What does "where for" mean in Rust?

Question

This SO answer uses a where syntax I'm unfamiliar with:

fn readfile<'a, T: ?Sized>(filename: &str) -> Result<Box<Outer<'a, T>>, Box<std::error::Error>>
where
    for<'de> T: Deserialize<'de> + 'a
{
   ...

What does the for mean?

Where is this documented?

Answer 1

This is so-called higher-ranked trait bounds:

for<'a> can be read as "for all choices of 'a"

And it is defined by Rust RFC 0387.