core: rustc: improve compile message for unsized types

Author: liigo

src/libcore/marker.rs

@@ -47,7 +47,8 @@ impl<T> !Send for *mut T { }
 /// Types with a constant size known at compile-time.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "sized"]
-#[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"]
+#[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time. \
+                            Try to use `&{Self}` or `Box<{Self}>`?"]
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
 pub trait Sized {
     // Empty.