Merging upstream version 1.66.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 0 insertions, 1885 deletions

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
deleted file mode 100644
index def592c46..000000000
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ /dev/null
@@ -1,1885 +0,0 @@
-//! # Type Coercion
-//!
-//! Under certain circumstances we will coerce from one type to another,
-//! for example by auto-borrowing. This occurs in situations where the
-//! compiler has a firm 'expected type' that was supplied from the user,
-//! and where the actual type is similar to that expected type in purpose
-//! but not in representation (so actual subtyping is inappropriate).
-//!
-//! ## Reborrowing
-//!
-//! Note that if we are expecting a reference, we will *reborrow*
-//! even if the argument provided was already a reference. This is
-//! useful for freezing mut things (that is, when the expected type is &T
-//! but you have &mut T) and also for avoiding the linearity
-//! of mut things (when the expected is &mut T and you have &mut T). See
-//! the various `src/test/ui/coerce/*.rs` tests for
-//! examples of where this is useful.
-//!
-//! ## Subtle note
-//!
-//! When inferring the generic arguments of functions, the argument
-//! order is relevant, which can lead to the following edge case:
-//!
-//! ```ignore (illustrative)
-//! fn foo<T>(a: T, b: T) {
-//!     // ...
-//! }
-//!
-//! foo(&7i32, &mut 7i32);
-//! // This compiles, as we first infer `T` to be `&i32`,
-//! // and then coerce `&mut 7i32` to `&7i32`.
-//!
-//! foo(&mut 7i32, &7i32);
-//! // This does not compile, as we first infer `T` to be `&mut i32`
-//! // and are then unable to coerce `&7i32` to `&mut i32`.
-//! ```
-
-use crate::astconv::AstConv;
-use crate::check::FnCtxt;
-use rustc_errors::{
-    struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
-};
-use rustc_hir as hir;
-use rustc_hir::def_id::DefId;
-use rustc_hir::intravisit::{self, Visitor};
-use rustc_hir::Expr;
-use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_infer::infer::{Coercion, InferOk, InferResult};
-use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt};
-use rustc_middle::lint::in_external_macro;
-use rustc_middle::ty::adjustment::{
-    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
-};
-use rustc_middle::ty::error::TypeError;
-use rustc_middle::ty::relate::RelateResult;
-use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::visit::TypeVisitable;
-use rustc_middle::ty::{self, ToPredicate, Ty, TypeAndMut};
-use rustc_session::parse::feature_err;
-use rustc_span::symbol::sym;
-use rustc_span::{self, BytePos, DesugaringKind, Span};
-use rustc_target::spec::abi::Abi;
-use rustc_trait_selection::infer::InferCtxtExt as _;
-use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
-use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
-
-use smallvec::{smallvec, SmallVec};
-use std::ops::Deref;
-
-struct Coerce<'a, 'tcx> {
-    fcx: &'a FnCtxt<'a, 'tcx>,
-    cause: ObligationCause<'tcx>,
-    use_lub: bool,
-    /// Determines whether or not allow_two_phase_borrow is set on any
-    /// autoref adjustments we create while coercing. We don't want to
-    /// allow deref coercions to create two-phase borrows, at least initially,
-    /// but we do need two-phase borrows for function argument reborrows.
-    /// See #47489 and #48598
-    /// See docs on the "AllowTwoPhase" type for a more detailed discussion
-    allow_two_phase: AllowTwoPhase,
-}
-
-impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
-    type Target = FnCtxt<'a, 'tcx>;
-    fn deref(&self) -> &Self::Target {
-        &self.fcx
-    }
-}
-
-type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
-
-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);
-            }
-            _ => {}
-        }
-
-        match *a.kind() {
-            ty::FnDef(..) => {
-                // Function items are coercible to any closure
-                // type; function pointers are not (that would
-                // require double indirection).
-                // Additionally, we permit coercion of function
-                // items to drop the unsafe qualifier.
-                self.coerce_from_fn_item(a, b)
-            }
-            ty::FnPtr(a_f) => {
-                // We permit coercion of fn pointers to drop the
-                // unsafe qualifier.
-                self.coerce_from_fn_pointer(a, a_f, b)
-            }
-            ty::Closure(closure_def_id_a, substs_a) => {
-                // Non-capturing closures are coercible to
-                // function pointers or unsafe function pointers.
-                // It cannot convert closures that require unsafe.
-                self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
-            }
-            _ => {
-                // Otherwise, just use unification rules.
-                self.unify_and(a, b, identity)
-            }
-        }
-    }
-
-    /// Coercing *from* an inference variable. In this case, we have no information
-    /// about the source type, so we can't really do a true coercion and we always
-    /// fall back to subtyping (`unify_and`).
-    fn coerce_from_inference_variable(
-        &self,
-        a: Ty<'tcx>,
-        b: Ty<'tcx>,
-        make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    ) -> CoerceResult<'tcx> {
-        debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
-        assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
-        assert!(self.shallow_resolve(b) == b);
-
-        if b.is_ty_var() {
-            // Two unresolved type variables: create a `Coerce` predicate.
-            let target_ty = if self.use_lub {
-                self.next_ty_var(TypeVariableOrigin {
-                    kind: TypeVariableOriginKind::LatticeVariable,
-                    span: self.cause.span,
-                })
-            } else {
-                b
-            };
-
-            let mut obligations = Vec::with_capacity(2);
-            for &source_ty in &[a, b] {
-                if source_ty != target_ty {
-                    obligations.push(Obligation::new(
-                        self.cause.clone(),
-                        self.param_env,
-                        ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
-                            a: source_ty,
-                            b: target_ty,
-                        }))
-                        .to_predicate(self.tcx()),
-                    ));
-                }
-            }
-
-            debug!(
-                "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
-                target_ty, obligations
-            );
-            let adjustments = make_adjustments(target_ty);
-            InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
-        } else {
-            // One unresolved type variable: just apply subtyping, we may be able
-            // to do something useful.
-            self.unify_and(a, b, make_adjustments)
-        }
-    }
-
-    /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
-    /// To match `A` with `B`, autoderef will be performed,
-    /// calling `deref`/`deref_mut` where necessary.
