Group unused import warnings per path list

Given a file

```rust
use std::collections::{BinaryHeap, BTreeMap, BTreeSet};

fn main() {}
```

Show a single warning, instead of three for each unused import:

```nocode
warning: unused imports, #[warn(unused_imports)] on by default
 --> foo.rs:1:24
  |
1 | use std::collections::{BinaryHeap, BTreeMap, BTreeSet};
  |                        ^^^^^^^^^^  ^^^^^^^^  ^^^^^^^^
```

Include support for lints pointing at `MultilineSpan`s, instead of just
`Span`s.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index d99850332c3..d6e45aa0b9f 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -52,7 +52,7 @@ pub type FileName = String;
 /// able to use many of the functions on spans in codemap and you cannot assume
 /// that the length of the span = hi - lo; there may be space in the BytePos
 /// range between files.
-#[derive(Clone, Copy, Hash, PartialEq, Eq)]
+#[derive(Clone, Copy, Hash, PartialEq, Eq, Ord, PartialOrd)]
 pub struct Span {
     pub lo: BytePos,
     pub hi: BytePos,
@@ -67,7 +67,7 @@ pub struct Span {
 ///   the error, and would be rendered with `^^^`.
 /// - they can have a *label*. In this case, the label is written next
 ///   to the mark in the snippet when we render.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct MultiSpan {
     primary_spans: Vec<Span>,
     span_labels: Vec<(Span, String)>,
@@ -254,7 +254,7 @@ impl From<Span> for MultiSpan {
     }
 }
 
-#[derive(PartialEq, Eq, Clone, Debug, Hash, RustcEncodable, RustcDecodable, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Hash, RustcEncodable, RustcDecodable, Copy, Ord, PartialOrd)]
 pub struct ExpnId(pub u32);
 
 pub const NO_EXPANSION: ExpnId = ExpnId(!0);