Teach dropck about resume arguments

src/test/ui/generator/dropck-resume.rs

@@ -0,0 +1,33 @@
+#![feature(generators, generator_trait)]
+
+use std::ops::{Generator, GeneratorState};
+use std::pin::Pin;
+
+struct SetToNone<'a: 'b, 'b>(&'b mut Option<&'a i32>);
+
+impl<'a, 'b> Drop for SetToNone<'a, 'b> {
+    fn drop(&mut self) {
+        *self.0 = None;
+    }
+}
+
+fn drop_using_generator() -> i32 {
+    let mut y = Some(&0);
+    let z = &mut y;
+    let r;
+    {
+        let mut g = move |r| {
+            let _s = SetToNone(r);
+            yield;
+        };
+        let mut g = Pin::new(&mut g);
+        g.as_mut().resume(z);
+        r = y.as_ref().unwrap();
+        //~^ ERROR cannot borrow `y` as immutable because it is also borrowed as mutable
+    }
+    **r
+}
+
+fn main() {
+    println!("{}", drop_using_generator());
+}

src/test/ui/generator/dropck-resume.stderr

@@ -0,0 +1,15 @@
+error[E0502]: cannot borrow `y` as immutable because it is also borrowed as mutable
+  --> $DIR/dropck-resume.rs:25:13
+   |
+LL |     let z = &mut y;
+   |             ------ mutable borrow occurs here
+...
+LL |         r = y.as_ref().unwrap();
+   |             ^ immutable borrow occurs here
+LL |
+LL |     }
+   |     - mutable borrow might be used here, when `g` is dropped and runs the destructor for generator
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.

src/test/ui/generator/retain-resume-ref.stderr

@@ -4,9 +4,10 @@ error[E0499]: cannot borrow `thing` as mutable more than once at a time
 LL |     gen.as_mut().resume(&mut thing);
    |                         ---------- first mutable borrow occurs here
 LL |     gen.as_mut().resume(&mut thing);
-   |                  ------ ^^^^^^^^^^ second mutable borrow occurs here
-   |                  |
-   |                  first borrow later used by call
+   |                         ^^^^^^^^^^ second mutable borrow occurs here
+LL |
+LL | }
+   | - first borrow might be used here, when `gen` is dropped and runs the destructor for generator
 
 error: aborting due to previous error