rollup merge of #27605: AlisdairO/diagnostics387

As title :-)
Part of #24407.

r? @Manishearth

src/librustc_borrowck/diagnostics.rs

@@ -158,6 +158,60 @@ fn main(){
     x = 5;
 }
 ```
+"##,
+
+E0387: r##"
+This error occurs when an attempt is made to mutate or mutably reference data
+that a closure has captured immutably. Examples of this error are shown below:
+
+```
+// Accepts a function or a closure that captures its environment immutably.
+// Closures passed to foo will not be able to mutate their closed-over state.
+fn foo<F: Fn()>(f: F) { }
+
+// Attempts to mutate closed-over data.  Error message reads:
+// `cannot assign to data in a captured outer variable...`
+fn mutable() {
+    let mut x = 0u32;
+    foo(|| x = 2);
+}
+
+// Attempts to take a mutable reference to closed-over data.  Error message
+// reads: `cannot borrow data mutably in a captured outer variable...`
+fn mut_addr() {
+    let mut x = 0u32;
+    foo(|| { let y = &mut x; });
+}
+```
+
+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.
+
+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:
+
+```
+use std::cell::Cell;
+
+fn mutable() {
+    let x = 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/
 "##
 
 }
@@ -166,7 +220,6 @@ register_diagnostics! {
     E0383, // partial reinitialization of uninitialized structure
     E0385, // {} in an aliasable location
     E0386, // {} in an immutable container
-    E0387, // {} in a captured outer variable in an `Fn` closure
     E0388, // {} in a static location
     E0389  // {} in a `&` reference
 }