.unify(derefd_ty_a, b) { + Ok(ok) => { + found = Some(ok); + break; + } + Err(err) => { + if first_error.is_none() { + first_error = Some(err); + } + } + } + } + + // Extract type or return an error. We return the first error + // we got, which should be from relating the "base" type + // (e.g., in example above, the failure from relating `Vec<T>` + // to the target type), since that should be the least + // confusing. + let Some(InferOk { value: ty, mut obligations }) = found else { + let err = first_error.expect("coerce_borrowed_pointer had no error"); + debug!("coerce_borrowed_pointer: failed with err = {:?}", err); + return Err(err); + }; + + if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 { + // As a special case, if we would produce `&'a *x`, that's + // a total no-op. We end up with the type `&'a T` just as + // we started with. In that case, just skip it + // altogether. This is just an optimization. + // + // Note that for `&mut`, we DO want to reborrow -- + // otherwise, this would be a move, which might be an + // error. For example `foo(self.x)` where `self` and + // `self.x` both have `&mut `type would be a move of + // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`, + // which is a borrow. + assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U + return success(vec![], ty, obligations); + } + + let InferOk { value: mut adjustments, obligations: o } = + self.adjust_steps_as_infer_ok(&autoderef); + obligations.extend(o); + obligations.extend(autoderef.into_obligations()); + + // Now apply the autoref. We have to extract the region out of + // the final ref type we got. + let ty::Ref(r_borrow, _, _) = ty.kind() else { + span_bug!(span, "expected a ref type, got {:?}", ty); + }; + let mutbl = match mutbl_b { + hir::Mutability::Not => AutoBorrowMutability::Not, + hir::Mutability::Mut => { + AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase } + } + }; + adjustments.push(Adjustment { + kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)), + target: ty, + }); + + debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments); + + success(adjustments, ty, obligations) + } + + // &[T; n] or &mut [T; n] -> &[T] + // or &mut [T; n] -> &mut [T] + // or &Concrete -> &Trait, etc. + #[instrument(skip(self), level = "debug")] + fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> { + source = self.shallow_resolve(source); + target = self.shallow_resolve(target); + debug!(?source, ?target); + + // These 'if' statements require some explanation. + // The `CoerceUnsized` trait is special - it is only + // possible to write `impl CoerceUnsized<B> for A` where + // A and B have 'matching' fields. This rules out the following + // two types of blanket impls: + // + // `impl<T> CoerceUnsized<T> for SomeType` + // `impl<T> CoerceUnsized<SomeType> for T` + // + // Both of these trigger a special `CoerceUnsized`-related error (E0376) + // + // We can take advantage of this fact to avoid performing unnecessary work. + // If either `source` or `target` is a type variable, then any applicable impl + // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`) + // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for + // SomeType`). + // + // However, these are exactly the kinds of impls which are forbidden by + // the compiler! Therefore, we can be sure that coercion will always fail + // when either the source or target type is a type variable. This allows us + // to skip performing any trait selection, and immediately bail out. + if source.is_ty_var() { + debug!("coerce_unsized: source is a TyVar, bailing out"); + return Err(TypeError::Mismatch); + } + if target.is_ty_var() { + debug!("coerce_unsized: target is a TyVar, bailing out"); + return Err(TypeError::Mismatch); + } + + let traits = + (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait()); + let (Some(unsize_did), Some(coerce_unsized_did)) = traits else { + debug!("missing Unsize or CoerceUnsized traits"); + return Err(TypeError::Mismatch); + }; + + // Note, we want to avoid unnecessary unsizing. We don't want to coerce to + // a DST unless we have to. This currently comes out in the wash since + // we can't unify [T] with U. But to properly support DST, we need to allow + // that, at which point we will need extra checks on the target here. + + // Handle reborrows before selecting `Source: CoerceUnsized<Target>`. + let reborrow = match (source.kind(), target.kind()) { + (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => { + coerce_mutbls(mutbl_a, mutbl_b)?; + + let coercion = Coercion(self.cause.span); + let r_borrow = self.next_region_var(coercion); + let mutbl = match mutbl_b { + hir::Mutability::Not => AutoBorrowMutability::Not, + hir::Mutability::Mut => AutoBorrowMutability::Mut { + // We don't allow two-phase borrows here, at least for initial + // implementation. If it happens that this coercion is a function argument, + // the reborrow in coerce_borrowed_ptr will pick it up. + allow_two_phase_borrow: AllowTwoPhase::No, + }, + }; + Some(( + Adjustment { kind: Adjust::Deref(None), target: ty_a }, + Adjustment { + kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)), + target: self + .tcx + .