1 files changed, 58 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0392.md b/compiler/rustc_error_codes/src/error_codes/E0392.md
new file mode 100644
index 000000000..f373d8945
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0392.md
@@ -0,0 +1,58 @@
+A type or lifetime parameter has been declared but is not actually used.
+
+Erroneous code example:
+
+```compile_fail,E0392
+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:
+
+```compile_fail,E0392
+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.
+
+[PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html