add long explanation for E0183

Author: cameron1024

compiler/rustc_error_codes/src/error_codes.rs

@@ -92,6 +92,7 @@ E0164: include_str!("./error_codes/E0164.md"),
 E0165: include_str!("./error_codes/E0165.md"),
 E0170: include_str!("./error_codes/E0170.md"),
 E0178: include_str!("./error_codes/E0178.md"),
+E0183: include_str!("./error_codes/E0183.md"),
 E0184: include_str!("./error_codes/E0184.md"),
 E0185: include_str!("./error_codes/E0185.md"),
 E0186: include_str!("./error_codes/E0186.md"),
@@ -512,7 +513,6 @@ E0785: include_str!("./error_codes/E0785.md"),
 //  E0173, // manual implementations of unboxed closure traits are experimental
 //  E0174,
 //  E0182, // merged into E0229
-    E0183,
 //  E0187, // cannot 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

compiler/rustc_error_codes/src/error_codes/E0183.md

@@ -0,0 +1,39 @@
+Manual implemetation of a `Fn*` trait.
+
+Erroneous code example:
+
+```compile_fail,E0183
+struct MyClosure {
+    foo: i32
+}
+
+impl FnOnce<()> for MyClosure {  // error
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
+        println!("{}", self.foo);
+    }
+}
+```
+
+Manually implementing `Fn`, `FnMut` or `FnOnce` is unstable
+and requires `#![feature(fn_traits, unboxed_closures)]`.
+
+```
+#![feature(fn_traits, unboxed_closures)]
+
+struct MyClosure {
+    foo: i32
+}
+
+impl FnOnce<()> for MyClosure {  // ok!
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
+        println!("{}", self.foo);
+    }
+}
+```
+
+The argumements must be a tuple representing the argument list.
+For more info, see the [tracking issue][iss29625]:
+
+[iss29625]: https://github.com/rust-lang/rust/issues/29625

src/test/ui/feature-gates/feature-gate-unboxed-closures-manual-impls.stderr

@@ -101,5 +101,5 @@ LL | impl FnOnce<()> for Baz {
 
 error: aborting due to 12 previous errors
 
-Some errors have detailed explanations: E0229, E0658.
-For more information about an error, try `rustc --explain E0229`.
+Some errors have detailed explanations: E0183, E0229, E0658.
+For more information about an error, try `rustc --explain E0183`.

src/test/ui/feature-gates/feature-gate-unboxed-closures.stderr

@@ -26,4 +26,5 @@ LL | impl FnOnce<(u32, u32)> for Test {
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0658`.
+Some errors have detailed explanations: E0183, E0658.
+For more information about an error, try `rustc --explain E0183`.