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+A mutable reference was used in a constant.
+
+Erroneous code example:
+
+```compile_fail,E0764
+#![feature(const_mut_refs)]
+
+fn main() {
+    const OH_NO: &'static mut usize = &mut 1; // error!
+}
+```
+
+Mutable references (`&mut`) can only be used in constant functions, not statics
+or constants. This limitation exists to prevent the creation of constants that
+have a mutable reference in their final value. If you had a constant of
+`&mut i32` type, you could modify the value through that reference, making the
+constant essentially mutable.
+
+While there could be a more fine-grained scheme in the future that allows
+mutable references if they are not "leaked" to the final value, a more
+conservative approach was chosen for now. `const fn` do not have this problem,
+as the borrow checker will prevent the `const fn` from returning new mutable
+references.
+
+Remember: you cannot use a function call inside a constant or static. However,
+you can totally use it in constant functions:
+
+```
+#![feature(const_mut_refs)]
+
+const fn foo(x: usize) -> usize {
+    let mut y = 1;
+    let z = &mut y;
+    *z += x;
+    y
+}
+
+fn main() {
+    const FOO: usize = foo(10); // ok!
+}
+```