---

yaml
---
r: 232268
b: refs/heads/try
c: 78d2833
h: refs/heads/master
v: v3

Author: AlisdairO

[refs]

@@ -1,7 +1,7 @@
 ---
 refs/heads/master: edeb4f1c86cbf6af8ef9874d4b3af50f721ea1b8
 refs/heads/snap-stage3: 1af31d4974e33027a68126fa5a5a3c2c6491824f
-refs/heads/try: d83a0fdf7da8b93b21de6ff2809e81a69447a9a5
+refs/heads/try: 78d28336feb9f46ac5929023f45acdbb3f3379e5
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503

branches/try/src/librustc_borrowck/diagnostics.rs

@@ -125,12 +125,28 @@ The problem here is that foo is defined as accepting a parameter of type `Fn`.
 Closures passed into foo will thus be inferred to be of type `Fn`, meaning that
 they capture their context immutably.
 
-The solution is to capture the data mutably. This can be done by defining `foo`
-to take FnMut rather than Fn:
+If the definition of `foo` is under your control, the simplest solution is to
+capture the data mutably. This can be done by defining `foo` to take FnMut
+rather than Fn:
 
 ```
 fn foo<F: FnMut()>(f: F) { }
 ```
+
+Alternatively, we can consider using the `Cell` and `RefCell` types to achieve
+interior mutability through a shared reference. Our example's `mutable` function
+could be redefined as below:
+
+```
+fn mutable() {
+    let x = std::cell::Cell::new(0u32);
+    foo(|| x.set(2));
+}
+```
+
+You can read more about cell types in the API documentation:
+
+https://doc.rust-lang.org/std/cell/
 "##
 
 }