Auto merge of #27099 - AlisdairO:diagnostics3, r=Manishearth

Per the title.  I've linked to the reference at http://doc.rust-lang.org/reference.html#type-parameters-1, but I'm not sure that's such a good link - but there doesn't seem to be a great deal of explanation elsewhere in the reference either...

Author: bors

src/librustc_typeck/diagnostics.rs

@@ -2109,6 +2109,66 @@ E0380: r##"
 Default impls are only allowed for traits with no methods or associated items.
 For more information see the [opt-in builtin traits RFC](https://github.com/rust
 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
+"##,
+
+E0392: r##"
+This error indicates that a type or lifetime parameter has been declared
+but not actually used.  Here is an example that demonstrates the error:
+
+```
+enum Foo<T> {
+    Bar
+}
+```
+
+If the type parameter was included by mistake, this error can be fixed
+by simply removing the type parameter, as shown below:
+
+```
+enum Foo {
+    Bar
+}
+```
+
+Alternatively, if the type parameter was intentionally inserted, it must be
+used. A simple fix is shown below:
+
+```
+enum Foo<T> {
+    Bar(T)
+}
+```
+
+This error may also commonly be found when working with unsafe code. For
+example, when using raw pointers one may wish to specify the lifetime for
+which the pointed-at data is valid. An initial attempt (below) causes this
+error:
+
+```
+struct Foo<'a, T> {
+    x: *const T
+}
+```
+
+We want to express the constraint that Foo should not outlive `'a`, because
+the data pointed to by `T` is only valid for that lifetime. The problem is
+that there are no actual uses of `'a`. It's possible to work around this
+by adding a PhantomData type to the struct, using it to tell the compiler
+to act as if the struct contained a borrowed reference `&'a T`:
+
+```
+use std::marker::PhantomData;
+
+struct Foo<'a, T: 'a> {
+    x: *const T,
+    phantom: PhantomData<&'a T>
+}
+```
+
+PhantomData can also be used to express information about unused type
+parameters. You can read more about it in the API documentation:
+
+https://doc.rust-lang.org/std/marker/struct.PhantomData.html
 "##
 
 }
@@ -2211,7 +2271,6 @@ register_diagnostics! {
     E0390, // only a single inherent implementation marked with
            // `#[lang = \"{}\"]` is allowed for the `{}` primitive
     E0391, // unsupported cyclic reference between types/traits detected
-    E0392, // parameter `{}` is never used
     E0393, // the type parameter `{}` must be explicitly specified in an object
            // type because its default value `{}` references the type `Self`"
     E0399, // trait items need to be implemented because the associated