From 55752a4bdeae5dad77875b7702a14882fd12725b Mon Sep 17 00:00:00 2001
From: Nathan Kleyn <nathan@nathankleyn.com>
Date: Wed, 12 Aug 2015 20:46:38 +0100
Subject: [PATCH 1/3] Add detailed diagnostics for E0386.

This adds detailed diagnostics for E0386, 'cannot assign to data in an
immutable container'.
---
 src/librustc_borrowck/diagnostics.rs | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/librustc_borrowck/diagnostics.rs b/src/librustc_borrowck/diagnostics.rs
index 850701e704613..1aa735f8c639e 100644
--- a/src/librustc_borrowck/diagnostics.rs
+++ b/src/librustc_borrowck/diagnostics.rs
@@ -160,6 +160,27 @@ fn main(){
 ```
 "##,
 
+E0386: r##"
+This error occurs when an attempt is made to mutate the target of a mutable
+reference stored inside an immutable container.
+
+For example, this can happen when storing a `&mut` inside an immutable `Box`:
+
+```
+let mut x: i64 = 1;
+let y: Box<_> = Box::new(&mut x);
+**y = 2; // error, cannot assign to data in an immutable container
+```
+
+This error can be fixed by making the container mutable:
+
+```
+let mut x: i64 = 1;
+let mut y: Box<_> = Box::new(&mut x);
+**y = 2;
+```
+"##,
+
 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:
@@ -219,7 +240,6 @@ https://doc.rust-lang.org/std/cell/
 register_diagnostics! {
     E0383, // partial reinitialization of uninitialized structure
     E0385, // {} in an aliasable location
-    E0386, // {} in an immutable container
     E0388, // {} in a static location
     E0389  // {} in a `&` reference
 }

From 75c6428e1bf2f5564392276642d962163d8a9809 Mon Sep 17 00:00:00 2001
From: Nathan Kleyn <nathan@nathankleyn.com>
Date: Thu, 13 Aug 2015 09:25:56 +0100
Subject: [PATCH 2/3] Add details about types with interior mutability to
 E0386.

Types with interior mutability like `Cell` and `RefCell` can be used to
skirt the restriction on mutating mutable values inside an immutable
container.
---
 src/librustc_borrowck/diagnostics.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/librustc_borrowck/diagnostics.rs b/src/librustc_borrowck/diagnostics.rs
index 1aa735f8c639e..0d0456cfb42cc 100644
--- a/src/librustc_borrowck/diagnostics.rs
+++ b/src/librustc_borrowck/diagnostics.rs
@@ -179,6 +179,14 @@ let mut x: i64 = 1;
 let mut y: Box<_> = Box::new(&mut x);
 **y = 2;
 ```
+
+It can also be fixed by using a type with interior mutability, such as `Cell` or
+`RefCell`:
+
+```
+let y: Cell<_> = Cell::new(1);
+y.set(2);
+```
 "##,
 
 E0387: r##"

From b0b29521896d280a2f5340edc901a812ad618266 Mon Sep 17 00:00:00 2001
From: Nathan Kleyn <nathan@nathankleyn.com>
Date: Thu, 13 Aug 2015 10:17:18 +0100
Subject: [PATCH 3/3] Improve interior mutability example for E0386.

---
 src/librustc_borrowck/diagnostics.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/librustc_borrowck/diagnostics.rs b/src/librustc_borrowck/diagnostics.rs
index 0d0456cfb42cc..d7762896debd7 100644
--- a/src/librustc_borrowck/diagnostics.rs
+++ b/src/librustc_borrowck/diagnostics.rs
@@ -184,7 +184,8 @@ It can also be fixed by using a type with interior mutability, such as `Cell` or
 `RefCell`:
 
 ```
-let y: Cell<_> = Cell::new(1);
+let x: i64 = 1;
+let y: Box<Cell<_>> = Box::new(Cell::new(x));
 y.set(2);
 ```
 "##,