Rollup merge of rust-lang#27220 - AlisdairO:diagnostics120, r=Manishearth

As title!

I should probably be bunching these up a bit more, but I'm not sure when my time is going to disappear on me.  Once my schedule stabilises I'll try to start batching them into larger PRs.

Part of rust-lang#24407.
r? @Manishearth

Author: steveklabnik

src/librustc_typeck/diagnostics.rs

@@ -1316,6 +1316,45 @@ fn main() {
 ```
 "##,
 
+E0120: r##"
+An attempt was made to implement Drop on a trait, which is not allowed: only
+structs and enums can implement Drop. An example causing this error:
+
+```
+trait MyTrait {}
+
+impl Drop for MyTrait {
+    fn drop(&mut self) {}
+}
+```
+
+A workaround for this problem is to wrap the trait up in a struct, and implement
+Drop on that. An example is shown below:
+
+```
+trait MyTrait {}
+struct MyWrapper<T: MyTrait> { foo: T }
+
+impl <T: MyTrait> Drop for MyWrapper<T> {
+    fn drop(&mut self) {}
+}
+
+```
+
+Alternatively, wrapping trait objects requires something like the following:
+
+```
+trait MyTrait {}
+
+//or Box<MyTrait>, if you wanted an owned trait object
+struct MyWrapper<'a> { foo: &'a MyTrait }
+
+impl <'a> Drop for MyWrapper<'a> {
+    fn drop(&mut self) {}
+}
+```
+"##,
+
 E0121: r##"
 In order to be consistent with Rust's lack of global type inference, type
 placeholders are disallowed by design in item signatures.
@@ -2251,7 +2290,6 @@ register_diagnostics! {
     E0103,
     E0104,
     E0118,
-    E0120,
     E0122,
     E0123,
     E0127,