implement Step for Idx types

This way, we can iterate over a `Range<T>` where `T: Idx`

Author: nikomatsakis

src/librustc/lib.rs

@@ -69,6 +69,7 @@
 #![feature(trusted_len)]
 #![feature(vec_remove_item)]
 #![feature(catch_expr)]
+#![feature(step_trait)]
 #![feature(integer_atomics)]
 #![feature(test)]
 #![feature(in_band_lifetimes)]

src/librustc_data_structures/indexed_vec.rs

@@ -89,6 +89,35 @@ macro_rules! newtype_index {
             }
         }
 
+        impl ::std::iter::Step for $type {
+            fn steps_between(start: &Self, end: &Self) -> Option<usize> {
+                <usize as ::std::iter::Step>::steps_between(
+                    &Idx::index(*start),
+                    &Idx::index(*end),
+                )
+            }
+
+            fn replace_one(&mut self) -> Self {
+                ::std::mem::replace(self, Self::new(1))
+            }
+
+            fn replace_zero(&mut self) -> Self {
+                ::std::mem::replace(self, Self::new(0))
+            }
+
+            fn add_one(&self) -> Self {
+                Self::new(Idx::index(*self) + 1)
+            }
+
+            fn sub_one(&self) -> Self {
+                Self::new(Idx::index(*self) - 1)
+            }
+
+            fn add_usize(&self, u: usize) -> Option<Self> {
+                Idx::index(*self).checked_add(u).map(Self::new)
+            }
+        }
+
         newtype_index!(
             @handle_debug
             @derives      [$($derives,)*]

src/librustc_mir/lib.rs

@@ -36,6 +36,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(specialization)]
 #![feature(try_trait)]
 #![feature(unicode_internals)]
+#![feature(step_trait)]
 
 #![recursion_limit="256"]