-    fn coerce_borrowed_pointer(
-        &self,
-        a: Ty<'tcx>,
-        b: Ty<'tcx>,
-        r_b: ty::Region<'tcx>,
-        mutbl_b: hir::Mutability,
-    ) -> CoerceResult<'tcx> {
-        debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
-
-        // If we have a parameter of type `&M T_a` and the value
-        // provided is `expr`, we will be adding an implicit borrow,
-        // meaning that we convert `f(expr)` to `f(&M *expr)`.  Therefore,
-        // to type check, we will construct the type that `&M*expr` would
-        // yield.
-
-        let (r_a, mt_a) = match *a.kind() {
-            ty::Ref(r_a, ty, mutbl) => {
-                let mt_a = ty::TypeAndMut { ty, mutbl };
-                coerce_mutbls(mt_a.mutbl, mutbl_b)?;
-                (r_a, mt_a)
-            }
-            _ => return self.unify_and(a, b, identity),
-        };
-
-        let span = self.cause.span;
-
-        let mut first_error = None;
-        let mut r_borrow_var = None;
-        let mut autoderef = self.autoderef(span, a);
-        let mut found = None;
-
-        for (referent_ty, autoderefs) in autoderef.by_ref() {
-            if autoderefs == 0 {
-                // Don't let this pass, otherwise it would cause
-                // &T to autoref to &&T.
-                continue;
-            }
-
-            // At this point, we have deref'd `a` to `referent_ty`.  So
-            // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
-            // In the autoderef loop for `&'a mut Vec<T>`, we would get
-            // three callbacks:
-            //
-            // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
-            // - `Vec<T>` -- 1 deref
-            // - `[T]` -- 2 deref
-            //
-            // At each point after the first callback, we want to
-            // check to see whether this would match out target type
-            // (`&'b mut [T]`) if we autoref'd it. We can't just
-            // compare the referent types, though, because we still
-            // have to consider the mutability. E.g., in the case
-            // we've been considering, we have an `&mut` reference, so
-            // the `T` in `[T]` needs to be unified with equality.
-            //
-            // Therefore, we construct reference types reflecting what
-            // the types will be after we do the final auto-ref and
-            // compare those. Note that this means we use the target
-            // mutability [1], since it may be that we are coercing
-            // from `&mut T` to `&U`.
-            //
-            // One fine point concerns the region that we use. We
-            // choose the region such that the region of the final
-            // type that results from `unify` will be the region we
-            // want for the autoref:
-            //
-            // - if in sub mode, that means we want to use `'b` (the
-            //   region from the target reference) for both
-            //   pointers [2]. This is because sub mode (somewhat
-            //   arbitrarily) returns the subtype region.  In the case
-            //   where we are coercing to a target type, we know we
-            //   want to use that target type region (`'b`) because --
-            //   for the program to type-check -- it must be the
-            //   smaller of the two.
-            //   - One fine point. It may be surprising that we can
-            //     use `'b` without relating `'a` and `'b`. The reason
-            //     that this is ok is that what we produce is
-            //     effectively a `&'b *x` expression (if you could
-            //     annotate the region of a borrow), and regionck has
-            //     code that adds edges from the region of a borrow
-            //     (`'b`, here) into the regions in the borrowed
-            //     expression (`*x`, here).  (Search for "link".)
-            // - if in lub mode, things can get fairly complicated. The
-            //   easiest thing is just to make a fresh
-            //   region variable [4], which effectively means we defer
-            //   the decision to region inference (and regionck, which will add
-            //   some more edges to this variable). However, this can wind up
-            //   creating a crippling number of variables in some cases --
-            //   e.g., #32278 -- so we optimize one particular case [3].
-            //   Let me try to explain with some examples:
-            //   - The "running example" above represents the simple case,
-            //     where we have one `&` reference at the outer level and
-            //     ownership all the rest of the way down. In this case,
-            //     we want `LUB('a, 'b)` as the resulting region.
-            //   - However, if there are nested borrows, that region is
-            //     too strong. Consider a coercion from `&'a &'x Rc<T>` to
-            //     `&'b T`. In this case, `'a` is actually irrelevant.
-            //     The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
-            //     we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
-            //     (The errors actually show up in borrowck, typically, because
-            //     this extra edge causes the region `'a` to be inferred to something
-            //     too big, which then results in borrowck errors.)
-            //   - We could track the innermost shared reference, but there is already
-            //     code in regionck that has the job of creating links between
-            //     the region of a borrow and the regions in the thing being
-            //     borrowed (here, `'a` and `'x`), and it knows how to handle
-            //     all the various cases. So instead we just make a region variable
-            //     and let regionck figure it out.
-            let r = if !self.use_lub {
-                r_b // [2] above
-            } else if autoderefs == 1 {
-                r_a // [3] above
-            } else {
-                if r_borrow_var.is_none() {
-                    // create var lazily, at most once
-                    let coercion = Coercion(span);
-                    let r = self.next_region_var(coercion);
-                    r_borrow_var = Some(r); // [4] above
-                }
-                r_borrow_var.unwrap()
-            };
-            let derefd_ty_a = self.tcx.mk_ref(
-                r,
-                TypeAndMut {
-                    ty: referent_ty,
-                    mutbl: mutbl_b, // [1] above
-                },
-            );
-            match self.unify(derefd_ty_a, b) {
-                Ok(ok) => {
-                    found = Some(ok);
-                    break;
-                }
-                Err(err) => {
-                    if first_error.is_none() {
-                        first_error = Some(err);
-                    }
-                }
-            }
-        }
-
-        // Extract type or return an error. We return the first error
-        // we got, which should be from relating the "base" type
-        // (e.g., in example above, the failure from relating `Vec<T>`
-        // to the target type), since that should be the least
-        // confusing.
-        let Some(InferOk { value: ty, mut obligations }) = found else {
-            let err = first_error.expect("coerce_borrowed_pointer had no error");
-            debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
-            return Err(err);
-        };
-
-        if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
-            // As a special case, if we would produce `&'a *x`, that's
-            // a total no-op. We end up with the type `&'a T` just as
-            // we started with.  In that case, just skip it
-            // altogether. This is just an optimization.
