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

compiler/rustc_hir_typeck/src/coercion.rs

@@ -269,13 +269,18 @@ 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()
+                    && let Ok(pin_coerce) =
+                        self.commit_if_ok(|_| self.coerce_pin_ref_to_ref(a, b, mutbl_b))
+                {
+                    return Ok(pin_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 self.tcx.features().pin_ergonomics()
+                && let Some((_, ty::Pinnedness::Pinned, mut_b, _)) = b.maybe_pinned_ref() =>
             {
-                let pin_coerce = self.commit_if_ok(|_| self.coerce_to_pin_ref(a, b));
+                let pin_coerce = self.commit_if_ok(|_| self.coerce_to_pin_ref(a, b, mut_b));
                 if pin_coerce.is_ok() {
                     return pin_coerce;
                 }
@@ -790,61 +795,103 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         Ok(())
     }
 
-    /// Applies reborrowing for `Pin`
+    /// Create an obligation for `ty: Unpin`.
+    fn unpin_obligation(&self, ty: Ty<'tcx>) -> PredicateObligation<'tcx> {
+        let pred = ty::TraitRef::new(
+            self.tcx,
+            self.tcx.require_lang_item(hir::LangItem::Unpin, self.cause.span),
+            [ty],
+        );
+        PredicateObligation::new(self.tcx, self.cause.clone(), self.param_env, pred)
+    }
+
+    /// Coerces from a pinned reference to a normal reference.
+    #[instrument(skip(self), level = "trace")]
+    fn coerce_pin_ref_to_ref(
+        &self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        mut_b: hir::Mutability,
+    ) -> CoerceResult<'tcx> {
+        debug_assert!(self.shallow_resolve(a) == a);
+        debug_assert!(self.shallow_resolve(b) == b);
+        debug_assert!(self.tcx.features().pin_ergonomics());
+
+        let Some((a_ty, ty::Pinnedness::Pinned, mut_a, r_a)) = a.maybe_pinned_ref() else {
+            debug!("not fitting pinned ref to ref coercion (`{:?}` -> `{:?}`)", a, b);
+            return Err(TypeError::Mismatch);
+        };
+
+        coerce_mutbls(mut_a, mut_b)?;
+
+        let a = Ty::new_ref(self.tcx, r_a, a_ty, mut_b);
+        let mut coerce = self.unify_and(
+            a,
+            b,
+            [Adjustment { kind: Adjust::Deref(DerefAdjustKind::Pin), target: a_ty }],
+            Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::new(mut_b, self.allow_two_phase))),
+            ForceLeakCheck::No,
+        )?;
+        coerce.obligations.push(self.unpin_obligation(a_ty));
+        Ok(coerce)
+    }
+
+    /// Applies reborrowing and auto-borrowing that results to `Pin<&T>` or `Pin<&mut T>`:
     ///
-    /// We currently only support reborrowing `Pin<&mut T>` as `Pin<&mut T>`. This is accomplished
-    /// by inserting a call to `Pin::as_mut` during MIR building.
+    /// Currently we only support the following coercions:
+    /// - Reborrowing `Pin<&mut T>` -> `Pin<&mut T>`
+    /// - Reborrowing `Pin<&T>` -> `Pin<&T>`
+    /// - Auto-borrowing `&mut T` -> `Pin<&mut T>` where `T: Unpin`
+    /// - Auto-borrowing `&mut T` -> `Pin<&T>` where `T: Unpin`
+    /// - Auto-borrowing `&T` -> `Pin<&T>` where `T: Unpin`
     ///
     /// In the future we might want to support other reborrowing coercions, such as:
-    /// - `Pin<&mut T>` as `Pin<&T>`
-    /// - `Pin<&T>` as `Pin<&T>`
     /// - `Pin<Box<T>>` as `Pin<&T>`
     /// - `Pin<Box<T>>` as `Pin<&mut T>`
     #[instrument(skip(self), level = "trace")]
-    fn coerce_to_pin_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+    fn coerce_to_pin_ref(
+        &self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        mut_b: hir::Mutability,
+    ) -> CoerceResult<'tcx> {
         debug_assert!(self.shallow_resolve(a) == a);
         debug_assert!(self.shallow_resolve(b) == b);
+        debug_assert!(self.tcx.features().pin_ergonomics());
 
