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+#### Note: this error code is no longer emitted by the compiler.
+
+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;
+```
+
+It can also be fixed by using a type with interior mutability, such as `Cell`
+or `RefCell`:
+
+```
+use std::cell::Cell;
+
+let x: i64 = 1;
+let y: Box<Cell<_>> = Box::new(Cell::new(x));
+y.set(2);
+```