Function parallel_prime_caches 

pub fn parallel_prime_caches(
    db: &RootDatabase,
    scope: &[Crate],
    num_worker_threads: usize,
    cb: &(dyn Fn(ParallelPrimeCachesProgress) + Sync),
)

Warm caches for scope.

scope must be closed under transitive dependencies: the scheduler only follows reverse-dep edges within scope. Out-of-scope crates can still be primed by salsa on demand when a scope crate’s queries reach into them. Callers that want to prime everything pass &all_crates(db).