fix the error code document

compiler/rustc_error_codes/src/error_codes/E0802.md

@@ -6,6 +6,8 @@ Erroneous code examples:
 The target data is not a `struct`.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 enum NotStruct<'a, T: ?Sized> {
     Variant(&'a T),
@@ -16,6 +18,8 @@ The target data has a layout that is not transparent, or `repr(transparent)`
 in other words.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 struct NotTransparent<'a, #[pointee] T: ?Sized> {
     ptr: &'a T,
@@ -25,6 +29,8 @@ struct NotTransparent<'a, #[pointee] T: ?Sized> {
 The target data has no data field.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 #[repr(transparent)]
 struct NoField<'a, #[pointee] T: ?Sized> {}
@@ -33,6 +39,8 @@ struct NoField<'a, #[pointee] T: ?Sized> {}
 The target data is not generic over any data, or has no generic type parameter.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 #[repr(transparent)]
 struct NoGeneric<'a>(&'a u8);
@@ -42,6 +50,8 @@ The target data has multiple generic type parameters, but none is designated as
 a pointee for coercion.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 #[repr(transparent)]
 struct AmbiguousPointee<'a, T1: ?Sized, T2: ?Sized> {
@@ -53,6 +63,8 @@ The target data has multiple generic type parameters that are designated as
 pointees for coercion.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 #[repr(transparent)]
 struct TooManyPointees<
@@ -65,6 +77,8 @@ struct TooManyPointees<
 The type parameter that is designated as a pointee is not marked `?Sized`.
 
 ```compile_fail,E0802
+#![feature(coerce_pointee)]
+use std::marker::CoercePointee;
 #[derive(CoercePointee)]
 #[repr(transparent)]
 struct NoMaybeSized<'a, #[pointee] T> {