-            //
-            // Note that for `&mut`, we DO want to reborrow --
-            // otherwise, this would be a move, which might be an
-            // error. For example `foo(self.x)` where `self` and
-            // `self.x` both have `&mut `type would be a move of
-            // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
-            // which is a borrow.
-            assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U
-            return success(vec![], ty, obligations);
-        }
-
-        let InferOk { value: mut adjustments, obligations: o } =
-            self.adjust_steps_as_infer_ok(&autoderef);
-        obligations.extend(o);
-        obligations.extend(autoderef.into_obligations());
-
-        // Now apply the autoref. We have to extract the region out of
-        // the final ref type we got.
-        let ty::Ref(r_borrow, _, _) = ty.kind() else {
-            span_bug!(span, "expected a ref type, got {:?}", ty);
-        };
-        let mutbl = match mutbl_b {
-            hir::Mutability::Not => AutoBorrowMutability::Not,
-            hir::Mutability::Mut => {
-                AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
-            }
-        };
-        adjustments.push(Adjustment {
-            kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
-            target: ty,
-        });
-
-        debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
-
-        success(adjustments, ty, obligations)
-    }
-
-    // &[T; n] or &mut [T; n] -> &[T]
-    // or &mut [T; n] -> &mut [T]
-    // or &Concrete -> &Trait, etc.
-    #[instrument(skip(self), level = "debug")]
-    fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
-        source = self.shallow_resolve(source);
-        target = self.shallow_resolve(target);
-        debug!(?source, ?target);
-
-        // These 'if' statements require some explanation.
-        // The `CoerceUnsized` trait is special - it is only
-        // possible to write `impl CoerceUnsized<B> for A` where
-        // A and B have 'matching' fields. This rules out the following
-        // two types of blanket impls:
-        //
-        // `impl<T> CoerceUnsized<T> for SomeType`
-        // `impl<T> CoerceUnsized<SomeType> for T`
-        //
-        // Both of these trigger a special `CoerceUnsized`-related error (E0376)
-        //
-        // We can take advantage of this fact to avoid performing unnecessary work.
-        // If either `source` or `target` is a type variable, then any applicable impl
-        // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
-        // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
-        // SomeType`).
-        //
-        // However, these are exactly the kinds of impls which are forbidden by
-        // the compiler! Therefore, we can be sure that coercion will always fail
-        // when either the source or target type is a type variable. This allows us
-        // to skip performing any trait selection, and immediately bail out.
-        if source.is_ty_var() {
-            debug!("coerce_unsized: source is a TyVar, bailing out");
-            return Err(TypeError::Mismatch);
-        }
-        if target.is_ty_var() {
-            debug!("coerce_unsized: target is a TyVar, bailing out");
-            return Err(TypeError::Mismatch);
-        }
-
-        let traits =
-            (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
-        let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
-            debug!("missing Unsize or CoerceUnsized traits");
-            return Err(TypeError::Mismatch);
-        };
-
-        // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
-        // a DST unless we have to. This currently comes out in the wash since
-        // we can't unify [T] with U. But to properly support DST, we need to allow
-        // that, at which point we will need extra checks on the target here.
-
-        // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
-        let reborrow = match (source.kind(), target.kind()) {
-            (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
-                coerce_mutbls(mutbl_a, mutbl_b)?;
-
-                let coercion = Coercion(self.cause.span);
-                let r_borrow = self.next_region_var(coercion);
-                let mutbl = match mutbl_b {
-                    hir::Mutability::Not => AutoBorrowMutability::Not,
-                    hir::Mutability::Mut => AutoBorrowMutability::Mut {
-                        // We don't allow two-phase borrows here, at least for initial
-                        // implementation. If it happens that this coercion is a function argument,
-                        // the reborrow in coerce_borrowed_ptr will pick it up.
-                        allow_two_phase_borrow: AllowTwoPhase::No,
-                    },
-                };
-                Some((
-                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
-                    Adjustment {
-                        kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
-                        target: self
-                            .tcx
-                            .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
-                    },
-                ))
-            }
-            (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
-                coerce_mutbls(mt_a, mt_b)?;
-
-                Some((
-                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
-                    Adjustment {
-                        kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
-                        target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
-                    },
-                ))
-            }
-            _ => None,
-        };
-        let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
-
-        // Setup either a subtyping or a LUB relationship between
-        // the `CoerceUnsized` target type and the expected type.
-        // We only have the latter, so we use an inference variable
-        // for the former and let type inference do the rest.
-        let origin = TypeVariableOrigin {
-            kind: TypeVariableOriginKind::MiscVariable,
-            span: self.cause.span,
-        };
-        let coerce_target = self.next_ty_var(origin);
-        let mut coercion = self.unify_and(coerce_target, target, |target| {
-            let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
-            match reborrow {
-                None => vec![unsize],
-                Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
-            }
-        })?;
-
-        let mut selcx = traits::SelectionContext::new(self);
-
-        // Create an obligation for `Source: CoerceUnsized<Target>`.
-        let cause = ObligationCause::new(
-            self.cause.span,
-            self.body_id,
-            ObligationCauseCode::Coercion { source, target },
-        );
-
-        // Use a FIFO queue for this custom fulfillment procedure.
-        //
-        // A Vec (or SmallVec) is not a natural choice for a queue. However,
-        // this code path is hot, and this queue usually has a max length of 1
-        // and almost never more than 3. By using a SmallVec we avoid an
-        // allocation, at the (very small) cost of (occasionally) having to
-        // shift subsequent elements down when removing the front element.
-        let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
-            self.tcx,
-            self.fcx.param_env,
-            cause,
-            coerce_unsized_did,
-            0,
-            coerce_source,
-            &[coerce_target.into()]
-        )];
-
-        let mut has_unsized_tuple_coercion = false;
-        let mut has_trait_upcasting_coercion = None;
-
-        // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
-        // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
-        // inference might unify those two inner type variables later.
