1 files changed, 30 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0502.md b/compiler/rustc_error_codes/src/error_codes/E0502.md
new file mode 100644
index 000000000..dc3ffdfdd
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0502.md
@@ -0,0 +1,30 @@
+A variable already borrowed as immutable was borrowed as mutable.
+
+Erroneous code example:
+
+```compile_fail,E0502
+fn bar(x: &mut i32) {}
+fn foo(a: &mut i32) {
+    let y = &a; // a is borrowed as immutable.
+    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
+            //        as immutable
+    println!("{}", y);
+}
+```
+
+To fix this error, ensure that you don't have any other references to the
+variable before trying to access it mutably:
+
+```
+fn bar(x: &mut i32) {}
+fn foo(a: &mut i32) {
+    bar(a);
+    let y = &a; // ok!
+    println!("{}", y);
+}
+```
+
+For more information on Rust's ownership system, take a look at 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