E0316 - Error codes index
Description: Book listing all Rust error codes

# Error code E0316

A `where` clause contains a nested quantification over lifetimes.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    trait Tr<'a, 'b> {}

    fn foo<T>(t: T)
    where
        for<'a> &'a T: for<'b> Tr<'a, 'b>, // error: nested quantification
    {
    }
    }

Rust syntax allows lifetime quantifications in two places within `where` clauses: Quantifying over the trait bound only (as in `Ty: for<'l> Trait<'l>`) and quantifying over the whole clause (as in `for<'l> &'l Ty: Trait<'l>`). Using both in the same clause leads to a nested lifetime quantification, which is not supported.

The following example compiles, because the clause with the nested quantification has been rewritten to use only one `for<>`:

    #![allow(unused)]
    fn main() {
    trait Tr<'a, 'b> {}

    fn foo<T>(t: T)
    where
        for<'a, 'b> &'a T: Tr<'a, 'b>, // ok
    {
    }
    }