-        let traits = [coerce_unsized_did, unsize_did];
-        while !queue.is_empty() {
-            let obligation = queue.remove(0);
-            debug!("coerce_unsized resolve step: {:?}", obligation);
-            let bound_predicate = obligation.predicate.kind();
-            let trait_pred = match bound_predicate.skip_binder() {
-                ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => {
-                    if unsize_did == trait_pred.def_id() {
-                        let self_ty = trait_pred.self_ty();
-                        let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
-                        if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
-                            (self_ty.kind(), unsize_ty.kind())
-                            && data_a.principal_def_id() != data_b.principal_def_id()
-                        {
-                            debug!("coerce_unsized: found trait upcasting coercion");
-                            has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
-                        }
-                        if let ty::Tuple(..) = unsize_ty.kind() {
-                            debug!("coerce_unsized: found unsized tuple coercion");
-                            has_unsized_tuple_coercion = true;
-                        }
-                    }
-                    bound_predicate.rebind(trait_pred)
-                }
-                _ => {
-                    coercion.obligations.push(obligation);
-                    continue;
-                }
-            };
-            match selcx.select(&obligation.with(trait_pred)) {
-                // Uncertain or unimplemented.
-                Ok(None) => {
-                    if trait_pred.def_id() == unsize_did {
-                        let trait_pred = self.resolve_vars_if_possible(trait_pred);
-                        let self_ty = trait_pred.skip_binder().self_ty();
-                        let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
-                        debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
-                        match (&self_ty.kind(), &unsize_ty.kind()) {
-                            (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
-                                if self.type_var_is_sized(*v) =>
-                            {
-                                debug!("coerce_unsized: have sized infer {:?}", v);
-                                coercion.obligations.push(obligation);
-                                // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
-                                // for unsizing.
-                            }
-                            _ => {
-                                // Some other case for `$0: Unsize<Something>`. Note that we
-                                // hit this case even if `Something` is a sized type, so just
-                                // don't do the coercion.
-                                debug!("coerce_unsized: ambiguous unsize");
-                                return Err(TypeError::Mismatch);
-                            }
-                        }
-                    } else {
-                        debug!("coerce_unsized: early return - ambiguous");
-                        return Err(TypeError::Mismatch);
-                    }
-                }
-                Err(traits::Unimplemented) => {
-                    debug!("coerce_unsized: early return - can't prove obligation");
-                    return Err(TypeError::Mismatch);
-                }
-
-                // Object safety violations or miscellaneous.
-                Err(err) => {
-                    self.report_selection_error(obligation.clone(), &obligation, &err, false);
-                    // Treat this like an obligation and follow through
-                    // with the unsizing - the lack of a coercion should
-                    // be silent, as it causes a type mismatch later.
-                }
-
-                Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
-            }
-        }
-
-        if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
-            feature_err(
-                &self.tcx.sess.parse_sess,
-                sym::unsized_tuple_coercion,
-                self.cause.span,
-                "unsized tuple coercion is not stable enough for use and is subject to change",
-            )
-            .emit();
-        }
-
-        if let Some((sub, sup)) = has_trait_upcasting_coercion
-            && !self.tcx().features().trait_upcasting
-        {
-            // Renders better when we erase regions, since they're not really the point here.
-            let (sub, sup) = self.tcx.erase_regions((sub, sup));
-            let mut err = feature_err(
-                &self.tcx.sess.parse_sess,
-                sym::trait_upcasting,
-                self.cause.span,
-                &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
-            );
-            err.note(&format!("required when coercing `{source}` into `{target}`"));
-            err.emit();
-        }
-
-        Ok(coercion)
-    }
-
-    fn coerce_from_safe_fn<F, G>(
-        &self,
-        a: Ty<'tcx>,
-        fn_ty_a: ty::PolyFnSig<'tcx>,
-        b: Ty<'tcx>,
-        to_unsafe: F,
-        normal: G,
-    ) -> CoerceResult<'tcx>
-    where
-        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-        G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    {
-        self.commit_if_ok(|snapshot| {
-            let result = if let ty::FnPtr(fn_ty_b) = b.kind()
-                && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
-                    (fn_ty_a.unsafety(), fn_ty_b.unsafety())
-            {
-                let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
-                self.unify_and(unsafe_a, b, to_unsafe)
-            } else {
-                self.unify_and(a, b, normal)
-            };
-
-            // FIXME(#73154): This is a hack. Currently LUB can generate
-            // unsolvable constraints. Additionally, it returns `a`
-            // unconditionally, even when the "LUB" is `b`. In the future, we
-            // want the coerced type to be the actual supertype of these two,
-            // but for now, we want to just error to ensure we don't lock
-            // ourselves into a specific behavior with NLL.
-            self.leak_check(false, snapshot)?;
-
-            result
-        })
-    }
-
-    fn coerce_from_fn_pointer(
-        &self,
-        a: Ty<'tcx>,
-        fn_ty_a: ty::PolyFnSig<'tcx>,
-        b: Ty<'tcx>,
-    ) -> CoerceResult<'tcx> {
-        //! Attempts to coerce from the type of a Rust function item
-        //! into a closure or a `proc`.
-        //!
-
-        let b = self.shallow_resolve(b);
-        debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
-
-        self.coerce_from_safe_fn(
-            a,
-            fn_ty_a,
-            b,
-            simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
-            identity,
-        )
-    }
-
-    fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
-        //! Attempts to coerce from the type of a Rust function item
-        //! into a closure or a `proc`.
