"a string to search for" and "a string for which to search" to my mind are equivalent in conversational English but in academic writing students are often asked to avoid ending clauses on a preposition - after all, they are _pre_positional phrases and the traditionally placed to be in a prefix notation to an object.

I believe this change preserves the intent and meaning while avoiding phrasing that may be perceived as awkward to some readers. By way of motivation, I teach both writing and Rust at University and am slated to quote this exact passage, this term, to students whom I have previously taught technical writing.