Begin experimental support for pin reborrowing

compiler/rustc_borrowck/messages.ftl

@@ -207,7 +207,7 @@ borrowck_simd_intrinsic_arg_const =
         *[other] {$arg}th
     } argument of `{$intrinsic}` is required to be a `const` item
 
-borrowck_suggest_create_freash_reborrow =
+borrowck_suggest_create_fresh_reborrow =
     consider reborrowing the `Pin` instead of moving it
 
 borrowck_suggest_iterate_over_slice =

compiler/rustc_borrowck/src/session_diagnostics.rs

@@ -415,7 +415,7 @@ pub(crate) enum CaptureReasonSuggest<'tcx> {
         span: Span,
     },
     #[suggestion(
-        borrowck_suggest_create_freash_reborrow,
+        borrowck_suggest_create_fresh_reborrow,
         applicability = "maybe-incorrect",
         code = ".as_mut()",
         style = "verbose"

compiler/rustc_feature/src/unstable.rs

@@ -558,6 +558,8 @@ declare_features! (
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows specifying nop padding on functions for dynamic patching.
     (unstable, patchable_function_entry, "1.81.0", Some(123115)),
+    /// Experimental features that make `Pin` more ergonomic.
+    (incomplete, pin_ergonomics, "CURRENT_RUSTC_VERSION", Some(130494)),
     /// Allows postfix match `expr.match { ... }`
     (unstable, postfix_match, "1.79.0", Some(121618)),
     /// Allows macro attributes on expressions, statements and non-inline modules.

compiler/rustc_hir_typeck/src/coercion.rs

@@ -214,6 +214,12 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
                 return self.coerce_dyn_star(a, b, predicates, region);
             }
+            ty::Adt(pin, _)
+                if self.tcx.features().pin_ergonomics
+                    && self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) =>
+            {
+                return self.coerce_pin(a, b);
+            }
             _ => {}
         }
 
@@ -774,6 +780,62 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         })
     }
 
+    /// Applies reborrowing for `Pin`
+    ///
+    /// 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.
+    ///
+    /// 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_pin(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        // 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)
+                }
+            }
+        };
+
+        let (pin, a_region, a_ty, mut_a) = extract_pin_mut(a)?;
+        let (_, b_region, _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()]),
+        );
+
+        // 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, |_inner_ty| {
+            vec![Adjustment { kind: Adjust::ReborrowPin(b_region, mut_b), target: b }]
+        })
+    }
+
     fn coerce_from_safe_fn<F, G>(
         &self,
         a: Ty<'tcx>,

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -780,6 +780,16 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 adjustment::Adjust::Borrow(ref autoref) => {
                     self.walk_autoref(expr, &place_with_id, autoref);
                 }
+
+                adjustment::Adjust::ReborrowPin(_, mutbl) => {
+                    // Reborrowing a Pin is like a combinations of a deref and a borrow, so we do
+                    // both.
+                    let bk = match mutbl {
+                        ty::Mutability::Not => ty::BorrowKind::ImmBorrow,
+                        ty::Mutability::Mut => ty::BorrowKind::MutBorrow,
+                    };
+                    self.delegate.borrow_mut().borrow(&place_with_id, place_with_id.hir_id, bk);
+                }
             }
             place_with_id = self.cat_expr_adjusted(expr, place_with_id, adjustment)?;
         }
@@ -1284,6 +1294,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
             adjustment::Adjust::NeverToAny
             | adjustment::Adjust::Pointer(_)
             | adjustment::Adjust::Borrow(_)
+            | adjustment::Adjust::ReborrowPin(..)
             | adjustment::Adjust::DynStar => {
                 // Result is an rvalue.
                 Ok(self.cat_rvalue(expr.hir_id, target))

compiler/rustc_middle/src/ty/adjustment.rs

@@ -104,6 +104,9 @@ pub enum Adjust<'tcx> {
 
     /// Cast into a dyn* object.
     DynStar,
+
+    /// Take a pinned reference and reborrow as a `Pin<&mut T>` or `Pin<&T>`.
+    ReborrowPin(ty::Region<'tcx>, hir::Mutability),
 }
 
