Support coercion between references and pinned references

Author: frank-king

compiler/rustc_hir_analysis/src/autoderef.rs

@@ -17,6 +17,8 @@ pub enum AutoderefKind {
     Builtin,
     /// A type which must dispatch to a `Deref` implementation.
     Overloaded,
+    /// A pinned reference type, such as `Pin<&T>` and `Pin<&mut T>`.
+    Pin,
 }
 struct AutoderefSnapshot<'tcx> {
     at_start: bool,

compiler/rustc_hir_typeck/src/autoderef.rs

@@ -6,7 +6,8 @@ use itertools::Itertools;
 use rustc_hir_analysis::autoderef::{Autoderef, AutoderefKind};
 use rustc_infer::infer::InferOk;
 use rustc_infer::traits::PredicateObligations;
-use rustc_middle::ty::adjustment::{Adjust, Adjustment, OverloadedDeref};
+use rustc_middle::bug;
+use rustc_middle::ty::adjustment::{Adjust, Adjustment, DerefAdjustKind, OverloadedDeref};
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 
@@ -45,22 +46,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             steps.iter().skip(1).map(|&(ty, _)| ty).chain(iter::once(autoderef.final_ty()));
         let steps: Vec<_> = steps
             .iter()
-            .map(|&(source, kind)| {
-                if let AutoderefKind::Overloaded = kind {
-                    self.try_overloaded_deref(autoderef.span(), source).and_then(
-                        |InferOk { value: method, obligations: o }| {
+            .map(|&(source, kind)| match kind {
+                AutoderefKind::Overloaded => {
+                    self.try_overloaded_deref(autoderef.span(), source)
+                        .and_then(|InferOk { value: method, obligations: o }| {
                             obligations.extend(o);
                             // FIXME: we should assert the sig is &T here... there's no reason for this to be fallible.
                             if let ty::Ref(_, _, mutbl) = *method.sig.output().kind() {
-                                Some(OverloadedDeref { mutbl, span: autoderef.span() })
+                                Some(DerefAdjustKind::Overloaded(OverloadedDeref {
+                                    mutbl,
+                                    span: autoderef.span(),
+                                }))
                             } else {
                                 None
                             }
-                        },
-                    )
-                } else {
-                    None
+                        })
+                        .unwrap_or(DerefAdjustKind::Builtin)
                 }
+                AutoderefKind::Pin => {
+                    bug!("Pin autoderef kind should not be present in the steps")
+                }
+                AutoderefKind::Builtin => DerefAdjustKind::Builtin,
             })
             .zip_eq(targets)
             .map(|(autoderef, target)| Adjustment { kind: Adjust::Deref(autoderef), target })

compiler/rustc_hir_typeck/src/coercion.rs

@@ -50,7 +50,8 @@ use rustc_infer::traits::{
 };
 use rustc_middle::span_bug;
 use rustc_middle::ty::adjustment::{
-    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCoercion,
+    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, DerefAdjustKind,
+    PointerCoercion,
 };
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt};
@@ -231,12 +232,21 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 return self.coerce_raw_ptr(a, b, b_mutbl);
             }
             ty::Ref(r_b, _, mutbl_b) => {
+                if self.tcx.features().pin_ergonomics()
+                    && a.pinned_ty().is_some_and(|ty| ty.is_ref())
+                    && let Ok(coerce) = self.commit_if_ok(|_| self.coerce_pin_or_ref(a, b))
+                {
+                    return Ok(coerce);
+                }
                 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
             }
             ty::Adt(pin, _)
                 if self.tcx.features().pin_ergonomics()
                     && self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) =>
             {
+                if a.is_ref() && b.pinned_ty().is_some_and(|ty| ty.is_ref()) {
+                    return self.coerce_pin_or_ref(a, b);
+                }
                 let pin_coerce = self.commit_if_ok(|_| self.coerce_pin_ref(a, b));
                 if pin_coerce.is_ok() {
                     return pin_coerce;
@@ -606,7 +616,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);
 
                 Some((
-                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
+                    Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: ty_a },
                     Adjustment {
                         kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
                         target: Ty::new_ref(self.tcx, r_borrow, ty_a, mutbl_b),
@@ -617,7 +627,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 coerce_mutbls(mt_a, mt_b)?;
 
                 Some((
-                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
+                    Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: ty_a },
                     Adjustment {
                         kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
                         target: Ty::new_ptr(self.tcx, ty_a, mt_b),
@@ -856,6 +866,61 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b))
     }
 
