//! # 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 `tests/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(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::FnCtxt; use rustc_errors::{ struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, }; use rustc_hir as hir; use rustc_hir::def_id::DefId; use rustc_hir::intravisit::{self, Visitor}; use rustc_hir::Expr; use rustc_hir_analysis::astconv::AstConv; use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind}; use rustc_infer::infer::{Coercion, InferOk, InferResult}; use rustc_infer::traits::Obligation; 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, 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::TypeErrCtxtExt as _; use rustc_trait_selection::traits::{ self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt, }; 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>, Ty<'tcx>)>; struct CollectRetsVisitor<'tcx> { ret_exprs: Vec<&'tcx hir::Expr<'tcx>>, } impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> { fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) { if let hir::ExprKind::Ret(_) = expr.kind { self.ret_exprs.push(expr); } intravisit::walk_expr(self, expr); } } /// 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, ()> { if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) } } /// Do not require any adjustments, i.e. coerce `x -> x`. fn identity(_: Ty<'_>) -> Vec> { vec![] } fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec> { move |target| vec![Adjustment { kind, target }] } /// This always returns `Ok(...)`. fn success<'tcx>( adj: Vec>, 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(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx> where F: FnOnce(Ty<'tcx>) -> Vec>, { 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() { // Best-effort try to unify these types -- we're already on the error path, // so this will have the side-effect of making sure we have no ambiguities // due to `[type error]` and `_` not coercing together. let _ = self.commit_if_ok(|_| self.at(&self.cause, self.param_env).eq(a, b)); 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(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); } ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => { return self.coerce_dyn_star(a, b, predicates, region); } _ => {} } 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>, ) -> 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.tcx(), self.cause.clone(), self.param_env, ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate { a: source_ty, b: target_ty, })), )); } } 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` to `&'b mut [T]`. // In the autoderef loop for `&'a mut Vec`, we would get // three callbacks: // // - `&'a mut Vec` -- 0 derefs, just ignore it // - `Vec` -- 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` 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` // 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.is_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!(mutbl_b.is_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 = AutoBorrowMutability::new(mutbl_b, 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); // We don't apply any coercions incase either the source or target // aren't sufficiently well known but tend to instead just equate // them both. 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`. 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); // 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. let mutbl = AutoBorrowMutability::new(mutbl_b, 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, |(_, 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`. 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] )]; 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::Clause(ty::Clause::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(selcx.tcx(), 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` where we know that `$0: Sized`, try going // for unsizing. } _ => { // Some other case for `$0: Unsize`. 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.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err); // 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_dyn_star( &self, a: Ty<'tcx>, b: Ty<'tcx>, predicates: &'tcx ty::List>, b_region: ty::Region<'tcx>, ) -> CoerceResult<'tcx> { if !self.tcx.features().dyn_star { return Err(TypeError::Mismatch); } if let ty::Dynamic(a_data, _, _) = a.kind() && let ty::Dynamic(b_data, _, _) = b.kind() && a_data.principal_def_id() == b_data.principal_def_id() { return self.unify_and(a, b, |_| vec![]); } // Check the obligations of the cast -- for example, when casting // `usize` to `dyn* Clone + 'static`: let mut obligations: Vec<_> = predicates .iter() .map(|predicate| { // For each existential predicate (e.g., `?Self: Clone`) substitute // the type of the expression (e.g., `usize` in our example above) // and then require that the resulting predicate (e.g., `usize: Clone`) // holds (it does). let predicate = predicate.with_self_ty(self.tcx, a); Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate) }) .chain([ // Enforce the region bound (e.g., `usize: 'static`, in our example). Obligation::new( self.tcx, self.cause.clone(), self.param_env, ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::TypeOutlives( ty::OutlivesPredicate(a, b_region), ))), ), ]) .collect(); // Enforce that the type is `usize`/pointer-sized. obligations.push(Obligation::new( self.tcx, self.cause.clone(), self.param_env, ty::Binder::dummy( self.tcx.at(self.cause.span).mk_trait_ref(hir::LangItem::PointerSized, [a]), ), )); Ok(InferOk { value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b), obligations, }) } fn coerce_from_safe_fn( &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>, G: FnOnce(Ty<'tcx>) -> Vec>, { 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.at(&self.cause, self.param_env).normalize(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.at(&self.cause, self.param_env).normalize(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>, ) -> 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 ocx = ObligationCtxt::new_in_snapshot(self); ocx.register_obligations(ok.obligations); ocx.select_where_possible().