Forbid the upper indices of IndexVec indices to allow for niche optimizations

src/test/run-pass-fulldeps/newtype_index.rs

@@ -0,0 +1,20 @@
+#![feature(rustc_attrs, step_trait, rustc_private)]
+
+#[macro_use] extern crate rustc_data_structures;
+extern crate rustc_serialize;
+
+use rustc_data_structures::indexed_vec::Idx;
+
+newtype_index!(struct MyIdx { MAX = 0xFFFF_FFFA });
+
+use std::mem::size_of;
+
+fn main() {
+    assert_eq!(size_of::<MyIdx>(), 4);
+    assert_eq!(size_of::<Option<MyIdx>>(), 4);
+    assert_eq!(size_of::<Option<Option<MyIdx>>>(), 4);
+    assert_eq!(size_of::<Option<Option<Option<MyIdx>>>>(), 4);
+    assert_eq!(size_of::<Option<Option<Option<Option<MyIdx>>>>>(), 4);
+    assert_eq!(size_of::<Option<Option<Option<Option<Option<MyIdx>>>>>>(), 4);
+    assert_eq!(size_of::<Option<Option<Option<Option<Option<Option<MyIdx>>>>>>>(), 8);
+}