The Rust Programming Language Pig Latin Exercise
Be aware that Rust typically uses 4 spaces
It's fine if you consistently use 2 spaces (especially if you override it in rustfmt.toml), but just be aware that the standard is different.
Collect directly to a String
Instead of collecting to a Vec and then copying that over to a new Vec (within String), collect to a String directly:
pub fn translate(s: &amp;str) -&gt; String {
    s.split_word_bounds().map(translate_word).collect()
}

Use Chars::as_str
When you use str::chars, the specific iterator that it returns is called Chars. It has a handy function to get the remaining part, so you don't need to allocate a new string:
'a'..='z' =&gt; format!(&quot;{}-{}ay&quot;, it.as_str(), first),

Use Cow
This is a bit of an advanced optimization that you don't need to do—the Book doesn't even mention it once.
Currently, you allocate a new String even when the output is identical to the input. Instead, return Cow&lt;str&gt;: if the first character isn't a letter, you can return Cow::Borrowed(s), which points to the existing &amp;str. If it does start with a letter, return Cow::Owned(format!(...)), which has the same overhead as it did before. Here, I'm using .into() instead of writing Cow::Owned and Cow::Borrowed explicitly. You can do either.
fn translate_word(s: &amp;str) -&gt; Cow&lt;str&gt; {
    let mut it = s.chars();
    let first = it.next().unwrap();
    match first.to_ascii_lowercase() {
        'a' | 'e' | 'i' | 'o' | 'u' =&gt; format!(&quot;{}-hay&quot;, s).into(),
        'a'..='z' =&gt; format!(&quot;{}-{}ay&quot;, it.as_str(), first).into(),
        _ =&gt; s.into(),
    }
}

Final code
https://play.rust-lang.org/?version=stable&amp;mode=debug&amp;edition=2018&amp;gist=78daca7b7adab4587436cacf35bca90e