-
-        let b = self.shallow_resolve(b);
-        let InferOk { value: b, mut obligations } =
-            self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
-        debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
-
-        match b.kind() {
-            ty::FnPtr(b_sig) => {
-                let a_sig = a.fn_sig(self.tcx);
-                if let ty::FnDef(def_id, _) = *a.kind() {
-                    // Intrinsics are not coercible to function pointers
-                    if self.tcx.is_intrinsic(def_id) {
-                        return Err(TypeError::IntrinsicCast);
-                    }
-
-                    // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
-
-                    if b_sig.unsafety() == hir::Unsafety::Normal
-                        && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
-                    {
-                        return Err(TypeError::TargetFeatureCast(def_id));
-                    }
-                }
-
-                let InferOk { value: a_sig, obligations: o1 } =
-                    self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
-                obligations.extend(o1);
-
-                let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
-                let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
-                    a_fn_pointer,
-                    a_sig,
-                    b,
-                    |unsafe_ty| {
-                        vec![
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
-                                target: a_fn_pointer,
-                            },
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
-                                target: unsafe_ty,
-                            },
-                        ]
-                    },
-                    simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
-                )?;
-
-                obligations.extend(o2);
-                Ok(InferOk { value, obligations })
-            }
-            _ => self.unify_and(a, b, identity),
-        }
-    }
-
-    fn coerce_closure_to_fn(
-        &self,
-        a: Ty<'tcx>,
-        closure_def_id_a: DefId,
-        substs_a: SubstsRef<'tcx>,
-        b: Ty<'tcx>,
-    ) -> CoerceResult<'tcx> {
-        //! Attempts to coerce from the type of a non-capturing closure
-        //! into a function pointer.
-        //!
-
-        let b = self.shallow_resolve(b);
-
-        match b.kind() {
-            // At this point we haven't done capture analysis, which means
-            // that the ClosureSubsts just contains an inference variable instead
-            // of tuple of captured types.
-            //
-            // All we care here is if any variable is being captured and not the exact paths,
-            // so we check `upvars_mentioned` for root variables being captured.
-            ty::FnPtr(fn_ty)
-                if self
-                    .tcx
-                    .upvars_mentioned(closure_def_id_a.expect_local())
-                    .map_or(true, |u| u.is_empty()) =>
-            {
-                // We coerce the closure, which has fn type
-                //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
-                // to
-                //     `fn(arg0,arg1,...) -> _`
-                // or
-                //     `unsafe fn(arg0,arg1,...) -> _`
-                let closure_sig = substs_a.as_closure().sig();
-                let unsafety = fn_ty.unsafety();
-                let pointer_ty =
-                    self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
-                debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
-                self.unify_and(
-                    pointer_ty,
-                    b,
-                    simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
-                )
-            }
-            _ => self.unify_and(a, b, identity),
-        }
-    }
-
-    fn coerce_unsafe_ptr(
-        &self,
-        a: Ty<'tcx>,
-        b: Ty<'tcx>,
-        mutbl_b: hir::Mutability,
-    ) -> CoerceResult<'tcx> {
-        debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
-
-        let (is_ref, mt_a) = match *a.kind() {
-            ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
-            ty::RawPtr(mt) => (false, mt),
-            _ => return self.unify_and(a, b, identity),
-        };
-        coerce_mutbls(mt_a.mutbl, mutbl_b)?;
-
-        // Check that the types which they point at are compatible.
-        let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
-        // Although references and unsafe ptrs have the same
-        // representation, we still register an Adjust::DerefRef so that
-        // regionck knows that the region for `a` must be valid here.
-        if is_ref {
-            self.unify_and(a_unsafe, b, |target| {
-                vec![
-                    Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
-                    Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
-                ]
-            })
-        } else if mt_a.mutbl != mutbl_b {
-            self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
-        } else {
-            self.unify_and(a_unsafe, b, identity)
-        }
-    }
-}
-
-impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
-    /// Attempt to coerce an expression to a type, and return the
-    /// adjusted type of the expression, if successful.
-    /// Adjustments are only recorded if the coercion succeeded.
-    /// The expressions *must not* have any pre-existing adjustments.
-    pub fn try_coerce(
-        &self,
-        expr: &hir::Expr<'_>,
-        expr_ty: Ty<'tcx>,
-        target: Ty<'tcx>,
-        allow_two_phase: AllowTwoPhase,
-        cause: Option<ObligationCause<'tcx>>,
-    ) -> RelateResult<'tcx, Ty<'tcx>> {
-        let source = self.resolve_vars_with_obligations(expr_ty);
-        debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
-
-        let cause =
-            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
-        let coerce = Coerce::new(self, cause, allow_two_phase);
-        let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
-
-        let (adjustments, _) = self.register_infer_ok_obligations(ok);
-        self.apply_adjustments(expr, adjustments);
-        Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
-    }
-
-    /// Same as `try_coerce()`, but without side-effects.
-    ///
-    /// Returns false if the coercion creates any obligations that result in
-    /// errors.
-    pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
-        let source = self.resolve_vars_with_obligations(expr_ty);
-        debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
-
-        let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
-        // We don't ever need two-phase here since we throw out the result of the coercion
-        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
-        self.probe(|_| {
-            let Ok(ok) = coerce.coerce(source, target) else {
-                return false;
-            };
-            let mut fcx = traits::FulfillmentContext::new_in_snapshot();
-            fcx.register_predicate_obligations(self, ok.obligations);
-            fcx.select_where_possible(&self).is_empty()
-        })
-    }
-
-    /// Given a type and a target type, this function will calculate and return
-    /// how many dereference steps needed to achieve `expr_ty <: target`. If
-    /// it's not possible, return `None`.
-    pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
-        let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
-        // We don't ever need two-phase here since we throw out the result of the coercion
-        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
-        coerce
-            .autoderef(rustc_span::DUMMY_SP, expr_ty)
-            .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
-    }
-
-    /// Given a type, this function will calculate and return the type given
-    /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
-    ///
-    /// This function is for diagnostics only, since it does not register
-    /// trait or region sub-obligations. (presumably we could, but it's not
-    /// particularly important for diagnostics...)
-    pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
-        self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
-            self.infcx
-                .type_implements_trait(
-                    self.tcx.lang_items().deref_mut_trait()?,
-                    expr_ty,
-                    ty::List::empty(),
-                    self.param_env,
-                )
-                .may_apply()
-                .then(|| deref_ty)
-        })
-    }
-
-    /// Given some expressions, their known unified type and another expression,
-    /// tries to unify the types, potentially inserting coercions on any of the
-    /// provided expressions and returns their LUB (aka "common supertype").