+    /// Coerce pinned reference to regular reference or vice versa
+    ///
+    /// - `Pin<&mut T>` <-> `&mut T` when `T: Unpin`
+    /// - `Pin<&T>` <-> `&T` when `T: Unpin`
+    /// - `Pin<&mut T>` <-> `Pin<&T>` when `T: Unpin`
+    #[instrument(skip(self), level = "trace")]
+    fn coerce_pin_or_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        let span = self.cause.span;
+        let Some((a_ty, a_pinnedness, a_mutbl, a_region)) = a.maybe_pinned_ref() else {
+            span_bug!(span, "expect pinned reference or reference, found {:?}", a);
+        };
+        let Some((_b_ty, b_pinnedness, b_mutbl, _b_region)) = b.maybe_pinned_ref() else {
+            span_bug!(span, "expect pinned reference or reference, found {:?}", b);
+        };
+        use ty::Pinnedness::*;
+        if a_pinnedness == b_pinnedness {
+            span_bug!(span, "expect different pinnedness, found {:?} and {:?}", a, b);
+        }
+
+        coerce_mutbls(a_mutbl, b_mutbl)?;
+
+        let (deref, borrow) = match (a_pinnedness, b_pinnedness) {
+            (Not, Not) | (Pinned, Pinned) => {
+                span_bug!(span, "expect different pinnedness, found {:?} and {:?}", a, b)
+            }
+            (Pinned, Not) => {
+                let mutbl = AutoBorrowMutability::new(b_mutbl, AllowTwoPhase::Yes);
+                (DerefAdjustKind::Pin, AutoBorrow::Ref(mutbl))
+            }
+            (Not, Pinned) => (DerefAdjustKind::Builtin, AutoBorrow::Pin(b_mutbl)),
+        };
+        let mut coerce = self.unify_and(
+            // update a with b's pinnedness and mutability since we'll be coercing pinnedness and mutability
+            match b_pinnedness {
+                Pinned => Ty::new_pinned_ref(self.tcx, a_region, a_ty, b_mutbl),
+                Not => Ty::new_ref(self.tcx, a_region, a_ty, b_mutbl),
+            },
+            b,
+            [Adjustment { kind: Adjust::Deref(deref), target: a_ty }],
+            Adjust::Borrow(borrow),
+        )?;
+
+        // Create an obligation for `a_ty: Unpin`.
+        let cause =
+            self.cause(self.cause.span, ObligationCauseCode::Coercion { source: a, target: b });
+        let pred = ty::TraitRef::new(
+            self.tcx,
+            self.tcx.require_lang_item(hir::LangItem::Unpin, self.cause.span),
+            [a_ty],
+        );
+        let obligation = Obligation::new(self.tcx, cause, self.fcx.param_env, pred);
+        coerce.obligations.push(obligation);
+        Ok(coerce)
+    }
+
     fn coerce_from_safe_fn(
         &self,
         fn_ty_a: ty::PolyFnSig<'tcx>,
@@ -1039,7 +1104,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             self.unify_and(
                 a_raw,
                 b,
-                [Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }],
+                [Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: mt_a.ty }],
                 Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
             )
         } else if mt_a.mutbl != mutbl_b {

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -23,6 +23,7 @@ use rustc_middle::hir::place::ProjectionKind;
 // Export these here so that Clippy can use them.
 pub use rustc_middle::hir::place::{Place, PlaceBase, PlaceWithHirId, Projection};
 use rustc_middle::mir::FakeReadCause;
+use rustc_middle::ty::adjustment::DerefAdjustKind;
 use rustc_middle::ty::{
     self, BorrowKind, Ty, TyCtxt, TypeFoldable, TypeVisitableExt as _, adjustment,
 };
@@ -829,14 +830,14 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                     self.consume_or_copy(&place_with_id, place_with_id.hir_id);
                 }
 
-                adjustment::Adjust::Deref(None) => {}
+                adjustment::Adjust::Deref(DerefAdjustKind::Builtin | DerefAdjustKind::Pin) => {}
 
