[E0183] Add missing error code explanation

src/librustc_typeck/error_codes.rs

@@ -1588,6 +1588,38 @@ fn bar(foo: Foo) -> u32 {
 ```
 "##,
 
+E0183: r##"
+Fn* traits (`unboxed closures`) are experimental. 
+See [tracking issue #29625][iss29625] for the status of the feature.
+
+Functions must have exactly one (non self) argument, a tuple representing
+the argument list [1]. 
+
+Here's an example of this error:
+```compile_fail,E0183
+extern "rust-call" fn echo(arg: (u32,)) -> (u32,) {
+    arg
+}
+```
+
+This feature can be enabled with:
+```
+#![feature(unboxed_closures)]
+```
+
+Here's the above example fixed:
+```
+#![feature(unboxed_closures)]
+
+extern "rust-call" fn echo(arg: (u32,)) -> (u32,) {
+    arg
+}
+```
+
+[1]: https://doc.rust-lang.org/unstable-book/language-features/unboxed-closures.html
+[iss29625]: https://github.com/rust-lang/rust/issues/29625
+"##,
+
 E0184: r##"
 Explicitly implementing both Drop and Copy for a type is currently disallowed.
 This feature can make some sense in theory, but the current implementation is
@@ -4671,7 +4703,6 @@ register_diagnostics! {
 //  E0173, // manual implementations of unboxed closure traits are experimental
 //  E0174,
 //  E0182, // merged into E0229
-    E0183,
 //  E0187, // can't infer the kind of the closure
 //  E0188, // can not cast an immutable reference to a mutable pointer
 //  E0189, // deprecated: can only cast a boxed pointer to a boxed object