-    ///
-    /// This is really an internal helper. From outside the coercion
-    /// module, you should instantiate a `CoerceMany` instance.
-    fn try_find_coercion_lub<E>(
-        &self,
-        cause: &ObligationCause<'tcx>,
-        exprs: &[E],
-        prev_ty: Ty<'tcx>,
-        new: &hir::Expr<'_>,
-        new_ty: Ty<'tcx>,
-    ) -> RelateResult<'tcx, Ty<'tcx>>
-    where
-        E: AsCoercionSite,
-    {
-        let prev_ty = self.resolve_vars_with_obligations(prev_ty);
-        let new_ty = self.resolve_vars_with_obligations(new_ty);
-        debug!(
-            "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
-            prev_ty,
-            new_ty,
-            exprs.len()
-        );
-
-        // The following check fixes #88097, where the compiler erroneously
-        // attempted to coerce a closure type to itself via a function pointer.
-        if prev_ty == new_ty {
-            return Ok(prev_ty);
-        }
-
-        // Special-case that coercion alone cannot handle:
-        // Function items or non-capturing closures of differing IDs or InternalSubsts.
-        let (a_sig, b_sig) = {
-            #[allow(rustc::usage_of_ty_tykind)]
-            let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
-                if let &ty::Closure(closure_def_id, _substs) = ty {
-                    self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
-                } else {
-                    false
-                }
-            };
-            if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
-                (None, None)
-            } else {
-                match (prev_ty.kind(), new_ty.kind()) {
-                    (ty::FnDef(..), ty::FnDef(..)) => {
-                        // Don't reify if the function types have a LUB, i.e., they
-                        // are the same function and their parameters have a LUB.
-                        match self
-                            .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
-                        {
-                            // We have a LUB of prev_ty and new_ty, just return it.
-                            Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
-                            Err(_) => {
-                                (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
-                            }
-                        }
-                    }
-                    (ty::Closure(_, substs), ty::FnDef(..)) => {
-                        let b_sig = new_ty.fn_sig(self.tcx);
-                        let a_sig = self
-                            .tcx
-                            .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
-                        (Some(a_sig), Some(b_sig))
-                    }
-                    (ty::FnDef(..), ty::Closure(_, substs)) => {
-                        let a_sig = prev_ty.fn_sig(self.tcx);
-                        let b_sig = self
-                            .tcx
-                            .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
-                        (Some(a_sig), Some(b_sig))
-                    }
-                    (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
-                        Some(self.tcx.signature_unclosure(
-                            substs_a.as_closure().sig(),
-                            hir::Unsafety::Normal,
-                        )),
-                        Some(self.tcx.signature_unclosure(
-                            substs_b.as_closure().sig(),
-                            hir::Unsafety::Normal,
-                        )),
-                    ),
-                    _ => (None, None),
-                }
-            }
-        };
-        if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
-            // Intrinsics are not coercible to function pointers.
-            if a_sig.abi() == Abi::RustIntrinsic
-                || a_sig.abi() == Abi::PlatformIntrinsic
-                || b_sig.abi() == Abi::RustIntrinsic
-                || b_sig.abi() == Abi::PlatformIntrinsic
-            {
-                return Err(TypeError::IntrinsicCast);
-            }
-            // The signature must match.
-            let a_sig = self.normalize_associated_types_in(new.span, a_sig);
-            let b_sig = self.normalize_associated_types_in(new.span, b_sig);
-            let sig = self
-                .at(cause, self.param_env)
-                .trace(prev_ty, new_ty)
-                .lub(a_sig, b_sig)
-                .map(|ok| self.register_infer_ok_obligations(ok))?;
-
-            // Reify both sides and return the reified fn pointer type.
-            let fn_ptr = self.tcx.mk_fn_ptr(sig);
-            let prev_adjustment = match prev_ty.kind() {
-                ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
-                ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
-                _ => unreachable!(),
-            };
-            let next_adjustment = match new_ty.kind() {
-                ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
-                ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
-                _ => unreachable!(),
-            };
-            for expr in exprs.iter().map(|e| e.as_coercion_site()) {
-                self.apply_adjustments(
-                    expr,
-                    vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
-                );
-            }
-            self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
-            return Ok(fn_ptr);
-        }
-
-        // Configure a Coerce instance to compute the LUB.
-        // We don't allow two-phase borrows on any autorefs this creates since we
-        // probably aren't processing function arguments here and even if we were,
-        // they're going to get autorefed again anyway and we can apply 2-phase borrows
-        // at that time.
-        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
-        coerce.use_lub = true;
-
-        // First try to coerce the new expression to the type of the previous ones,
-        // but only if the new expression has no coercion already applied to it.
-        let mut first_error = None;
-        if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
-            let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
-            match result {
-                Ok(ok) => {
-                    let (adjustments, target) = self.register_infer_ok_obligations(ok);
-                    self.apply_adjustments(new, adjustments);
-                    debug!(
-                        "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
-                        new_ty, prev_ty, target
-                    );
-                    return Ok(target);
-                }
-                Err(e) => first_error = Some(e),
-            }
-        }
-
-        // Then try to coerce the previous expressions to the type of the new one.
-        // This requires ensuring there are no coercions applied to *any* of the
-        // previous expressions, other than noop reborrows (ignoring lifetimes).
-        for expr in exprs {
-            let expr = expr.as_coercion_site();
-            let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
-                &[
-                    Adjustment { kind: Adjust::Deref(_), .. },
-                    Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
-                ] => {
-                    match *self.node_ty(expr.hir_id).kind() {
-                        ty::Ref(_, _, mt_orig) => {
-                            let mutbl_adj: hir::Mutability = mutbl_adj.into();
-                            // Reborrow that we can safely ignore, because
-                            // the next adjustment can only be a Deref
-                            // which will be merged into it.
-                            mutbl_adj == mt_orig
-                        }
-                        _ => false,
-                    }
-                }
-                &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
-                _ => false,
-            };
-
-            if !noop {
-                debug!(
-                    "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
-                    expr,
-                );
-
-                return self
-                    .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
-                    .map(|ok| self.register_infer_ok_obligations(ok));
-            }
-        }
-
-        match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
-            Err(_) => {
-                // Avoid giving strange errors on failed attempts.