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }), + }, + )) + } + (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => { + coerce_mutbls(mt_a, mt_b)?; + + Some(( + Adjustment { kind: Adjust::Deref(None), target: ty_a }, + Adjustment { + kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)), + target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }), + }, + )) + } + _ => None, + }; + let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target); + + // Setup either a subtyping or a LUB relationship between + // the `CoerceUnsized` target type and the expected type. + // We only have the latter, so we use an inference variable + // for the former and let type inference do the rest. + let origin = TypeVariableOrigin { + kind: TypeVariableOriginKind::MiscVariable, + span: self.cause.span, + }; + let coerce_target = self.next_ty_var(origin); + let mut coercion = self.unify_and(coerce_target, target, |target| { + let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target }; + match reborrow { + None => vec![unsize], + Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize], + } + })?; + + let mut selcx = traits::SelectionContext::new(self); + + // Create an obligation for `Source: CoerceUnsized<Target>`. + let cause = ObligationCause::new( + self.cause.span, + self.body_id, + ObligationCauseCode::Coercion { source, target }, + ); + + // Use a FIFO queue for this custom fulfillment procedure. + // + // A Vec (or SmallVec) is not a natural choice for a queue. However, + // this code path is hot, and this queue usually has a max length of 1 + // 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<[_; 4]> = smallvec![traits::predicate_for_trait_def( + self.tcx, + self.fcx.param_env, + cause, + coerce_unsized_did, + 0, + coerce_source, + &[coerce_target.into()] + )]; + + let mut has_unsized_tuple_coercion = false; + let mut has_trait_upcasting_coercion = None; + + // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid + // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where + // inference might unify those two inner type variables later. + let traits = [coerce_unsized_did, unsize_did]; + while !queue.is_empty() { + let obligation = queue.remove(0); + debug!("coerce_unsized resolve step: {:?}", obligation); + let bound_predicate = obligation.predicate.kind(); + let trait_pred = match bound_predicate.skip_binder() { + ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => { + if unsize_did == trait_pred.def_id() { + let self_ty = trait_pred.self_ty(); + let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty(); + if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) = + (self_ty.kind(), unsize_ty.kind()) + && data_a.principal_def_id() != data_b.principal_def_id() + { + debug!("coerce_unsized: found trait upcasting coercion"); + has_trait_upcasting_coercion = Some((self_ty, unsize_ty)); + } + if let ty::Tuple(..) = unsize_ty.kind() { + debug!("coerce_unsized: found unsized tuple coercion"); + has_unsized_tuple_coercion = true; + } + } + bound_predicate.rebind(trait_pred) + } + _ => { + coercion.obligations.push(obligation); + continue; + } + }; + match selcx.select(&obligation.with(trait_pred)) { + // Uncertain or unimplemented. + Ok(None) => { + if trait_pred.def_id() == unsize_did { + let trait_pred = self.resolve_vars_if_possible(trait_pred); + let self_ty = trait_pred.skip_binder().self_ty(); + let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty(); + debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred); + match (&self_ty.kind(), &unsize_ty.kind()) { + (ty::Infer(ty::TyVar(v)), ty::Dynamic(..)) + if self.type_var_is_sized(*v) => + { + debug!("coerce_unsized: have sized infer {:?}", v); + coercion.obligations.push(obligation); + // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going + // for unsizing. + } + _ => { + // Some other case for `$0: Unsize<Something>`. Note that we + // hit this case even if `Something` is a sized type, so just + // don't do the coercion. + debug!("coerce_unsized: ambiguous unsize"); + return Err(TypeError::Mismatch); + } + } + } else { + debug!("coerce_unsized: early return - ambiguous"); + return Err(TypeError::Mismatch); + } + } + Err(traits::Unimplemented) => { + debug!("coerce_unsized: early return - can't prove obligation"); + return Err(TypeError::Mismatch); + } + + // Object safety violations or miscellaneous. + Err(err) => { + self.report_selection_error(obligation.clone(), &obligation, &err, false); + // Treat this like an obligation and follow through + // with the unsizing - the lack of a coercion should + // be silent, as it causes a type mismatch later. + } + + Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()), + } + } + + if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion { + feature_err( + &self.tcx.sess.parse_sess, + sym::unsized_tuple_coercion, + self.cause.span, + "unsized tuple coercion is not stable enough for use and is subject to change", + ) + .emit(); + } + + if let Some((sub, sup)) = has_trait_upcasting_coercion + && !self.tcx().features().trait_upcasting + { + // Renders better when we erase regions, since they're not really the point here. + let (sub, sup) = self.tcx.erase_regions((sub, sup)); + let mut err = feature_err( + &self.tcx.sess.parse_sess, + sym::trait_upcasting, + self.cause.span, + &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"), + ); + err.note(&format!