Add error explanation for E0328.

Author: linclark

src/librustc_typeck/diagnostics.rs

@@ -3087,6 +3087,29 @@ impl Foo for Bar {
 ```
 "##,
 
+E0328: r##"
+The Unsize trait should not be implemented directly. All implementations of
+Unsize are provided automatically by the compiler.
+
+Here's an example of this error:
+
+```compile_fail,E0328
+#![feature(unsize)]
+
+use std::marker::Unsize;
+
+pub struct MyType;
+
+impl<T> Unsize<T> for MyType {}
+```
+
+If you are defining your own smart pointer type and would like to enable
+conversion from a sized to an unsized type with the [DST coercion system]
+(https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md), use
+[`CoerceUnsized`](https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html)
+instead.
+"##,
+
 E0329: r##"
 An attempt was made to access an associated constant through either a generic
 type parameter or `Self`. This is not supported yet. An example causing this
@@ -4195,7 +4218,6 @@ register_diagnostics! {
 //  E0249,
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
     E0320, // recursive overflow during dropck
-    E0328, // cannot implement Unsize explicitly
 //  E0372, // coherence not object safe
     E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
            // between structures with the same definition