-                if let Some(e) = first_error {
-                    Err(e)
-                } else {
-                    self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
-                        .map(|ok| self.register_infer_ok_obligations(ok))
-                }
-            }
-            Ok(ok) => {
-                let (adjustments, target) = self.register_infer_ok_obligations(ok);
-                for expr in exprs {
-                    let expr = expr.as_coercion_site();
-                    self.apply_adjustments(expr, adjustments.clone());
-                }
-                debug!(
-                    "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
-                    prev_ty, new_ty, target
-                );
-                Ok(target)
-            }
-        }
-    }
-}
-
-/// CoerceMany encapsulates the pattern you should use when you have
-/// many expressions that are all getting coerced to a common
-/// type. This arises, for example, when you have a match (the result
-/// of each arm is coerced to a common type). It also arises in less
-/// obvious places, such as when you have many `break foo` expressions
-/// that target the same loop, or the various `return` expressions in
-/// a function.
-///
-/// The basic protocol is as follows:
-///
-/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
-///   This will also serve as the "starting LUB". The expectation is
-///   that this type is something which all of the expressions *must*
-///   be coercible to. Use a fresh type variable if needed.
-/// - For each expression whose result is to be coerced, invoke `coerce()` with.
-///   - In some cases we wish to coerce "non-expressions" whose types are implicitly
-///     unit. This happens for example if you have a `break` with no expression,
-///     or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
-///   - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
-///     from you so that you don't have to worry your pretty head about it.
-///     But if an error is reported, the final type will be `err`.
-///   - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
-///     previously coerced expressions.
-/// - When all done, invoke `complete()`. This will return the LUB of
-///   all your expressions.
-///   - WARNING: I don't believe this final type is guaranteed to be
-///     related to your initial `expected_ty` in any particular way,
-///     although it will typically be a subtype, so you should check it.
-///   - Invoking `complete()` may cause us to go and adjust the "adjustments" on
-///     previously coerced expressions.
-///
-/// Example:
-///
-/// ```ignore (illustrative)
-/// let mut coerce = CoerceMany::new(expected_ty);
-/// for expr in exprs {
-///     let expr_ty = fcx.check_expr_with_expectation(expr, expected);
-///     coerce.coerce(fcx, &cause, expr, expr_ty);
-/// }
-/// let final_ty = coerce.complete(fcx);
-/// ```
-pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
-    expected_ty: Ty<'tcx>,
-    final_ty: Option<Ty<'tcx>>,
-    expressions: Expressions<'tcx, 'exprs, E>,
-    pushed: usize,
-}
-
-/// The type of a `CoerceMany` that is storing up the expressions into
-/// a buffer. We use this in `check/mod.rs` for things like `break`.
-pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
-
-enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
-    Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
-    UpFront(&'exprs [E]),
-}
-
-impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
-    /// The usual case; collect the set of expressions dynamically.
-    /// If the full set of coercion sites is known before hand,
-    /// consider `with_coercion_sites()` instead to avoid allocation.
-    pub fn new(expected_ty: Ty<'tcx>) -> Self {
-        Self::make(expected_ty, Expressions::Dynamic(vec![]))
-    }
-
-    /// As an optimization, you can create a `CoerceMany` with a
-    /// pre-existing slice of expressions. In this case, you are
-    /// expected to pass each element in the slice to `coerce(...)` in
-    /// order. This is used with arrays in particular to avoid
-    /// needlessly cloning the slice.
-    pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
-        Self::make(expected_ty, Expressions::UpFront(coercion_sites))
-    }
-
-    fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
-        CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
-    }
-
-    /// Returns the "expected type" with which this coercion was
-    /// constructed. This represents the "downward propagated" type
-    /// that was given to us at the start of typing whatever construct
-    /// we are typing (e.g., the match expression).
-    ///
-    /// Typically, this is used as the expected type when
-    /// type-checking each of the alternative expressions whose types
-    /// we are trying to merge.
-    pub fn expected_ty(&self) -> Ty<'tcx> {
-        self.expected_ty
-    }
-
-    /// Returns the current "merged type", representing our best-guess
-    /// at the LUB of the expressions we've seen so far (if any). This
-    /// isn't *final* until you call `self.complete()`, which will return
-    /// the merged type.
-    pub fn merged_ty(&self) -> Ty<'tcx> {
-        self.final_ty.unwrap_or(self.expected_ty)
-    }
-
-    /// Indicates that the value generated by `expression`, which is
-    /// of type `expression_ty`, is one of the possibilities that we
-    /// could coerce from. This will record `expression`, and later
-    /// calls to `coerce` may come back and add adjustments and things
-    /// if necessary.
-    pub fn coerce<'a>(
-        &mut self,
-        fcx: &FnCtxt<'a, 'tcx>,
-        cause: &ObligationCause<'tcx>,
-        expression: &'tcx hir::Expr<'tcx>,
-        expression_ty: Ty<'tcx>,
-    ) {
-        self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
-    }
-
-    /// Indicates that one of the inputs is a "forced unit". This
-    /// occurs in a case like `if foo { ... };`, where the missing else
-    /// generates a "forced unit". Another example is a `loop { break;
-    /// }`, where the `break` has no argument expression. We treat
-    /// these cases slightly differently for error-reporting
-    /// purposes. Note that these tend to correspond to cases where
-    /// the `()` expression is implicit in the source, and hence we do
-    /// not take an expression argument.
-    ///
-    /// The `augment_error` gives you a chance to extend the error
-    /// message, in case any results (e.g., we use this to suggest
-    /// removing a `;`).
-    pub fn coerce_forced_unit<'a>(
-        &mut self,
-        fcx: &FnCtxt<'a, 'tcx>,
-        cause: &ObligationCause<'tcx>,
-        augment_error: &mut dyn FnMut(&mut Diagnostic),
-        label_unit_as_expected: bool,
-    ) {
-        self.coerce_inner(
-            fcx,
-            cause,
-            None,
-            fcx.tcx.mk_unit(),
-            Some(augment_error),
-            label_unit_as_expected,
-        )
-    }
-
-    /// The inner coercion "engine". If `expression` is `None`, this
-    /// is a forced-unit case, and hence `expression_ty` must be
-    /// `Nil`.
