Add back the E0022 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -25,6 +25,7 @@ E0016: include_str!("./error_codes/E0016.md"),
 E0017: include_str!("./error_codes/E0017.md"),
 E0018: include_str!("./error_codes/E0018.md"),
 E0019: include_str!("./error_codes/E0019.md"),
+E0022: include_str!("./error_codes/E0022.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),
 E0026: include_str!("./error_codes/E0026.md"),

src/librustc_error_codes/error_codes/E0022.md

@@ -0,0 +1,17 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Constant functions are not allowed to mutate anything. Thus, binding to an
+argument with a mutable pattern is not allowed. For example,
+
+```
+const fn foo(mut x: u8) {
+    // do stuff
+}
+```
+
+Is incorrect because the function body may not mutate `x`.
+
+Remove any mutable bindings from the argument list to fix this error. In case
+you need to mutate the argument, try lazily initializing a global variable
+instead of using a `const fn`, or refactoring the code to a functional style to
+avoid mutation if possible.