Rollup merge of #69634 - GuillaumeGomez:clean-up-e0378, r=Dylan-DPC

clean up E0378 explanation

r? @Dylan-DPC

Author: Dylan-DPC

src/librustc_error_codes/error_codes/E0378.md

@@ -1,10 +1,28 @@
+The `DispatchFromDyn` trait was implemented on something which is not a pointer
+or a newtype wrapper around a pointer.
+
+Erroneous code example:
+
+```compile-fail,E0378
+#![feature(dispatch_from_dyn)]
+use std::ops::DispatchFromDyn;
+
+struct WrapperExtraField<T> {
+    ptr: T,
+    extra_stuff: i32,
+}
+
+impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
+where
+    T: DispatchFromDyn<U>,
+{}
+```
+
 The `DispatchFromDyn` trait currently can only be implemented for
 builtin pointer types and structs that are newtype wrappers around them
 — that is, the struct must have only one field (except for`PhantomData`),
 and that field must itself implement `DispatchFromDyn`.
 
-Examples:
-
 ```
 #![feature(dispatch_from_dyn, unsize)]
 use std::{
@@ -20,6 +38,8 @@ where
 {}
 ```
 
+Another example:
+
 ```
 #![feature(dispatch_from_dyn)]
 use std::{
@@ -37,21 +57,3 @@ where
     T: DispatchFromDyn<U>,
 {}
 ```
-
-Example of illegal `DispatchFromDyn` implementation
-(illegal because of extra field)
-
-```compile-fail,E0378
-#![feature(dispatch_from_dyn)]
-use std::ops::DispatchFromDyn;
-
-struct WrapperExtraField<T> {
-    ptr: T,
-    extra_stuff: i32,
-}
-
-impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
-where
-    T: DispatchFromDyn<U>,
-{}
-```