-    #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
-    pub(crate) fn coerce_inner<'a>(
-        &mut self,
-        fcx: &FnCtxt<'a, 'tcx>,
-        cause: &ObligationCause<'tcx>,
-        expression: Option<&'tcx hir::Expr<'tcx>>,
-        mut expression_ty: Ty<'tcx>,
-        augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
-        label_expression_as_expected: bool,
-    ) {
-        // Incorporate whatever type inference information we have
-        // until now; in principle we might also want to process
-        // pending obligations, but doing so should only improve
-        // compatibility (hopefully that is true) by helping us
-        // uncover never types better.
-        if expression_ty.is_ty_var() {
-            expression_ty = fcx.infcx.shallow_resolve(expression_ty);
-        }
-
-        // If we see any error types, just propagate that error
-        // upwards.
-        if expression_ty.references_error() || self.merged_ty().references_error() {
-            self.final_ty = Some(fcx.tcx.ty_error());
-            return;
-        }
-
-        // Handle the actual type unification etc.
-        let result = if let Some(expression) = expression {
-            if self.pushed == 0 {
-                // Special-case the first expression we are coercing.
-                // To be honest, I'm not entirely sure why we do this.
-                // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
-                fcx.try_coerce(
-                    expression,
-                    expression_ty,
-                    self.expected_ty,
-                    AllowTwoPhase::No,
-                    Some(cause.clone()),
-                )
-            } else {
-                match self.expressions {
-                    Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
-                        cause,
-                        exprs,
-                        self.merged_ty(),
-                        expression,
-                        expression_ty,
-                    ),
-                    Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
-                        cause,
-                        &coercion_sites[0..self.pushed],
-                        self.merged_ty(),
-                        expression,
-                        expression_ty,
-                    ),
-                }
-            }
-        } else {
-            // this is a hack for cases where we default to `()` because
-            // the expression etc has been omitted from the source. An
-            // example is an `if let` without an else:
-            //
-            //     if let Some(x) = ... { }
-            //
-            // we wind up with a second match arm that is like `_ =>
-            // ()`.  That is the case we are considering here. We take
-            // a different path to get the right "expected, found"
-            // message and so forth (and because we know that
-            // `expression_ty` will be unit).
-            //
-            // Another example is `break` with no argument expression.
-            assert!(expression_ty.is_unit(), "if let hack without unit type");
-            fcx.at(cause, fcx.param_env)
-                .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
-                .map(|infer_ok| {
-                    fcx.register_infer_ok_obligations(infer_ok);
-                    expression_ty
-                })
-        };
-
-        debug!(?result);
-        match result {
-            Ok(v) => {
-                self.final_ty = Some(v);
-                if let Some(e) = expression {
-                    match self.expressions {
-                        Expressions::Dynamic(ref mut buffer) => buffer.push(e),
-                        Expressions::UpFront(coercion_sites) => {
-                            // if the user gave us an array to validate, check that we got
-                            // the next expression in the list, as expected
-                            assert_eq!(
-                                coercion_sites[self.pushed].as_coercion_site().hir_id,
-                                e.hir_id
-                            );
-                        }
-                    }
-                    self.pushed += 1;
-                }
-            }
-            Err(coercion_error) => {
-                // 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.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.report_mismatched_types(cause, expected, found, ty_err);
-
-        let mut pointing_at_return_type = false;
-        let mut fn_output = None;
-
-        let parent_id = fcx.tcx.hir().get_parent_node(id);
-        let parent = fcx.tcx.hir().get(parent_id);
-        if let Some(expr) = expression
-            && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
-            && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
-        {
-            fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
-        }
-        // Verify that this is a tail expression of a function, otherwise the
-        // label pointing out the cause for the type coercion will be wrong
-        // as prior return coercions would not be relevant (#57664).
-        let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
-            pointing_at_return_type =
-                fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
-            if let (Some(cond_expr), true, false) = (
-                fcx.tcx.hir().get_if_cause(expr.hir_id),
-                expected.is_unit(),
-                pointing_at_return_type,
-            )
-                // If the block is from an external macro or try (`?`) desugaring, then
-                // do not suggest adding a semicolon, because there's nowhere to put it.
-                // See issues #81943 and #87051.
-                && matches!(
-                    cond_expr.span.desugaring_kind(),
-                    None | Some(DesugaringKind::WhileLoop)
-                ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
-                    && !matches!(
-                        cond_expr.kind,
-                        hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
-                    )
-            {
-                err.span_label(cond_expr.span, "expected this to be `()`");
-                if expr.can_have_side_effects() {
-                    fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
-                }
-            }
-            fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
-        } else {
-            fcx.get_fn_decl(parent_id)
-        };
-
-        if let Some((fn_decl, can_suggest)) = fn_decl {
-            if blk_id.is_none() {
-                pointing_at_return_type |= fcx.suggest_missing_return_type(
-                    &mut err,
-                    &fn_decl,
-                    expected,
-                    found,
-                    can_suggest,
-                    fcx.tcx.hir().local_def_id_to_hir_id(fcx.tcx.hir().get_parent_item(id)),
-                );
-            }
-            if !pointing_at_return_type {
-                fn_output = Some(&fn_decl.output); // `impl Trait` return type
-            }
-        }
-
-        let parent_id = fcx.tcx.hir().get_parent_item(id);
-        let parent_item = fcx.tcx.hir().get_by_def_id(parent_id);
-
-        if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
-            (expression, blk_id, fcx.get_node_fn_decl(parent_item))
-        {
-            fcx.suggest_missing_break_or_return_expr(
-                &mut err,
-                expr,
-                fn_decl,
-                expected,
-                found,
-                id,
-                fcx.tcx.hir().local_def_id_to_hir_id(parent_id),
-            );
-        }
-
-        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
-    }
-}