A where clause contains a nested quantification over lifetimes.
Erroneous code example:
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<>:
trait Tr<'a, 'b> {}
fn foo<T>(t: T)
where
for<'a, 'b> &'a T: Tr<'a, 'b>, // ok
{
}