Implement coercions between &pin (mut|const) T and &(mut) T when T: Unpin

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

@@ -6,6 +6,7 @@ use itertools::Itertools;
 use rustc_hir_analysis::autoderef::{Autoderef, AutoderefKind};
 use rustc_infer::infer::InferOk;
 use rustc_infer::traits::PredicateObligations;
+use rustc_middle::bug;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, DerefAdjustKind, OverloadedDeref};
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
@@ -62,6 +63,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         })
                         .unwrap_or(DerefAdjustKind::Builtin)
                 }
+                AutoderefKind::Pin => {
+                    bug!("Pin autoderef kind should not be present in the steps")
+                }
                 AutoderefKind::Builtin => DerefAdjustKind::Builtin,
             })
             .zip_eq(targets)

compiler/rustc_hir_typeck/src/coercion.rs

@@ -269,12 +269,21 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 return self.coerce_to_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_maybe_pinned_ref(a, b))
+                {
+                    return Ok(coerce);
+                }
                 return self.coerce_to_ref(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_maybe_pinned_ref(a, b);
+                }
                 let pin_coerce = self.commit_if_ok(|_| self.coerce_to_pin_ref(a, b));
                 if pin_coerce.is_ok() {
                     return pin_coerce;
@@ -847,6 +856,62 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b), ForceLeakCheck::No)
     }
 
+    /// 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_maybe_pinned_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),
+            ForceLeakCheck::No,
+        )?;
+
+        // 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_fn_pointer(
         &self,
         a: Ty<'tcx>,

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -734,7 +734,7 @@ 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(DerefAdjustKind::Builtin) => {}
+                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`
@@ -798,6 +798,16 @@ 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),
+                );
+            }
         }
     }
 

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -279,6 +279,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 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_lint/src/autorefs.rs

@@ -174,7 +174,7 @@ fn has_implicit_borrow(Adjustment { kind, .. }: &Adjustment<'_>) -> Option<(Muta
         Adjust::NeverToAny
         | Adjust::Pointer(..)
         | Adjust::ReborrowPin(..)
-        | Adjust::Deref(DerefAdjustKind::Builtin)
-        | Adjust::Borrow(AutoBorrow::RawPtr(..)) => None,
+        | Adjust::Deref(DerefAdjustKind::Builtin | DerefAdjustKind::Pin)
+        | Adjust::Borrow(AutoBorrow::RawPtr(..) | AutoBorrow::Pin(..)) => None,
     }
 }

compiler/rustc_middle/src/ty/adjustment.rs

@@ -105,13 +105,15 @@ pub enum Adjust {
     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)`
@@ -196,6 +198,9 @@ 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

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -144,6 +144,19 @@ impl<'tcx> ThirBuildCx<'tcx> {
                 adjust_span(&mut expr);
                 ExprKind::Deref { arg: self.thir.exprs.push(expr) }
             }
+            Adjust::Deref(DerefAdjustKind::Pin) => {
+                adjust_span(&mut expr);
+                // pointer = ($expr).pointer
+                let pin_ty = expr.ty.pinned_ty().expect("Deref(Pin) with non-Pin type");
+                let pointer_target = ExprKind::Field {
+                    lhs: self.thir.exprs.push(expr),
+                    variant_index: FIRST_VARIANT,
+                    name: FieldIdx::ZERO,
+                };
+                let expr = Expr { temp_scope_id, ty: pin_ty, span, kind: pointer_target };
+                // expr = *pointer
+                ExprKind::Deref { arg: self.thir.exprs.push(expr) }
+            }
             Adjust::Deref(DerefAdjustKind::Overloaded(deref)) => {
                 // We don't need to do call adjust_span here since
                 // deref coercions always start with a built-in deref.
@@ -178,6 +191,37 @@ impl<'tcx> ThirBuildCx<'tcx> {
             Adjust::Borrow(AutoBorrow::RawPtr(mutability)) => {
                 ExprKind::RawBorrow { mutability, arg: self.thir.exprs.push(expr) }
             }
+            Adjust::Borrow(AutoBorrow::Pin(mutbl)) => {
+                // expr = &pin (mut|const|) arget
+                let borrow_kind = match mutbl {
+                    hir::Mutability::Mut => BorrowKind::Mut { kind: mir::MutBorrowKind::Default },
+                    hir::Mutability::Not => BorrowKind::Shared,
+                };
+                let new_pin_target =
+                    Ty::new_ref(self.tcx, self.tcx.lifetimes.re_erased, expr.ty, mutbl);
+                let arg = self.thir.exprs.push(expr);
+                let expr = self.thir.exprs.push(Expr {
+                    temp_scope_id,
+                    ty: new_pin_target,
+                    span,
+                    kind: ExprKind::Borrow { borrow_kind, arg },
+                });
+
+                // kind = Pin { pointer }
+                let pin_did = self.tcx.require_lang_item(rustc_hir::LangItem::Pin, span);
+                let args = self.tcx.mk_args(&[new_pin_target.into()]);
+                let kind = ExprKind::Adt(Box::new(AdtExpr {
+                    adt_def: self.tcx.adt_def(pin_did),
+                    variant_index: FIRST_VARIANT,
+                    args,
+                    fields: Box::new([FieldExpr { name: FieldIdx::ZERO, expr }]),
+                    user_ty: None,
+                    base: AdtExprBase::None,
+                }));
+
+                debug!(?kind);
+                kind
+            }
             Adjust::ReborrowPin(mutbl) => {
                 debug!("apply ReborrowPin adjustment");
                 // Rewrite `$expr` as `Pin { __pointer: &(mut)? *($expr).__pointer }`