 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`

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

@@ -74,6 +74,7 @@ impl<'tcx> Cx<'tcx> {
         self.thir.exprs.push(expr)
     }
 
+    #[instrument(level = "trace", skip(self, expr, span))]
     fn apply_adjustment(
         &mut self,
         hir_expr: &'tcx hir::Expr<'tcx>,
@@ -146,6 +147,67 @@ impl<'tcx> Cx<'tcx> {
                 ExprKind::RawBorrow { mutability, arg: self.thir.exprs.push(expr) }
             }
             Adjust::DynStar => ExprKind::Cast { source: self.thir.exprs.push(expr) },
+            Adjust::ReborrowPin(region, mutbl) => {
+                debug!("apply ReborrowPin adjustment");
+                // Rewrite `$expr` as `Pin { __pointer: &(mut)? *($expr).__pointer }`
+
+                // We'll need these types later on
+                let pin_ty_args = match expr.ty.kind() {
+                    ty::Adt(_, args) => args,
+                    _ => bug!("ReborrowPin with non-Pin type"),
+                };
+                let pin_ty = pin_ty_args.iter().next().unwrap().expect_ty();
+                let ptr_target_ty = match pin_ty.kind() {
+                    ty::Ref(_, ty, _) => *ty,
+                    _ => bug!("ReborrowPin with non-Ref type"),
+                };
+
+                // pointer = ($expr).__pointer
+                let pointer_target = ExprKind::Field {
+                    lhs: self.thir.exprs.push(expr),
+                    variant_index: FIRST_VARIANT,
+                    name: FieldIdx::from(0u32),
+                };
+                let arg = Expr { temp_lifetime, ty: pin_ty, span, kind: pointer_target };
+                let arg = self.thir.exprs.push(arg);
+
+                // arg = *pointer
+                let expr = ExprKind::Deref { arg };
+                let arg = self.thir.exprs.push(Expr {
+                    temp_lifetime,
+                    ty: ptr_target_ty,
+                    span,
+                    kind: expr,
+                });
+
+                // expr = &mut target
+                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, region, ptr_target_ty, mutbl);
+                let expr = self.thir.exprs.push(Expr {
+                    temp_lifetime,
+                    ty: new_pin_target,
+                    span,
+                    kind: ExprKind::Borrow { borrow_kind, arg },
+                });
+
+                // kind = Pin { __pointer: pointer }
+                let pin_did = self.tcx.require_lang_item(rustc_hir::LangItem::Pin, Some(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::from(0u32), expr }]),
+                    user_ty: None,
+                    base: None,
+                }));
+
+                debug!(?kind);
+                kind
+            }
         };
 
         Expr { temp_lifetime, ty: adjustment.target, span, kind }
@@ -1014,7 +1076,7 @@ impl<'tcx> Cx<'tcx> {
 
         // Reconstruct the output assuming it's a reference with the
         // same region and mutability as the receiver. This holds for
-        // `Deref(Mut)::Deref(_mut)` and `Index(Mut)::index(_mut)`.
+        // `Deref(Mut)::deref(_mut)` and `Index(Mut)::index(_mut)`.
         let ty::Ref(region, _, mutbl) = *self.thir[args[0]].ty.kind() else {
             span_bug!(span, "overloaded_place: receiver is not a reference");
         };

compiler/rustc_span/src/symbol.rs

@@ -1418,6 +1418,7 @@ symbols! {
         pic,
         pie,
         pin,
+        pin_ergonomics,
         platform_intrinsics,
         plugin,
         plugin_registrar,