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 { 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 `::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> { 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], 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( &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) = { let is_capturing_closure = |ty: Ty<'tcx>| { if let &ty::Closure(closure_def_id, _substs) = ty.kind() { self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some() } else { false } }; if is_capturing_closure(prev_ty) || is_capturing_closure(new_ty) { (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, b_sig) = self.normalize(new.span, (a_sig, 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>, 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) => { // Mark that we've failed to coerce the types here to suppress // any superfluous errors we might encounter while trying to // emit or provide suggestions on how to fix the initial error. fcx.set_tainted_by_errors( fcx.tcx.sess.delay_span_bug(cause.span, "coercion error but no error emitted"), ); 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.merged_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.merged_ty(), expression_ty) }; let (expected, found) = fcx.resolve_vars_if_possible((expected, found)); let mut err; let mut unsized_return = false; let mut visitor = CollectRetsVisitor { ret_exprs: vec![] }; 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().parent_id(blk_id); err = self.report_return_mismatched_types( cause, expected, found, coercion_error, fcx, parent_id, expression, Some(blk_id), ); if !fcx.tcx.features().unsized_locals { unsized_return = self.is_return_ty_unsized(fcx, blk_id); } if let Some(expression) = expression && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind { intravisit::walk_block(& mut visitor, loop_blk); } } ObligationCauseCode::ReturnValue(id) => { err = self.report_return_mismatched_types( cause, expected, found, coercion_error, fcx, id, expression, None, ); if !fcx.tcx.features().unsized_locals { let id = fcx.tcx.hir().parent_id(id); unsized_return = self.is_return_ty_unsized(fcx, id); } } _ => { err = fcx.err_ctxt().report_mismatched_types( cause, expected, found, coercion_error, ); } } 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), ); } if visitor.ret_exprs.len() > 0 && let Some(expr) = expression { self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs); } let reported = err.emit_unless(unsized_return); self.final_ty = Some(fcx.tcx.ty_error_with_guaranteed(reported)); } } } fn note_unreachable_loop_return( &self, err: &mut Diagnostic, expr: &hir::Expr<'tcx>, ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>, ) { let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;}; let mut span: MultiSpan = vec![loop_span].into(); span.push_span_label(loop_span, "this might have zero elements to iterate on"); const MAXITER: usize = 3; let iter = ret_exprs.iter().take(MAXITER); for ret_expr in iter { span.push_span_label( ret_expr.span, "if the loop doesn't execute, this value would never get returned", ); } err.span_note( span, "the function expects a value to always be returned, but loops might run zero times", ); if MAXITER < ret_exprs.len() { err.note(&format!( "if the loop doesn't execute, {} other values would never get returned", ret_exprs.len() - MAXITER )); } err.help( "return a value for the case when the loop has zero elements to iterate on, or \ consider changing the return type to account for that possibility", ); } 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, ) -> DiagnosticBuilder<'a, ErrorGuaranteed> { let mut err = fcx.err_ctxt().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().parent_id(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().get_parent_item(id).into(), ); } 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.def_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, parent_id.into(), ); } let ret_coercion_span = fcx.ret_coercion_span.get(); if let Some(sp) = ret_coercion_span // If the closure has an explicit return type annotation, or if // the closure's return type has been inferred from outside // requirements (such as an Fn* trait bound), then a type error // may occur at the first return expression we see in the closure // (if it conflicts with the declared return type). Skip adding a // note in this case, since it would be incorrect. && let Some(fn_sig) = fcx.body_fn_sig() && fn_sig.output().is_ty_var() { err.span_note( sp, &format!( "return type inferred to be `{}` here", expected ), ); } if let (Some(sp), Some(fn_output)) = (ret_coercion_span, 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 \ "; let trait_obj_msg = "for information on trait objects, see \ "; 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` 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 = fcx.astconv().ast_ty_to_ty( ty); // Get the `impl Trait`'s `DefId`. if let ty::Alias(ty::Opaque, ty::AliasTy { 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".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::>(); 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 = fcx.astconv().ast_ty_to_ty( 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 } }