("required when coercing `{source}` into `{target}`")); + err.emit(); + } + + Ok(coercion) + } + + fn coerce_from_safe_fn<F, G>( + &self, + a: Ty<'tcx>, + fn_ty_a: ty::PolyFnSig<'tcx>, + b: Ty<'tcx>, + to_unsafe: F, + normal: G, + ) -> CoerceResult<'tcx> + where + F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>, + G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>, + { + self.commit_if_ok(|snapshot| { + let result = if let ty::FnPtr(fn_ty_b) = b.kind() + && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) = + (fn_ty_a.unsafety(), fn_ty_b.unsafety()) + { + let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a); + self.unify_and(unsafe_a, b, to_unsafe) + } else { + self.unify_and(a, b, normal) + }; + + // FIXME(#73154): This is a hack. Currently LUB can generate + // unsolvable constraints. Additionally, it returns `a` + // unconditionally, even when the "LUB" is `b`. In the future, we + // want the coerced type to be the actual supertype of these two, + // but for now, we want to just error to ensure we don't lock + // ourselves into a specific behavior with NLL. + self.leak_check(false, snapshot)?; + + result + }) + } + + fn coerce_from_fn_pointer( + &self, + a: Ty<'tcx>, + fn_ty_a: ty::PolyFnSig<'tcx>, + b: Ty<'tcx>, + ) -> CoerceResult<'tcx> { + //! Attempts to coerce from the type of a Rust function item + //! into a closure or a `proc`. + //! + + let b = self.shallow_resolve(b); + debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b); + + self.coerce_from_safe_fn( + a, + fn_ty_a, + b, + simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)), + identity, + ) + } + + fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> { + //! Attempts to coerce from the type of a Rust function item + //! into a closure or a `proc`. + + let b = self.shallow_resolve(b); + let InferOk { value: b, mut obligations } = + self.normalize_associated_types_in_as_infer_ok(self.cause.span, b); + debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b); + + match b.kind() { + ty::FnPtr(b_sig) => { + let a_sig = a.fn_sig(self.tcx); + if let ty::FnDef(def_id, _) = *a.kind() { + // Intrinsics are not coercible to function pointers + if self.tcx.is_intrinsic(def_id) { + return Err(TypeError::IntrinsicCast); + } + + // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396). + + if b_sig.unsafety() == hir::Unsafety::Normal + && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty() + { + return Err(TypeError::TargetFeatureCast(def_id)); + } + } + + let InferOk { value: a_sig, obligations: o1 } = + self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig); + obligations.extend(o1); + + let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig); + let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn( + a_fn_pointer, + a_sig, + b, + |unsafe_ty| { + vec![ + Adjustment { + kind: Adjust::Pointer(PointerCast::ReifyFnPointer), + target: a_fn_pointer, + }, + Adjustment { + kind: Adjust::Pointer(PointerCast::UnsafeFnPointer), + target: unsafe_ty, + }, + ] + }, + simple(Adjust::Pointer(PointerCast::ReifyFnPointer)), + )?; + + obligations.extend(o2); + Ok(InferOk { value, obligations }) + } + _ => self.unify_and(a, b, identity), + } + } + + fn coerce_closure_to_fn( + &self, + a: Ty<'tcx>, + closure_def_id_a: DefId, + substs_a: SubstsRef<'tcx>, + b: Ty<'tcx>, + ) -> CoerceResult<'tcx> { + //! Attempts to coerce from the type of a non-capturing closure + //! into a function pointer. + //! + + let b = self.shallow_resolve(b); + + match b.kind() { + // At this point we haven't done capture analysis, which means + // that the ClosureSubsts just contains an inference variable instead + // of tuple of captured types. + // + // All we care here is if any variable is being captured and not the exact paths, + // so we check `upvars_mentioned` for root variables being captured. + ty::FnPtr(fn_ty) + if self + .tcx + .upvars_mentioned(closure_def_id_a.expect_local()) + .map_or(true, |u| u.is_empty()) => + { + // We coerce the closure, which has fn type + // `extern "rust-call" fn((arg0,arg1,...)) -> _` + // to + // `fn(arg0,arg1,...) -> _` + // or + // `unsafe fn(arg0,arg1,...) -> _` + let closure_sig = substs_a.as_closure().sig(); + let unsafety = fn_ty.unsafety(); + let pointer_ty = + self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety)); + debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty); + self.unify_and( + pointer_ty, + b, + simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))), + ) + } + _ => self.unify_and(a, b, identity), + } + } + + fn coerce_unsafe_ptr( + &self, + a: Ty<'tcx>, + b: Ty<'tcx>, + mutbl_b: hir::Mutability, + ) -> CoerceResult<'tcx> { + debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b); + + let (is_ref, mt_a) = match *a.kind() { + ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }), + ty::RawPtr(mt) => (false, mt), + _ => return self.unify_and(a, b, identity), + }; + coerce_mutbls(mt_a.mutbl, mutbl_b)?; + + // Check that the types which they point at are compatible. + let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty }); + // Although references and unsafe ptrs have the same + // representation, we still register an Adjust::DerefRef so that + // regionck knows that the region for `a` must be valid here. + if is_ref { + self.unify_and(a_unsafe, b, |target| { + vec![ + Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }, + Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target }, + ] + }) + } else if mt_a.mutbl != mutbl_b { + self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer))) + } else { + self.unify_and(a_unsafe, b, identity) + } + } +} + +impl<'a, 'tcx> FnCtxt<'a, 'tcx> { + /// Attempt to coerce an expression to a type, and return the + /// adjusted type of the expression, if successful. + /// Adjustments are only recorded if the coercion succeeded. + /// The expressions *must not* have any pre-existing adjustments. + pub fn try_coerce( + &self, + expr: &hir::Expr<'_>, + expr_ty: Ty<'tcx>, + target: Ty<'tcx>, + allow_two_phase: AllowTwoPhase, + cause: Option<ObligationCause<'tcx>>, + ) -> RelateResult<'tcx, Ty<'tcx>> { + let source = self.resolve_vars_with_obligations(expr_ty); + debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target); + + let cause = + cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable)); + let coerce = Coerce::new(self, cause, allow_two_phase); + let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?; + + let (adjustments, _) = self.register_infer_ok_obligations(ok); + self.apply_adjustments(expr, adjustments); + Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target }) + } + + /// Same as `try_coerce()`, but without side-effects. + /// + /// Returns false if the coercion creates any obligations that result in + /// errors. + pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool { + let source = self.resolve_vars_with_obligations(expr_ty); + debug!("coercion::can_with_predicates({:?} -> {:?})", source, target); + + let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable); + // We don't ever need two-phase here since we throw out the result of the coercion + let coerce = Coerce::new(self, cause, AllowTwoPhase::No); + self.probe(|_| { + let Ok(ok) = coerce.coerce(source, target) else { + return false; + }; + let mut fcx = traits::FulfillmentContext::new_in_snapshot(); + fcx.register_predicate_obligations(self, ok.obligations); + fcx.select_where_possible(&self).is_empty() + }) + } + + /// Given a type and a target type, this function will calculate and return + /// how many dereference steps needed to achieve `expr_ty <: target`. If + /// it's not possible, return `None`. + pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> { + let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable); + // We don't ever need two-phase here since we throw out the result of the coercion + let coerce = Coerce::new(self, cause, AllowTwoPhase::No); + coerce + .autoderef(rustc_span::DUMMY_SP, expr_ty) + .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps)) + } + + /// Given a type, this function will calculate and return the type given + /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`. + /// + /// This function is for diagnostics only, since it does not register + /// trait or region sub-obligations. (presumably we could, but it's not + /// particularly important for diagnostics...) + pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> { + self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| { + self.infcx + .type_implements_trait( + self.tcx.lang_items().deref_mut_trait()?, + expr_ty, + ty::List::empty(), + self.param_env, + ) + .may_apply() + .then(|| deref_ty) + }) + } + + /// Given some expressions, their known unified type and another expression, + /// tries to unify the types, potentially inserting coercions on any of the + /// provided expressions and returns their LUB (aka "common supertype"). + /// + /// This is really an internal helper. From outside the coercion + /// module, you should instantiate a `CoerceMany` instance. + fn try_find_coercion_lub<E>( + &self, + cause: &ObligationCause<'tcx>, + exprs: &[E], + prev_ty: Ty<'tcx>, + new: &hir::Expr<'_>, + new_ty: Ty<'tcx>, + ) -> RelateResult<'tcx, Ty<'tcx>> + where + E: AsCoercionSite, + { + let prev_ty = self.resolve_vars_with_obligations(prev_ty); + let new_ty = self.resolve_vars_with_obligations(new_ty); + debug!( + "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)", + prev_ty, + new_ty, + exprs.len() + ); + + // The following check fixes #88097, where the compiler erroneously + // attempted to coerce a closure type to itself via a function pointer. + if prev_ty == new_ty { + return Ok(prev_ty); + } + + // Special-case that coercion alone cannot handle: + // Function items or non-capturing closures of differing IDs or InternalSubsts. + let (a_sig, b_sig) = { + #[allow(rustc::usage_of_ty_tykind)] + let is_capturing_closure = |ty: &ty::TyKind<'tcx>| { + if let &ty::Closure(closure_def_id, _substs) = ty { + self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some() + } else { + false + } + }; + if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) { + (None, None) + } else { + match (prev_ty.kind(), new_ty.kind()) { + (ty::FnDef(..), ty::FnDef(..)) => { + // Don't reify if the function types have a LUB, i.e., they + // are the same function and their parameters have a LUB. + match self + .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty)) + { + // We have a LUB of prev_ty and new_ty, just return it. + Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)), + Err(_) => { + (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx))) + } + } + } + (ty::Closure(_, substs), ty::FnDef(..)) => { + let b_sig = new_ty.fn_sig(self.tcx); + let a_sig = self + .tcx + .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety()); + (Some(a_sig), Some(b_sig)) + } + (ty::FnDef(..), ty::Closure(_, substs)) => { + let a_sig = prev_ty.fn_sig(self.tcx); + let b_sig = self + .tcx + .