unused import lint is misidentifying/blaming globs

[pnkfelix]

Spawned off of "fun note" at end of #7663.

When a unused import (either specific or glob) is followed by a glob import that shadows it (and thus causes the first to go unused), the unused import lint is incorrectly blaming/identifying the latter glob as being unused, and failing to identify the former import as being unused.

Nearly exhaustive and certainly exhausting test case (multuse.rs):

mod A {
    pub fn p() { println("A::p"); }
}
mod B {
    pub fn p() { println("B::p"); }
}

mod C {
    pub fn q() { println("C::q"); }
}
mod D {
    pub fn q() { println("D::q"); }
}

mod E {
    pub fn r() { println("E::r"); }
}
mod F {
    pub fn r() { println("E::r"); }
}

mod G {
    pub fn s() { println("G::s"); }
    pub fn t() { println("G::t"); }
}
mod H {
    pub fn s() { println("H::s"); }
}

mod I {
    pub fn u() { println("I::u"); }
    pub fn v() { println("I::v"); }
}
mod J {
    pub fn u() { println("J::u"); }
    pub fn v() { println("J::v"); }
}

mod K {
    pub fn w() { println("K::w"); }
}
mod L {
    pub fn w() { println("L::w"); }
}

mod m {
   use A::p; // <-- this `p` is unused; correctly warns
   use B::p;
   use C::q; // <-- this `q` is unused, but NO WARNING
   use D::*; // <-- this `q` IS USED, yields false positive warning
   use E::*; // <-- this `r` is overridden and unused; correctly warns
   use F::r;
   use G::*; // <-- this `s` is overridden, no warning since t is used, as expected for glob.
   use H::*; // <-- this `s` is USED, yields false positive warning
   use I::*; // <-- this `v` is overridden, no warning since u is used, as expected for glob
   use J::v;
   use K::*; // <-- all imports here are overriden, none used, but NO WARNING
   use L::*; // <-- this `w` is USED, yields false positive warning

   #[main]
   fn my_main() {
       p();
       q();
       r();
       s();
       t();
       u();
       v();
       w();
   }
}

Result of the run (to confirm what the comments next to the uses say):

% rustc /tmp/multuse.rs && /tmp/multuse
/tmp/multuse.rs:47:7: 47:11 warning: unused import [-W unused-imports (default)]
/tmp/multuse.rs:47    use A::p; // <-- this `p` is unused; correctly warns
                          ^~~~
/tmp/multuse.rs:50:7: 50:12 warning: unused import [-W unused-imports (default)]
/tmp/multuse.rs:50    use D::*; // <-- this `q` IS USED, yields false positive warning
                          ^~~~~
/tmp/multuse.rs:51:7: 51:12 warning: unused import [-W unused-imports (default)]
/tmp/multuse.rs:51    use E::*; // <-- this `r` is overridden and unused; correctly warns
                          ^~~~~
/tmp/multuse.rs:54:7: 54:12 warning: unused import [-W unused-imports (default)]
/tmp/multuse.rs:54    use H::*; // <-- this `s` is USED, yields false positive warning
                          ^~~~~
/tmp/multuse.rs:58:7: 58:12 warning: unused import [-W unused-imports (default)]
/tmp/multuse.rs:58    use L::*; // <-- this `w` is USED, yields false positive warning
                          ^~~~~
warning: no debug symbols in executable (-arch x86_64)
B::p
D::q
E::r
H::s
G::t
I::u
J::v
L::w

[alexcrichton]

Just want to say, this is an excellent test