pin_ergonomics: allow reborrowing as Pin<&T>

Author: eholk

Diff for: compiler/rustc_hir_typeck/src/coercion.rs

@@ -798,9 +798,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // 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 ty = match ty.kind() {
+            let (pin, ty) = match ty.kind() {
                 ty::Adt(pin, args) if self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) => {
-                    args[0].expect_ty()
+                    (*pin, args[0].expect_ty())
                 }
                 _ => {
                     debug!("can't reborrow {:?} as pinned", ty);
@@ -809,24 +809,30 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             };
             // Make sure the T is something we understand (just `&mut U` for now)
             match ty.kind() {
-                ty::Ref(region, ty, ty::Mutability::Mut) => Ok((*region, *ty)),
+                ty::Ref(region, ty, mutbl) => Ok((pin, *region, *ty, *mutbl)),
                 _ => {
                     debug!("can't reborrow pin of inner type {:?}", ty);
                     Err(TypeError::Mismatch)
                 }
             }
         };
 
-        let (_, _a_ty) = extract_pin_mut(a)?;
-        let (b_region, _b_ty) = extract_pin_mut(b)?;
+        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, hir::Mutability::Mut),
-                target: b,
-            }]
+            vec![Adjustment { kind: Adjust::ReborrowPin(b_region, mut_b), target: b }]
         })
     }
 

Diff for: compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -181,22 +181,25 @@ impl<'tcx> Cx<'tcx> {
                 });
 
                 // 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: Ty::new_ref(self.tcx, region, ptr_target_ty, mutbl),
+                    ty: new_pin_target,
                     span,
-                    kind: ExprKind::Borrow {
-                        borrow_kind: BorrowKind::Mut { kind: mir::MutBorrowKind::Default },
-                        arg,
-                    },
+                    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: pin_ty_args,
+                    args,
                     fields: Box::new([FieldExpr { name: FieldIdx::from(0u32), expr }]),
                     user_ty: None,
                     base: None,

Diff for: tests/ui/async-await/pin-reborrow-arg.rs

@@ -15,13 +15,22 @@ impl Foo {
 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() {}

Diff for: 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() {}

Diff for: 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`.