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety()); + (Some(a_sig), Some(b_sig)) + } + (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => ( + Some(self.tcx.signature_unclosure( + substs_a.as_closure().sig(), + hir::Unsafety::Normal, + )), + Some(self.tcx.signature_unclosure( + substs_b.as_closure().sig(), + hir::Unsafety::Normal, + )), + ), + _ => (None, None), + } + } + }; + if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) { + // Intrinsics are not coercible to function pointers. + if a_sig.abi() == Abi::RustIntrinsic + || a_sig.abi() == Abi::PlatformIntrinsic + || b_sig.abi() == Abi::RustIntrinsic + || b_sig.abi() == Abi::PlatformIntrinsic + { + return Err(TypeError::IntrinsicCast); + } + // The signature must match. + let a_sig = self.normalize_associated_types_in(new.span, a_sig); + let b_sig = self.normalize_associated_types_in(new.span, b_sig); + let sig = self + .at(cause, self.param_env) + .trace(prev_ty, new_ty) + .lub(a_sig, b_sig) + .map(|ok| self.register_infer_ok_obligations(ok))?; + + // Reify both sides and return the reified fn pointer type. + let fn_ptr = self.tcx.mk_fn_ptr(sig); + let prev_adjustment = match prev_ty.kind() { + ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())), + ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer), + _ => unreachable!(), + }; + let next_adjustment = match new_ty.kind() { + ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())), + ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer), + _ => unreachable!(), + }; + for expr in exprs.iter().map(|e| e.as_coercion_site()) { + self.apply_adjustments( + expr, + vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }], + ); + } + self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]); + return Ok(fn_ptr); + } + + // Configure a Coerce instance to compute the LUB. + // We don't allow two-phase borrows on any autorefs this creates since we + // probably aren't processing function arguments here and even if we were, + // they're going to get autorefed again anyway and we can apply 2-phase borrows + // at that time. + let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No); + coerce.use_lub = true; + + // First try to coerce the new expression to the type of the previous ones, + // but only if the new expression has no coercion already applied to it. + let mut first_error = None; + if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) { + let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty)); + match result { + Ok(ok) => { + let (adjustments, target) = self.register_infer_ok_obligations(ok); + self.apply_adjustments(new, adjustments); + debug!( + "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})", + new_ty, prev_ty, target + ); + return Ok(target); + } + Err(e) => first_error = Some(e), + } + } + + // Then try to coerce the previous expressions to the type of the new one. + // This requires ensuring there are no coercions applied to *any* of the + // previous expressions, other than noop reborrows (ignoring lifetimes). + for expr in exprs { + let expr = expr.as_coercion_site(); + let noop = match self.typeck_results.borrow().expr_adjustments(expr) { + &[ + Adjustment { kind: Adjust::Deref(_), .. }, + Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }, + ] => { + match *self.node_ty(expr.hir_id).kind() { + ty::Ref(_, _, mt_orig) => { + let mutbl_adj: hir::Mutability = mutbl_adj.into(); + // Reborrow that we can safely ignore, because + // the next adjustment can only be a Deref + // which will be merged into it. + mutbl_adj == mt_orig + } + _ => false, + } + } + &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true, + _ => false, + }; + + if !noop { + debug!( + "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB", + expr, + ); + + return self + .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty)) + .map(|ok| self.register_infer_ok_obligations(ok)); + } + } + + match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) { + Err(_) => { + // Avoid giving strange errors on failed attempts. + if let Some(e) = first_error { + Err(e) + } else { + self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty)) + .map(|ok| self.register_infer_ok_obligations(ok)) + } + } + Ok(ok) => { + let (adjustments, target) = self.register_infer_ok_obligations(ok); + for expr in exprs { + let expr = expr.as_coercion_site(); + self.apply_adjustments(expr, adjustments.clone()); + } + debug!( + "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})", + prev_ty, new_ty, target + ); + Ok(target) + } + } + } +} + +/// CoerceMany encapsulates the pattern you should use when you have +/// many expressions that are all getting coerced to a common +/// type. This arises, for example, when you have a match (the result +/// of each arm is coerced to a common type). It also arises in less +/// obvious places, such as when you have many `break foo` expressions +/// that target the same loop, or the various `return` expressions in +/// a function. +/// +/// The basic protocol is as follows: +/// +/// - Instantiate the `CoerceMany` with an initial `expected_ty`. +/// This will also serve as the "starting LUB". The expectation is +/// that this type is something which all of the expressions *must* +/// be coercible to. Use a fresh type variable if needed. +/// - For each expression whose result is to be coerced, invoke `coerce()` with. +/// - In some cases we wish to coerce "non-expressions" whose types are implicitly +/// unit. This happens for example if you have a `break` with no expression, +/// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`. +/// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this +/// from you so that you don't have to worry your pretty head about it. +/// But if an error is reported, the final type will be `err`. +/// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on +/// previously coerced expressions. +/// - When all done, invoke `complete()`. This will return the LUB of +/// all your expressions. +/// - WARNING: I don't believe this final type is guaranteed to be +/// related to your initial `expected_ty` in any particular way, +/// although