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+//! # 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::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, 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::TypeErrCtxtExt 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>)>;
+
+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, ()> {
+ 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);
+ }
+ 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<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.err_ctxt().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_dyn_star(
+ &self,
+ a: Ty<'tcx>,
+ b: Ty<'tcx>,
+ predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+ 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()
+ {
+ if a_data.principal_def_id() == b_data.principal_def_id() {
+ return self.unify_and(a, b, |_| vec![]);
+ } else if !self.tcx().features().trait_upcasting {
+ let mut err = feature_err(
+ &self.tcx.sess.parse_sess,
+ sym::trait_upcasting,
+ self.cause.span,
+ &format!(
+ "cannot cast `{a}` to `{b}`, trait upcasting coercion is experimental"
+ ),
+ );
+ err.emit();
+ }
+ }
+
+ // Check the obligations of the cast -- for example, when casting
+ // `usize` to `dyn* Clone + 'static`:
+ let obligations = 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.cause.clone(), self.param_env, predicate)
+ })
+ // Enforce the region bound (e.g., `usize: 'static`, in our example).
+ .chain([Obligation::new(
+ self.cause.clone(),
+ self.param_env,
+ self.tcx.mk_predicate(ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
+ ty::OutlivesPredicate(a, b_region),
+ ))),
+ )])
+ .collect();
+
+ Ok(InferOk {
+ value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
+ obligations,
+ })
+ }
+
+ 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) => {
+ // 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();
+ 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().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);
+ }
+ 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.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.err_ctxt().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),
+ );
+ }
+
+ if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
+ self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
+ }
+ err.emit_unless(unsized_return);
+
+ self.final_ty = Some(fcx.tcx.ty_error());
+ }
+ }
+ }
+ 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<hir::HirId>,
+ ) -> 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().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().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.
+ && !fcx.return_type_pre_known
+ {
+ 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 \
+ <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
+ }
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-07-06 02:18:46 +0000