Wording in "Unsized Types" chapter of TRPL is unclear

[mkpankov]

Meaning, this implementation would only work for [references][ref], and not other types of pointers. With this impl, all pointers, including (at some point, there are some bugs to fix first) user-defined custom smart pointers, can use this impl.

"With this impl" seems to refer to previously mentioned

impl Foo for str {

but it's unclear from the sentence which impl is being referred to.

@steveklabnik says he meant impl Foo for str { (link)