                 // Autoderefs for overloaded Deref calls in fact reference
                 // their receiver. That is, if we have `(*x)` where `x`
                 // is of type `Rc<T>`, then this in fact is equivalent to
                 // `x.deref()`. Since `deref()` is declared with `&self`,
                 // this is an autoref of `x`.
-                adjustment::Adjust::Deref(Some(ref deref)) => {
+                adjustment::Adjust::Deref(DerefAdjustKind::Overloaded(deref)) => {
                     let bk = ty::BorrowKind::from_mutbl(deref.mutbl);
                     self.delegate.borrow_mut().borrow(&place_with_id, place_with_id.hir_id, bk);
                 }
@@ -893,6 +894,16 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                     ty::BorrowKind::from_mutbl(m),
                 );
             }
+
+            adjustment::AutoBorrow::Pin(m) => {
+                debug!("walk_autoref: expr.hir_id={} base_place={:?}", expr.hir_id, base_place);
+
+                self.delegate.borrow_mut().borrow(
+                    base_place,
+                    base_place.hir_id,
+                    ty::BorrowKind::from_mutbl(m),
+                );
+            }
         }
     }
 
@@ -1325,9 +1336,9 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
     {
         let target = self.cx.resolve_vars_if_possible(adjustment.target);
         match adjustment.kind {
-            adjustment::Adjust::Deref(overloaded) => {
+            adjustment::Adjust::Deref(deref_kind) => {
                 // Equivalent to *expr or something similar.
-                let base = if let Some(deref) = overloaded {
+                let base = if let DerefAdjustKind::Overloaded(deref) = deref_kind {
                     let ref_ty = Ty::new_ref(
                         self.cx.tcx(),
                         self.cx.tcx().lifetimes.re_erased,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -20,7 +20,9 @@ use rustc_hir_analysis::hir_ty_lowering::{
 use rustc_infer::infer::canonical::{Canonical, OriginalQueryValues, QueryResponse};
 use rustc_infer::infer::{DefineOpaqueTypes, InferResult};
 use rustc_lint::builtin::SELF_CONSTRUCTOR_FROM_OUTER_ITEM;
-use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
+use rustc_middle::ty::adjustment::{
+    Adjust, Adjustment, AutoBorrow, AutoBorrowMutability, DerefAdjustKind,
+};
 use rustc_middle::ty::{
     self, AdtKind, CanonicalUserType, GenericArgsRef, GenericParamDefKind, IsIdentity,
     SizedTraitKind, Ty, TyCtxt, TypeFoldable, TypeVisitable, TypeVisitableExt, UserArgs,
@@ -266,17 +268,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         debug!("apply_adjustments: adding `{:?}` as diverging type var", a.target);
                     }
                 }
-                Adjust::Deref(Some(overloaded_deref)) => {
+                Adjust::Deref(DerefAdjustKind::Overloaded(overloaded_deref)) => {
                     self.enforce_context_effects(
                         None,
                         expr.span,
                         overloaded_deref.method_call(self.tcx),
                         self.tcx.mk_args(&[expr_ty.into()]),
                     );
                 }
-                Adjust::Deref(None) => {
+                Adjust::Deref(DerefAdjustKind::Builtin) => {
                     // FIXME(const_trait_impl): We *could* enforce `&T: [const] Deref` here.
                 }
+                Adjust::Deref(DerefAdjustKind::Pin) => {
+                    // FIXME(const_trait_impl): We *could* enforce `Pin<&T>: [const] Deref` here.
+                }
                 Adjust::Pointer(_pointer_coercion) => {
                     // FIXME(const_trait_impl): We should probably enforce these.
                 }

compiler/rustc_hir_typeck/src/pat.rs

@@ -2857,7 +2857,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // the bad interactions of the given hack detailed in (note_1).
                 debug!("check_pat_ref: expected={:?}", expected);
                 match expected.maybe_pinned_ref() {
-                    Some((r_ty, r_pinned, r_mutbl))
+                    Some((r_ty, r_pinned, r_mutbl, _))
                         if ((ref_pat_matches_mut_ref && r_mutbl >= pat_mutbl)
                             || r_mutbl == pat_mutbl)
                             && pat_pinned == r_pinned =>

compiler/rustc_hir_typeck/src/place_op.rs

@@ -4,8 +4,8 @@ use rustc_infer::infer::InferOk;
 use rustc_infer::traits::{Obligation, ObligationCauseCode};
 use rustc_middle::span_bug;
 use rustc_middle::ty::adjustment::{
-    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, OverloadedDeref,
-    PointerCoercion,
+    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, DerefAdjustKind,
+    OverloadedDeref, PointerCoercion,
 };
 use rustc_middle::ty::{self, Ty};
 use rustc_span::{Span, sym};
@@ -298,7 +298,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.typeck_results.borrow_mut().adjustments_mut().remove(expr.hir_id);
             if let Some(mut adjustments) = previous_adjustments {
                 for adjustment in &mut adjustments {
-                    if let Adjust::Deref(Some(ref mut deref)) = adjustment.kind
+                    if let Adjust::Deref(DerefAdjustKind::Overloaded(ref mut deref)) =
+                        adjustment.kind
                         && let Some(ok) = self.try_mutable_overloaded_place_op(
                             expr.span,
                             source,