it will typically be a subtype, so you should check it. +/// - Invoking `complete()` may cause us to go and adjust the "adjustments" on +/// previously coerced expressions. +/// +/// Example: +/// +/// ```ignore (illustrative) +/// let mut coerce = CoerceMany::new(expected_ty); +/// for expr in exprs { +/// let expr_ty = fcx.check_expr_with_expectation(expr, expected); +/// coerce.coerce(fcx, &cause, expr, expr_ty); +/// } +/// let final_ty = coerce.complete(fcx); +/// ``` +pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> { + expected_ty: Ty<'tcx>, + final_ty: Option<Ty<'tcx>>, + expressions: Expressions<'tcx, 'exprs, E>, + pushed: usize, +} + +/// The type of a `CoerceMany` that is storing up the expressions into +/// a buffer. We use this in `check/mod.rs` for things like `break`. +pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>; + +enum Expressions<'tcx, 'exprs, E: AsCoercionSite> { + Dynamic(Vec<&'tcx hir::Expr<'tcx>>), + UpFront(&'exprs [E]), +} + +impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> { + /// The usual case; collect the set of expressions dynamically. + /// If the full set of coercion sites is known before hand, + /// consider `with_coercion_sites()` instead to avoid allocation. + pub fn new(expected_ty: Ty<'tcx>) -> Self { + Self::make(expected_ty, Expressions::Dynamic(vec![])) + } + + /// As an optimization, you can create a `CoerceMany` with a + /// pre-existing slice of expressions. In this case, you are + /// expected to pass each element in the slice to `coerce(...)` in + /// order. This is used with arrays in particular to avoid + /// needlessly cloning the slice. + pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self { + Self::make(expected_ty, Expressions::UpFront(coercion_sites)) + } + + fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self { + CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 } + } + + /// Returns the "expected type" with which this coercion was + /// constructed. This represents the "downward propagated" type + /// that was given to us at the start of typing whatever construct + /// we are typing (e.g., the match expression). + /// + /// Typically, this is used as the expected type when + /// type-checking each of the alternative expressions whose types + /// we are trying to merge. + pub fn expected_ty(&self) -> Ty<'tcx> { + self.expected_ty + } + + /// Returns the current "merged type", representing our best-guess + /// at the LUB of the expressions we've seen so far (if any). This + /// isn't *final* until you call `self.complete()`, which will return + /// the merged type. + pub fn merged_ty(&self) -> Ty<'tcx> { + self.final_ty.unwrap_or(self.expected_ty) + } + + /// Indicates that the value generated by `expression`, which is + /// of type `expression_ty`, is one of the possibilities that we + /// could coerce from. This will record `expression`, and later + /// calls to `coerce` may come back and add adjustments and things + /// if necessary. + pub fn coerce<'a>( + &mut self, + fcx: &FnCtxt<'a, 'tcx>, + cause: &ObligationCause<'tcx>, + expression: &'tcx hir::Expr<'tcx>, + expression_ty: Ty<'tcx>, + ) { + self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false) + } + + /// Indicates that one of the inputs is a "forced unit". This + /// occurs in a case like `if foo { ... };`, where the missing else + /// generates a "forced unit". Another example is a `loop { break; + /// }`, where the `break` has no argument expression. We treat + /// these cases slightly differently for error-reporting + /// purposes. Note that these tend to correspond to cases where + /// the `()` expression is implicit in the source, and hence we do + /// not take an expression argument. + /// + /// The `augment_error` gives you a chance to extend the error + /// message, in case any results (e.g., we use this to suggest + /// removing a `;`). + pub fn coerce_forced_unit<'a>( + &mut self, + fcx: &FnCtxt<'a, 'tcx>, + cause: &ObligationCause<'tcx>, + augment_error: &mut dyn FnMut(&mut Diagnostic), + label_unit_as_expected: bool, + ) { + self.coerce_inner( + fcx, + cause, + None, + fcx.tcx.mk_unit(), + Some(augment_error), + label_unit_as_expected, + ) + } + + /// The inner coercion "engine". If `expression` is `None`, this + /// is a forced-unit case, and hence `expression_ty` must be + /// `Nil`. + #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")] + pub(crate) fn coerce_inner<'a>( + &mut self, + fcx: &FnCtxt<'a, 'tcx>, + cause: &ObligationCause<'tcx>, + expression: Option<&'tcx hir::Expr<'tcx>>, + mut expression_ty: Ty<'tcx>, + augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>, + label_expression_as_expected: bool, + ) { + // Incorporate whatever type inference information we have + // until now; in principle we might also want to process + // pending obligations, but doing so should only improve + // compatibility (hopefully that is true) by helping us + // uncover never types better. + if expression_ty.is_ty_var() { + expression_ty = fcx.infcx.shallow_resolve(expression_ty); + } + + // If we see any error types, just propagate that error + // upwards. + if expression_ty.references_error() || self.merged_ty().references_error() { + self.final_ty = Some(fcx.tcx.ty_error()); + return; + } + + // Handle the actual type unification etc. + let result = if let Some(expression) = expression { + if self.pushed == 0 { + // Special-case the first expression we are coercing. + // To be honest, I'm not entirely sure why we do this. + // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why + fcx.try_coerce( + expression, + expression_ty, + self.expected_ty, + AllowTwoPhase::No, + Some(cause.clone()), + ) + } else { + match self.expressions { + Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub( + cause, + exprs, + self.merged_ty(), + expression, + expression_ty, + ), + Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub( + cause, + &coercion_sites[0..self.pushed], + self.merged_ty(), + expression, + expression_ty, + ), + } + } + } else { + // this is a hack for cases where we default to `()` because + // the expression etc has been omitted from the source. An + // example is an `if let` without an else: + // + // if let Some(x) = ... { } + // + // we wind up with a second match arm that is like `_ => + // ()`. That is the case we are considering here. We take + // a different path to get the right "expected, found" + // message and so forth (and because we know that + // `expression_ty` will be unit). + // + // Another example is `break` with no argument expression. + assert!(expression_ty.is_unit(), "if let hack without unit type"); + fcx.at(cause, fcx.param_env) + .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty()) + .map(|infer_ok| { + fcx.register_infer_ok_obligations(infer_ok); + expression_ty + }) + }; + + debug!(?result); + match result { + Ok(v) => { + self.final_ty = Some(v); + if let Some(e) = expression { + match self.expressions { + Expressions::Dynamic(ref mut buffer) => buffer.push(e), + Expressions::UpFront(coercion_sites) => { + // if the user gave us an array to validate, check that we got + // the next expression in the list, as expected + assert_eq!( + coercion_sites[self.pushed].as_coercion_site().hir_id, + e.hir_id + ); + } + } + self.pushed += 1; + } + } + Err(coercion_error) => { + let (expected, found) = if label_expression_as_expected { + // In the case where this is a "forced unit", like + // `break`, we want to call the `()` "expected" + // since it is implied by the syntax. + // (Note: not all force-units work this way.)" + (expression_ty, self.final_ty.unwrap_or(self.expected_ty)) + } else { + // Otherwise, the "expected" type for error + // reporting is the current unification type, + // which is basically the LUB of the expressions + // we've seen so far (combined with the expected + // type) + (self.final_ty.unwrap_or(self.expected_ty), expression_ty) + }; + + let mut err; + let mut unsized_return = false; + match *cause.code() { + ObligationCauseCode::ReturnNoExpression => { + err = struct_span_err!( + fcx.tcx.sess, + cause.span, + E0069, + "`return;` in a function whose return type is not `()`" + ); + err.span_label(cause.span, "return type is not `()`"); + } + ObligationCauseCode::BlockTailExpression(blk_id) => { + let parent_id = fcx.tcx.hir().get_parent_node(blk_id); + err = self.report_return_mismatched_types( + cause, + expected, + found, + coercion_error.clone(), + fcx, + parent_id, + expression, + Some(blk_id), + ); + if !fcx.tcx.features().unsized_locals { + unsized_return = self.is_return_ty_unsized(fcx, blk_id); + } + } + ObligationCauseCode::ReturnValue(id) => { + err = self.report_return_mismatched_types( + cause, + expected, + found, + coercion_error.clone(), + fcx, + id, + expression, + None, + ); + if !fcx.tcx.features().unsized_locals { + let id = fcx.tcx.hir().get_parent_node(id); + unsized_return = self.is_return_ty_unsized(fcx, id); + } + } + _ => { + err = fcx.report_mismatched_types( + cause, + expected, + found, + coercion_error.clone(), + ); + } + } + + if let Some(augment_error) = augment_error { + augment_error(&mut err); + } + + let is_insufficiently_polymorphic = + matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..)); + + if !is_insufficiently_polymorphic && let Some(expr) = expression { + fcx.emit_coerce_suggestions( + &mut err, + expr, + found, + expected, + None, + Some(coercion_error), + ); + } + + err.emit_unless(unsized_return); + + self.final_ty = Some(fcx.tcx.ty_error()); + } + } + } + + fn report_return_mismatched_types<'a>( + &self, + cause: &ObligationCause<'tcx>, + expected: Ty<'tcx>, + found: Ty<'tcx>, + ty_err: TypeError<'tcx>, + fcx: &FnCtxt<'a, 'tcx>, + id: hir::HirId, + expression: Option<&'tcx hir::Expr<'tcx>>, + blk_id: Option<hir::HirId>, + ) -> DiagnosticBuilder<'a, ErrorGuaranteed> { + let mut err = fcx.report_mismatched_types(cause, expected, found, ty_err); + + let mut pointing_at_return_type = false; + let mut fn_output = None; + + let parent_id = fcx.tcx.hir().get_parent_node(id); + let parent = fcx.tcx.hir().get(parent_id); + if let Some(expr) = expression + && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent + && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..)) + { + fcx.suggest_missing_semicolon(&mut err, expr, expected, true); + } + // Verify that this is a tail expression of a function, otherwise the + // label pointing out the cause for the type coercion will be wrong + // as prior return coercions would not be relevant (#57664). + let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) { + pointing_at_return_type = + fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id); + if let (Some(cond_expr), true, false) = ( + fcx.tcx.hir().get_if_cause(expr.hir_id), + expected.is_unit(), + pointing_at_return_type, + ) + // If the block is from an external macro or try (`?`) desugaring, then + // do not suggest adding a semicolon, because there's nowhere to put it. + // See issues #81943 and #87051. + && matches!