Add back the E0011 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -17,6 +17,7 @@ E0007: include_str!("./error_codes/E0007.md"),
 E0008: include_str!("./error_codes/E0008.md"),
 E0009: include_str!("./error_codes/E0009.md"),
 E0010: include_str!("./error_codes/E0010.md"),
+E0011: include_str!("./error_codes/E0011.md"),
 E0013: include_str!("./error_codes/E0013.md"),
 E0014: include_str!("./error_codes/E0014.md"),
 E0015: include_str!("./error_codes/E0015.md"),

src/librustc_error_codes/error_codes/E0011.md

@@ -0,0 +1,30 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Using a user-defined operator on const/static variable is restricted to what
+can be evaluated at compile-time. Using an user-defined operator could call a
+user-defined function, which is not allowed.
+
+Bad example:
+
+```compile_fail,E0015
+use std::ops::Index;
+
+struct Foo { a: u8 }
+
+impl ::std::ops::Index<u8> for Foo {
+    type Output = u8;
+    fn index<'a>(&'a self, idx: u8) -> &'a u8 { &self.a }
+}
+
+const a: Foo = Foo { a: 0u8 };
+const b: u8 = a[0]; // Index trait is defined by the user, bad!
+```
+
+Only operators on builtin types are allowed.
+
+Example:
+
+```
+const a: &'static [i32] = &[1, 2, 3];
+const b: i32 = a[0]; // Good!
+```