compiler/rustc_lint/src/autorefs.rs

@@ -1,6 +1,8 @@
 use rustc_ast::{BorrowKind, UnOp};
 use rustc_hir::{Expr, ExprKind, Mutability};
-use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, OverloadedDeref};
+use rustc_middle::ty::adjustment::{
+    Adjust, Adjustment, AutoBorrow, DerefAdjustKind, OverloadedDeref,
+};
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::sym;
 
@@ -165,12 +167,14 @@ fn peel_derefs_adjustments<'a>(mut adjs: &'a [Adjustment<'a>]) -> &'a [Adjustmen
 /// an implicit borrow (or has an implicit borrow via an overloaded deref).
 fn has_implicit_borrow(Adjustment { kind, .. }: &Adjustment<'_>) -> Option<(Mutability, bool)> {
     match kind {
-        &Adjust::Deref(Some(OverloadedDeref { mutbl, .. })) => Some((mutbl, true)),
+        &Adjust::Deref(DerefAdjustKind::Overloaded(OverloadedDeref { mutbl, .. })) => {
+            Some((mutbl, true))
+        }
         &Adjust::Borrow(AutoBorrow::Ref(mutbl)) => Some((mutbl.into(), false)),
         Adjust::NeverToAny
         | Adjust::Pointer(..)
         | Adjust::ReborrowPin(..)
-        | Adjust::Deref(None)
-        | Adjust::Borrow(AutoBorrow::RawPtr(..)) => None,
+        | Adjust::Deref(DerefAdjustKind::Builtin | DerefAdjustKind::Pin)
+        | Adjust::Borrow(AutoBorrow::RawPtr(..) | AutoBorrow::Pin(..)) => None,
     }
 }

compiler/rustc_lint/src/noop_method_call.rs

@@ -1,7 +1,7 @@
 use rustc_hir::def::DefKind;
 use rustc_hir::{Expr, ExprKind};
 use rustc_middle::ty;
-use rustc_middle::ty::adjustment::Adjust;
+use rustc_middle::ty::adjustment::{Adjust, DerefAdjustKind};
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::sym;
 
@@ -114,7 +114,10 @@ impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
 
         // If there is any user defined auto-deref step, then we don't want to warn.
         // https://github.com/rust-lang/rust-clippy/issues/9272
-        if arg_adjustments.iter().any(|adj| matches!(adj.kind, Adjust::Deref(Some(_)))) {
+        if arg_adjustments
+            .iter()
+            .any(|adj| matches!(adj.kind, Adjust::Deref(DerefAdjustKind::Overloaded(_))))
+        {
             return;
         }
 

compiler/rustc_middle/src/ty/adjustment.rs

@@ -96,17 +96,25 @@ pub enum Adjust {
     NeverToAny,
 
     /// Dereference once, producing a place.
-    Deref(Option<OverloadedDeref>),
+    Deref(DerefAdjustKind),
 
     /// Take the address and produce either a `&` or `*` pointer.
     Borrow(AutoBorrow),
 
     Pointer(PointerCoercion),
 
     /// Take a pinned reference and reborrow as a `Pin<&mut T>` or `Pin<&T>`.
+    // FIXME(pin_ergonomics): This can be replaced with a `Deref(Pin)` followed by a `Borrow(Pin)`
     ReborrowPin(hir::Mutability),
 }
 
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub enum DerefAdjustKind {
+    Builtin,
+    Overloaded(OverloadedDeref),
+    Pin,
+}
+
 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`
 /// call, with the signature `&'a T -> &'a U` or `&'a mut T -> &'a mut U`.
 /// The target type is `U` in both cases, with the region and mutability
@@ -189,6 +197,9 @@ pub enum AutoBorrow {
 
     /// Converts from T to *T.
     RawPtr(hir::Mutability),
+
+    /// Converts from T to Pin<&T>.
+    Pin(hir::Mutability),
 }
 
 /// Information for `CoerceUnsized` impls, storing information we