tests/ui/async-await/pin-reborrow-arg.rs

@@ -0,0 +1,36 @@
+//@ check-pass
+
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+use std::pin::Pin;
+
+struct Foo;
+
+impl Foo {
+    fn baz(self: Pin<&mut Self>) {
+    }
+}
+
+fn foo(_: Pin<&mut Foo>) {
+}
+
+fn foo_const(_: Pin<&Foo>) {
+}
+
+fn bar(x: Pin<&mut Foo>) {
+    foo(x);
+    foo(x); // for this to work we need to automatically reborrow,
+            // as if the user had written `foo(x.as_mut())`.
+
+    Foo::baz(x);
+    Foo::baz(x);
+
+    foo_const(x); // make sure we can reborrow &mut as &.
+
+    let x: Pin<&Foo> = Pin::new(&Foo);
+
+    foo_const(x); // make sure reborrowing from & to & works.
+}
+
+fn main() {}

tests/ui/async-await/pin-reborrow-const-as-mut.rs

@@ -0,0 +1,18 @@
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+// make sure we can't accidentally reborrow Pin<&T> as Pin<&mut T>
+
+use std::pin::Pin;
+
+struct Foo;
+
+fn foo(_: Pin<&mut Foo>) {
+}
+
+fn bar(x: Pin<&Foo>) {
+    foo(x); //~ ERROR mismatched types
+            //| ERROR types differ in mutability
+}
+
+fn main() {}

tests/ui/async-await/pin-reborrow-const-as-mut.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/pin-reborrow-const-as-mut.rs:14:9
+   |
+LL |     foo(x);
+   |     --- ^ types differ in mutability
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `Pin<&mut Foo>`
+              found struct `Pin<&Foo>`
+note: function defined here
+  --> $DIR/pin-reborrow-const-as-mut.rs:10:4
+   |
+LL | fn foo(_: Pin<&mut Foo>) {
+   |    ^^^ ----------------
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/async-await/pin-reborrow-once.rs

@@ -0,0 +1,13 @@
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+// Make sure with pin reborrowing that we can only get one mutable reborrow of a pinned reference.
+
+use std::pin::{pin, Pin};
+
+fn twice(_: Pin<&mut i32>, _: Pin<&mut i32>) {}
+
+fn main() {
+    let x = pin!(42);
+    twice(x, x); //~ ERROR cannot borrow
+}

tests/ui/async-await/pin-reborrow-once.stderr

@@ -0,0 +1,12 @@
+error[E0499]: cannot borrow `*x.__pointer` as mutable more than once at a time
+  --> $DIR/pin-reborrow-once.rs:12:14
+   |
+LL |     twice(x, x);
+   |     ----- -  ^ second mutable borrow occurs here
+   |     |     |
+   |     |     first mutable borrow occurs here
+   |     first borrow later used by call
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0499`.

tests/ui/async-await/pin-reborrow-self.rs

@@ -0,0 +1,24 @@
+//@ check-pass
+//@ignore-test
+
+// Currently ignored due to self reborrowing not being implemented for Pin
+
+#![feature(pin_ergonomics)]
+#![allow(incomplete_features)]
+
+use std::pin::Pin;
+
+struct Foo;
+
+impl Foo {
+    fn foo(self: Pin<&mut Self>) {
+    }
+}
+
+fn bar(x: Pin<&mut Foo>) {
+    x.foo();
+    x.foo(); // for this to work we need to automatically reborrow,
+             // as if the user had written `x.as_mut().foo()`.
+}
+
+fn main() {}

tests/ui/async-await/pin-reborrow-shorter.rs

@@ -0,0 +1,14 @@
+//@check-pass
+
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+use std::pin::Pin;
+
+fn shorter<'b, T: 'b>(_: Pin<&'b mut T>) {}
+
+fn test<'a: 'b, 'b, T: 'a>(x: Pin<&'a mut T>) {
+    shorter::<'b>(x);
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-pin_ergonomics.rs

@@ -0,0 +1,15 @@
+#![allow(dead_code, incomplete_features)]
+
+use std::pin::Pin;
+
+struct Foo;
+
+fn foo(_: Pin<&mut Foo>) {
+}
+
+fn bar(mut x: Pin<&mut Foo>) {
+    foo(x);
+    foo(x); //~ ERROR use of moved value: `x`
+}
+
+fn main() {}