Add info about undefined behavior to as_ref suggestions

Author: nathanwhit

src/librustc_typeck/check/method/suggest.rs

@@ -331,6 +331,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             err.note("try using `<*const T>::as_ref()` to get a reference to the \
                                       type behind the pointer: https://doc.rust-lang.org/std/\
                                       primitive.pointer.html#method.as_ref");
+                            err.note("using `<*const T>::as_ref()` on a pointer \
+                                      which is unaligned or points to invalid \
+                                      or uninitialized memory is undefined behavior");
                         }
                         err
                     }

src/test/ui/issues/issue-21596.stderr

@@ -5,6 +5,7 @@ LL |     println!("{}", z.to_string());
    |                      ^^^^^^^^^
    |
    = note: try using `<*const T>::as_ref()` to get a reference to the type behind the pointer: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
+   = note: using `<*const T>::as_ref()` on a pointer which is unaligned or points to invalid or uninitialized memory is undefined behavior
    = note: the method `to_string` exists but the following trait bounds were not satisfied:
            `*const u8 : std::string::ToString`