Allow lifetime parameters on generic types

[sfackler]

TL;DR

It'd be nice if something like this were possible:

trait Foo {}

trait Bar<F: Foo> {
    fn bar<'a>(&'a self) -> F<'a>;
}

This'd probably require some new syntax to limit F to Foos with lifetime parameters. Potentially related to #5922.

Use Case

In https://github.com/sfackler/rust-postgres, the PostgresConnection and PostgresTransaction types can both prepare statements. However, PostgresConnection.prepare and PostgresTransaction.prepare return different implementations of the PostgresStatement trait. Both implementations of PostgresStatement contain borrowed pointers to their parent PostgresConnection and the lifetime of a statement is semantically tied to a connection in any case. I'd like to have a trait to unify this functionality. It'd look something like

pub trait PostgresPreparer<S: PostgresStatement> {
    fn try_prepare<'a>(&'a self, query: &str) -> Result<S<'a>, PostgresDbError>;

    fn prepare<'a>(&'a self, query: &str) -> S<'a> {
        match try_prepare(query) {
            Ok(stmt) => stmt,
            Err(err) => fail2!("Error preparing statement: {}", err.to_str())
        }
    }

    fn try_update(&self, query: &str, params: &[&ToSql]) -> Result<uint, PostgresDbError> {
        do self.try_prepare(query).chain |stmt| {
            stmt.try_update(params)
        }
    }

    fn update(&self, query: &str, params: &[&ToSql]) -> uint {
        match self.try_update(query, params) {
            Ok(updated) => updated,
            Err(err) => fail2!("Error executing query: {}", err.to_str())
        }
    }

}

[sfackler]

Talking with @nikomatsakis in IRC, this requires higher kinded types.

[nikomatsakis]

Dup #8922