Auto merge of #44015 - kennytm:hasher, r=alexcrichton

 impl Hasher for {&mut Hasher, Box<Hasher>}

**Rationale:** The `Hash` trait has `fn hash<H: Hasher>(&self, state: &mut H)`, which can only accept a `Sized` hasher, even if the `Hasher` trait is object-safe. We cannot retroactively add the `?Sized` bound without breaking stability, thus implementing `Hasher` to a trait object reference is the next best solution.

**Warning:** These `impl` are insta-stable, and should need an FCP. I don't think a full RFC is necessary.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 8f3e71ef794..c5beb63d12e 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -50,3 +50,19 @@ fn hash<T: Hash>(t: &T) -> u64 {
     t.hash(&mut s);
     s.finish()
 }
+
+// FIXME: Instantiated functions with i128 in the signature is not supported in Emscripten.
+// See https://github.com/kripken/emscripten-fastcomp/issues/169
+#[cfg(not(target_os = "emscripten"))]
+#[test]
+fn test_boxed_hasher() {
+    let ordinary_hash = hash(&5u32);
+
+    let mut hasher_1 = Box::new(DefaultHasher::new());
+    5u32.hash(&mut hasher_1);
+    assert_eq!(ordinary_hash, hasher_1.finish());
+
+    let mut hasher_2 = Box::new(DefaultHasher::new()) as Box<Hasher>;
+    5u32.hash(&mut hasher_2);
+    assert_eq!(ordinary_hash, hasher_2.finish());
+}