add new error code

1 files changed, 39 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0764.md b/src/librustc_error_codes/error_codes/E0764.md
new file mode 100644
index 00000000000..e9061f988ac
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0764.md
@@ -0,0 +1,39 @@
+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.
+
+Erroneous code example:
+
+```compile_fail,E0764
+#![feature(const_fn)]
+#![feature(const_mut_refs)]
+
+fn main() {
+    const OH_NO: &'static mut usize = &mut 1; // error!
+}
+```
+
+Remember: you cannot use a function call inside a constant or static. However,
+you can totally use it in constant functions:
+
+```
+#![feature(const_fn)]
+#![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!
+}
+```