( + cond_expr.span.desugaring_kind(), + None | Some(DesugaringKind::WhileLoop) + ) && !in_external_macro(fcx.tcx.sess, cond_expr.span) + && !matches!( + cond_expr.kind, + hir::ExprKind::Match(.., hir::MatchSource::TryDesugar) + ) + { + err.span_label(cond_expr.span, "expected this to be `()`"); + if expr.can_have_side_effects() { + fcx.suggest_semicolon_at_end(cond_expr.span, &mut err); + } + } + fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main)) + } else { + fcx.get_fn_decl(parent_id) + }; + + if let Some((fn_decl, can_suggest)) = fn_decl { + if blk_id.is_none() { + pointing_at_return_type |= fcx.suggest_missing_return_type( + &mut err, + &fn_decl, + expected, + found, + can_suggest, + fcx.tcx.hir().local_def_id_to_hir_id(fcx.tcx.hir().get_parent_item(id)), + ); + } + if !pointing_at_return_type { + fn_output = Some(&fn_decl.output); // `impl Trait` return type + } + } + + let parent_id = fcx.tcx.hir().get_parent_item(id); + let parent_item = fcx.tcx.hir().get_by_def_id(parent_id); + + if let (Some(expr), Some(_), Some((fn_decl, _, _))) = + (expression, blk_id, fcx.get_node_fn_decl(parent_item)) + { + fcx.suggest_missing_break_or_return_expr( + &mut err, + expr, + fn_decl, + expected, + found, + id, + fcx.tcx.hir().local_def_id_to_hir_id(parent_id), + ); + } + + if let (Some(sp), Some(fn_output)) = (fcx.ret_coercion_span.get(), fn_output) { + self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output); + } + err + } + + fn add_impl_trait_explanation<'a>( + &self, + err: &mut Diagnostic, + cause: &ObligationCause<'tcx>, + fcx: &FnCtxt<'a, 'tcx>, + expected: Ty<'tcx>, + sp: Span, + fn_output: &hir::FnRetTy<'_>, + ) { + let return_sp = fn_output.span(); + err.span_label(return_sp, "expected because this return type..."); + err.span_label( + sp, + format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)), + ); + let impl_trait_msg = "for information on `impl Trait`, see \ + <https://doc.rust-lang.org/book/ch10-02-traits.html\ + #returning-types-that-implement-traits>"; + let trait_obj_msg = "for information on trait objects, see \ + <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\ + #using-trait-objects-that-allow-for-values-of-different-types>"; + err.note("to return `impl Trait`, all returned values must be of the same type"); + err.note(impl_trait_msg); + let snippet = fcx + .tcx + .sess + .source_map() + .span_to_snippet(return_sp) + .unwrap_or_else(|_| "dyn Trait".to_string()); + let mut snippet_iter = snippet.split_whitespace(); + let has_impl = snippet_iter.next().map_or(false, |s| s == "impl"); + // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe. + let mut is_object_safe = false; + if let hir::FnRetTy::Return(ty) = fn_output + // Get the return type. + && let hir::TyKind::OpaqueDef(..) = ty.kind + { + let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty); + // Get the `impl Trait`'s `DefId`. + if let ty::Opaque(def_id, _) = ty.kind() + // Get the `impl Trait`'s `Item` so that we can get its trait bounds and + // get the `Trait`'s `DefId`. + && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) = + fcx.tcx.hir().expect_item(def_id.expect_local()).kind + { + // Are of this `impl Trait`'s traits object safe? + is_object_safe = bounds.iter().all(|bound| { + bound + .trait_ref() + .and_then(|t| t.trait_def_id()) + .map_or(false, |def_id| { + fcx.tcx.object_safety_violations(def_id).is_empty() + }) + }) + } + }; + if has_impl { + if is_object_safe { + err.multipart_suggestion( + "you could change the return type to be a boxed trait object", + vec![ + (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()), + (return_sp.shrink_to_hi(), ">".to_string()), + ], + Applicability::MachineApplicable, + ); + let sugg = [sp, cause.span] + .into_iter() + .flat_map(|sp| { + [ + (sp.shrink_to_lo(), "Box::new(".to_string()), + (sp.shrink_to_hi(), ")".to_string()), + ] + .into_iter() + }) + .collect::<Vec<_>>(); + err.multipart_suggestion( + "if you change the return type to expect trait objects, box the returned \ + expressions", + sugg, + Applicability::MaybeIncorrect, + ); + } else { + err.help(&format!( + "if the trait `{}` were object safe, you could return a boxed trait object", + &snippet[5..] + )); + } + err.note(trait_obj_msg); + } + err.help("you could instead create a new `enum` with a variant for each returned type"); + } + + fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool { + if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id) + && let hir::FnRetTy::Return(ty) = fn_decl.output + && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty) + && let ty::Dynamic(..) = ty.kind() + { + return true; + } + false + } + + pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> { + if let Some(final_ty) = self.final_ty { + final_ty + } else { + // If we only had inputs that were of type `!` (or no + // inputs at all), then the final type is `!`. + assert_eq!(self.pushed, 0); + fcx.tcx.types.never + } + } +} + +/// Something that can be converted into an expression to which we can +/// apply a coercion. +pub trait AsCoercionSite { + fn as_coercion_site(&self) -> &hir::Expr<'_>; +} + +impl AsCoercionSite for hir::Expr<'_> { + fn as_coercion_site(&self) -> &hir::Expr<'_> { + self + } +} + +impl<'a, T> AsCoercionSite for &'a T +where + T: AsCoercionSite, +{ + fn as_coercion_site(&self) -> &hir::Expr<'_> { + (**self).as_coercion_site() + } +} + +impl AsCoercionSite for ! { + fn as_coercion_site(&self) -> &hir::Expr<'_> { + unreachable!() + } +} + +impl AsCoercionSite for hir::Arm<'_> { + fn as_coercion_site(&self) -> &hir::Expr<'_> { + &self.body + } +} |