Function retarget_modules 

pub fn retarget_modules(
    modules: &mut [Module],
    used_items: &UsedItems,
    resolver: &impl Resolver,
)

Retarget used identifiers to point at the corresponding declaration.

We call this after usage analysis, because it is mutating the modules, and we want to keep mutations and lookups separate if possible, to avoid multiple mut borrows.

Panics

• if an identifier has no corresponding declaration.