Rollup merge of rust-lang#149130 - frank-king:feature/pin-coerce, r=jackh726

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

This allows the following (mutual) coercions when `T: Unpin`:
- `&T` <-> `Pin<&T>`
- `&mut T` <-> `Pin<&mut T>`
- `&mut T` -> `Pin<&T>`
- `Pin<&mut T>` -> `&T`

Part of [Pin Ergonomics](rust-lang#130494).

Author: JonathanBrouwer

compiler/rustc_hir_typeck/src/coercion.rs

@@ -114,6 +114,26 @@ fn success<'tcx>(
     Ok(InferOk { value: (adj, target), obligations })
 }
 
+/// Data extracted from a reference (pinned or not) for coercion to a reference (pinned or not).
+struct CoerceMaybePinnedRef<'tcx> {
+    /// coercion source, must be a pinned (i.e. `Pin<&T>` or `Pin<&mut T>`) or normal reference (`&T` or `&mut T`)
+    a: Ty<'tcx>,
+    /// coercion target, must be a pinned (i.e. `Pin<&T>` or `Pin<&mut T>`) or normal reference (`&T` or `&mut T`)
+    b: Ty<'tcx>,
+    /// referent type of the source
+    a_ty: Ty<'tcx>,
+    /// pinnedness of the source
+    a_pin: ty::Pinnedness,
+    /// mutability of the source
+    a_mut: ty::Mutability,
+    /// region of the source
+    a_r: ty::Region<'tcx>,
+    /// pinnedness of the target
+    b_pin: ty::Pinnedness,
+    /// mutability of the target
+    b_mut: ty::Mutability,
+}
+
 /// Whether to force a leak check to occur in `Coerce::unify_raw`.
 /// Note that leak checks may still occur evn with `ForceLeakCheck::No`.
 ///
@@ -269,16 +289,13 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 return self.coerce_to_raw_ptr(a, b, b_mutbl);
             }
             ty::Ref(r_b, _, mutbl_b) => {
+                if let Some(pin_ref_to_ref) = self.maybe_pin_ref_to_ref(a, b) {
+                    return self.coerce_pin_ref_to_ref(pin_ref_to_ref);
+                }
                 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) =>
-            {
-                let pin_coerce = self.commit_if_ok(|_| self.coerce_to_pin_ref(a, b));
-                if pin_coerce.is_ok() {
-                    return pin_coerce;
-                }
+            _ if let Some(to_pin_ref) = self.maybe_to_pin_ref(a, b) => {
+                return self.coerce_to_pin_ref(to_pin_ref);
             }
             _ => {}
         }
@@ -790,61 +807,131 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         Ok(())
     }
 
