Add back the E0016 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -21,6 +21,7 @@ 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"),
+E0016: include_str!("./error_codes/E0016.md"),
 E0017: include_str!("./error_codes/E0017.md"),
 E0019: include_str!("./error_codes/E0019.md"),
 E0023: include_str!("./error_codes/E0023.md"),

src/librustc_error_codes/error_codes/E0016.md

@@ -0,0 +1,14 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Blocks in constants may only contain items (such as constant, function
+definition, etc...) and a tail expression. Erroneous code example:
+
+```
+const FOO: i32 = { let x = 0; x }; // 'x' isn't an item!
+```
+
+To avoid it, you have to replace the non-item object:
+
+```
+const FOO: i32 = { const X : i32 = 0; X };
+```