-        // We need to make sure the two types are compatible for coercion.
-        // Then we will build a ReborrowPin adjustment and return that as an InferOk.
-
-        // Right now we can only reborrow if this is a `Pin<&mut T>`.
-        let extract_pin_mut = |ty: Ty<'tcx>| {
-            // Get the T out of Pin<T>
-            let (pin, ty) = match ty.kind() {
-                ty::Adt(pin, args) if self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) => {
-                    (*pin, args[0].expect_ty())
-                }
-                _ => {
-                    debug!("can't reborrow {:?} as pinned", ty);
-                    return Err(TypeError::Mismatch);
-                }
-            };
-            // Make sure the T is something we understand (just `&mut U` for now)
-            match ty.kind() {
-                ty::Ref(region, ty, mutbl) => Ok((pin, *region, *ty, *mutbl)),
-                _ => {
-                    debug!("can't reborrow pin of inner type {:?}", ty);
-                    Err(TypeError::Mismatch)
-                }
+        // We need to deref the reference first before we reborrow it to a pinned reference.
+        let (deref, a_ty, mut_a, r_a, unpin_obligation) = match a.maybe_pinned_ref() {
+            // no `Unpin` required when reborrowing a pinned reference to a pinned reference
+            Some((a_ty, ty::Pinnedness::Pinned, mut_a, r_a)) => {
+                (DerefAdjustKind::Pin, a_ty, mut_a, r_a, None)
+            }
+            // `Unpin` required when reborrowing a non-pinned reference to a pinned reference
+            Some((a_ty, ty::Pinnedness::Not, mut_a, r_a)) => {
+                (DerefAdjustKind::Builtin, a_ty, mut_a, r_a, Some(self.unpin_obligation(a_ty)))
+            }
+            None => {
+                debug!("can't reborrow {:?} as pinned", a);
+                return Err(TypeError::Mismatch);
             }
         };
 
-        let (pin, a_region, a_ty, mut_a) = extract_pin_mut(a)?;
-        let (_, _, _b_ty, mut_b) = extract_pin_mut(b)?;
-
         coerce_mutbls(mut_a, mut_b)?;
 
         // update a with b's mutability since we'll be coercing mutability
-        let a = Ty::new_adt(
-            self.tcx,
-            pin,
-            self.tcx.mk_args(&[Ty::new_ref(self.tcx, a_region, a_ty, mut_b).into()]),
-        );
+        let a = Ty::new_pinned_ref(self.tcx, r_a, a_ty, mut_b);
 
         // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
         // add the adjustments.
-        self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b), ForceLeakCheck::No)
+        let mut coerce = self.unify_and(
+            a,
+            b,
+            [Adjustment { kind: Adjust::Deref(deref), target: a_ty }],
+            Adjust::Borrow(AutoBorrow::Pin(mut_b)),
+            ForceLeakCheck::No,
+        )?;
+
+        coerce.obligations.extend(unpin_obligation);
+        Ok(coerce)
     }
 
     fn coerce_from_fn_pointer(

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`
@@ -789,7 +789,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 );
             }
 
-            adjustment::AutoBorrow::RawPtr(m) => {
+            adjustment::AutoBorrow::RawPtr(m) | adjustment::AutoBorrow::Pin(m) => {
                 debug!("walk_autoref: expr.hir_id={} base_place={:?}", expr.hir_id, base_place);
 
                 self.delegate.borrow_mut().borrow(

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_middle/src/ty/sty.rs

@@ -1336,6 +1336,14 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Returns the type, pinnedness, mutability, and the region of a reference (`&T` or `&mut T`)
+    /// or a pinned-reference type (`Pin<&T>` or `Pin<&mut T>`).
+    ///
+    /// Regarding the [`pin_ergonomics`] feature, one of the goals is to make pinned references
+    /// (`Pin<&T>` and `Pin<&mut T>`) behaves similar to normal references (`&T` and `&mut T`).
+    /// This function is useful when references and pinned references are processed similarly.
+    ///
+    /// [`pin_ergonomics`]: https://github.com/rust-lang/rust/issues/130494
     pub fn maybe_pinned_ref(
         self,
     ) -> Option<(Ty<'tcx>, ty::Pinnedness, ty::Mutability, Region<'tcx>)> {

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 }`