-    /// Applies reborrowing for `Pin`
+    /// Create an obligation for `ty: Unpin`, where .
+    fn unpin_obligation(
+        &self,
+        source: Ty<'tcx>,
+        target: Ty<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> PredicateObligation<'tcx> {
+        let pred = ty::TraitRef::new(
+            self.tcx,
+            self.tcx.require_lang_item(hir::LangItem::Unpin, self.cause.span),
+            [ty],
+        );
+        let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
+        PredicateObligation::new(self.tcx, cause, self.param_env, pred)
+    }
+
+    /// Checks if the given types are compatible for coercion from a pinned reference to a normal reference.
+    fn maybe_pin_ref_to_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> Option<CoerceMaybePinnedRef<'tcx>> {
+        if !self.tcx.features().pin_ergonomics() {
+            return None;
+        }
+        if let Some((a_ty, a_pin @ ty::Pinnedness::Pinned, a_mut, a_r)) = a.maybe_pinned_ref()
+            && let Some((_, b_pin @ ty::Pinnedness::Not, b_mut, _)) = b.maybe_pinned_ref()
+        {
+            return Some(CoerceMaybePinnedRef { a, b, a_ty, a_pin, a_mut, a_r, b_pin, b_mut });
+        }
+        debug!("not fitting pinned ref to ref coercion (`{:?}` -> `{:?}`)", a, b);
+        None
+    }
+
+    /// Coerces from a pinned reference to a normal reference.
+    #[instrument(skip(self), level = "trace")]
+    fn coerce_pin_ref_to_ref(
+        &self,
+        CoerceMaybePinnedRef { a, b, a_ty, a_pin, a_mut, a_r, b_pin, b_mut }: CoerceMaybePinnedRef<
+            'tcx,
+        >,
+    ) -> CoerceResult<'tcx> {
+        debug_assert!(self.shallow_resolve(a) == a);
+        debug_assert!(self.shallow_resolve(b) == b);
+        debug_assert!(self.tcx.features().pin_ergonomics());
+        debug_assert_eq!(a_pin, ty::Pinnedness::Pinned);
+        debug_assert_eq!(b_pin, ty::Pinnedness::Not);
+
+        coerce_mutbls(a_mut, b_mut)?;
+
+        let unpin_obligation = self.unpin_obligation(a, b, a_ty);
+
+        let a = Ty::new_ref(self.tcx, a_r, a_ty, b_mut);
+        let mut coerce = self.unify_and(
+            a,
+            b,
+            [Adjustment { kind: Adjust::Deref(DerefAdjustKind::Pin), target: a_ty }],
+            Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::new(b_mut, self.allow_two_phase))),
+            ForceLeakCheck::No,
+        )?;
+        coerce.obligations.push(unpin_obligation);
+        Ok(coerce)
+    }
+
+    /// Checks if the given types are compatible for coercion to a pinned reference.
+    fn maybe_to_pin_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> Option<CoerceMaybePinnedRef<'tcx>> {
+        if !self.tcx.features().pin_ergonomics() {
+            return None;
+        }
+        if let Some((a_ty, a_pin, a_mut, a_r)) = a.maybe_pinned_ref()
+            && let Some((_, b_pin @ ty::Pinnedness::Pinned, b_mut, _)) = b.maybe_pinned_ref()
+        {
+            return Some(CoerceMaybePinnedRef { a, b, a_ty, a_pin, a_mut, a_r, b_pin, b_mut });
+        }
+        debug!("not fitting ref to pinned ref coercion (`{:?}` -> `{:?}`)", a, b);
+        None
+    }
+
+    /// 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,
+        CoerceMaybePinnedRef { a, b, a_ty, a_pin, a_mut, a_r, b_pin, b_mut }: CoerceMaybePinnedRef<
+            'tcx,
+        >,
+    ) -> CoerceResult<'tcx> {
         debug_assert!(self.shallow_resolve(a) == a);
         debug_assert!(self.shallow_resolve(b) == b);
-
-        // 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)
-                }
+        debug_assert!(self.tcx.features().pin_ergonomics());
+        debug_assert_eq!(b_pin, ty::Pinnedness::Pinned);
+
+        // We need to deref the reference first before we reborrow it to a pinned reference.
+        let (deref, unpin_obligation) = match a_pin {
+            // no `Unpin` required when reborrowing a pinned reference to a pinned reference
+            ty::Pinnedness::Pinned => (DerefAdjustKind::Pin, None),
+            // `Unpin` required when reborrowing a non-pinned reference to a pinned reference
+            ty::Pinnedness::Not => {
+                (DerefAdjustKind::Builtin, Some(self.unpin_obligation(a, b, a_ty)))
             }
         };
 
-        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)?;
+        coerce_mutbls(a_mut, b_mut)?;
 
         // 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, a_r, a_ty, b_mut);
 
         // 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(b_mut)),
+            ForceLeakCheck::No,
+        )?;
+
+        coerce.obligations.extend(unpin_obligation);
+        Ok(coerce)
     }
 
     fn coerce_from_fn_pointer(

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -736,7 +736,7 @@ 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`
@@ -791,7 +791,7 @@ 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

@@ -331,6 +331,9 @@ 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

@@ -173,7 +173,7 @@ 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

@@ -1365,6 +1365,14 @@ 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

@@ -143,6 +143,19 @@ 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.
@@ -177,6 +190,37 @@ 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 }`