new lint: Unused trait bound(s)

What it does

For any trait bounds that's not used on the function body, create an "unused" warning

Lint Name

unused_trait_bound

Category

pedantic

Advantage

• More "tight" code, we can be sure that each bounds will be used
  • and maybe in other words: more cleaner code since unused bound can be safely deleted

Drawbacks

• Might not be suitable for library APIs, since removing bounds (even if it's unused) might be a breaking change
  • It might need some thought on how it would interact with negative_impls

Example

trait Database {
    fn database(&self);
}

trait Redis {
    fn redis(&self);
}

fn get_user<C>(conn: C)
where
    C: Database + Redis, // this `Redis` bound should be warned
{
    // we just so happen to forgot to check for Redis cache
    conn.database()
}

Could be written as:

trait Database {
    fn database(&self);
}

trait Redis {
    fn redis(&self);
}

fn get_user<C>(conn: C)
where
    C: Database, // either remove the bound, or
{
    // use the bound
    // conn.redis()
    conn.database()
}

No implementation files, tests, or entry points are identified in the issue. Start by reviewing the proposed lint behavior and the linked Rust negative_impls discussion, then determine the relevant Clippy lint location and test pattern. Done means the lint reliably warns for unused trait bounds while accounting for the API and negative-impl concerns described here.