Suggest Pin::as_mut when encountering borrow error

Author: estebank

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -1133,6 +1133,17 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                 place_name, partially_str, loop_message
                             ),
                         );
+                        if let ty::Adt(def, ..)
+                            = moved_place.ty(self.body, self.infcx.tcx).ty.kind()
+                            && Some(def.did()) == self.infcx.tcx.lang_items().pin_type()
+                        {
+                            err.span_suggestion_verbose(
+                                fn_call_span.shrink_to_lo(),
+                                "consider reborrowing the `Pin` instead of moving it",
+                                "as_mut().".to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
                     }
                     let tcx = self.infcx.tcx;
                     // Avoid pointing to the same function in multiple different

src/test/ui/moves/move-fn-self-receiver.stderr

@@ -67,6 +67,10 @@ note: `Foo::use_pin_box_self` takes ownership of the receiver `self`, which move
    |
 LL |     fn use_pin_box_self(self: Pin<Box<Self>>) {}
    |                         ^^^^
+help: consider reborrowing the `Pin` instead of moving it
+   |
+LL |     pin_box_foo.as_mut().use_pin_box_self();
+   |                 +++++++++
 
 error[E0505]: cannot move out of `mut_foo` because it is borrowed
   --> $DIR/move-fn-self-receiver.rs:50:5

src/test/ui/moves/pin-mut-reborrow.fixed

@@ -0,0 +1,15 @@
+// run-rustfix
+use std::pin::Pin;
+
+struct Foo;
+
+impl Foo {
+    fn foo(self: Pin<&mut Self>) {}
+}
+
+fn main() {
+    let mut foo = Foo;
+    let mut foo = Pin::new(&mut foo);
+    foo.as_mut().foo();
+    foo.foo(); //~ ERROR use of moved value
+}

src/test/ui/moves/pin-mut-reborrow.rs

@@ -0,0 +1,15 @@
+// run-rustfix
+use std::pin::Pin;
+
+struct Foo;
+
+impl Foo {
+    fn foo(self: Pin<&mut Self>) {}
+}
+
+fn main() {
+    let mut foo = Foo;
+    let mut foo = Pin::new(&mut foo);
+    foo.foo();
+    foo.foo(); //~ ERROR use of moved value
+}

src/test/ui/moves/pin-mut-reborrow.stderr

@@ -0,0 +1,23 @@
+error[E0382]: use of moved value: `foo`
+  --> $DIR/pin-mut-reborrow.rs:14:5
+   |
+LL |     let mut foo = Pin::new(&mut foo);
+   |         ------- move occurs because `foo` has type `Pin<&mut Foo>`, which does not implement the `Copy` trait
+LL |     foo.foo();
+   |         ----- `foo` moved due to this method call
+LL |     foo.foo();
+   |     ^^^ value used here after move
+   |
+note: `Foo::foo` takes ownership of the receiver `self`, which moves `foo`
+  --> $DIR/pin-mut-reborrow.rs:7:12
+   |
+LL |     fn foo(self: Pin<&mut Self>) {}
+   |            ^^^^
+help: consider reborrowing the `Pin` instead of moving it
+   |
+LL |     foo.as_mut().foo();
+   |         +++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0382`.