Add regression test for ICE that happened on incr comp

An ICE happened when certain code is compiled in incremental compilation
mode and there are two `Ident`s that have the same `StableHash` value
but are considered different by `Eq` and `Hash`.

The `Ident` issue is now fixed.

Author: spastorino

src/test/ui/associated-type-bounds/traits-assoc-type-macros.rs

@@ -0,0 +1,43 @@
+// check-pass
+// compile-flags:-Cincremental=tmp/traits-assoc-type-macros
+
+// This test case makes sure that we can compile with incremental compilation
+// enabled when there are macros, traits, inheritance and associated types involved.
+
+trait Deserializer {
+    type Error;
+}
+
+trait Deserialize {
+    fn deserialize<D>(_: D) -> D::Error
+    where
+        D: Deserializer;
+}
+
+macro_rules! impl_deserialize {
+    ($name:ident) => {
+        impl Deserialize for $name {
+            fn deserialize<D>(_: D) -> D::Error
+            where
+                D: Deserializer,
+            {
+                loop {}
+            }
+        }
+    };
+}
+
+macro_rules! formats {
+    {
+        $($name:ident,)*
+    } => {
+        $(
+            pub struct $name;
+
+            impl_deserialize!($name);
+        )*
+    }
+}
+formats! { Foo, Bar, }
+
+fn main() {}