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+A variable was borrowed as mutable more than once.
+
+Erroneous code example:
+
+```compile_fail,E0499
+let mut i = 0;
+let mut x = &mut i;
+let mut a = &mut i;
+x;
+// error: cannot borrow `i` as mutable more than once at a time
+```
+
+Please note that in Rust, you can either have many immutable references, or one
+mutable reference. For more details you may want to read the
+[References & Borrowing][references-and-borrowing] section of the Book.
+
+[references-and-borrowing]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
+
+Example:
+
+```
+let mut i = 0;
+let mut x = &mut i; // ok!
+
+// or:
+let mut i = 0;
+let a = &i; // ok!
+let b = &i; // still ok!
+let c = &i; // still ok!
+b;
+a;
+```