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

src/librustc/lib.rs

@@ -59,6 +59,8 @@
 #![feature(optin_builtin_traits)]
 #![feature(refcell_replace_swap)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(rustc_attrs)]
+#![cfg_attr(stage0, feature(attr_literals))]
 #![feature(slice_patterns)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(specialization)]

src/librustc/middle/region.rs

@@ -114,7 +114,7 @@ const SCOPE_DATA_NODE: u32 = !0;
 const SCOPE_DATA_CALLSITE: u32 = !1;
 const SCOPE_DATA_ARGUMENTS: u32 = !2;
 const SCOPE_DATA_DESTRUCTION: u32 = !3;
-const SCOPE_DATA_REMAINDER_MAX: u32 = !4;
+// be sure to add the MAX of FirstStatementIndex if you add more constants here
 
 #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, RustcEncodable, RustcDecodable)]
 pub enum ScopeData {
@@ -160,9 +160,7 @@ pub struct BlockRemainder {
 }
 
 newtype_index! {
-    pub struct FirstStatementIndex {
-        MAX = SCOPE_DATA_REMAINDER_MAX
-    }
+    pub struct FirstStatementIndex;
 }
 
 impl_stable_hash_for!(struct ::middle::region::FirstStatementIndex { private });

src/librustc_data_structures/indexed_vec.rs

@@ -72,7 +72,7 @@ macro_rules! newtype_index {
         newtype_index!(
             // Leave out derives marker so we can use its absence to ensure it comes first
             @type         [$name]
-            @max          [::std::u32::MAX - 1]
+            @max          [0xFFFF_FFFE]
             @vis          [$v]
             @debug_format ["{}"]);
     );
@@ -82,7 +82,7 @@ macro_rules! newtype_index {
         newtype_index!(
             // Leave out derives marker so we can use its absence to ensure it comes first
             @type         [$name]
-            @max          [::std::u32::MAX - 1]
+            @max          [0xFFFF_FFFE]
             @vis          [$v]
             @debug_format ["{}"]
                           $($tokens)+);
@@ -97,6 +97,7 @@ macro_rules! newtype_index {
      @vis          [$v:vis]
      @debug_format [$debug_format:tt]) => (
         #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, $($derives),*)]
+        #[rustc_layout_scalar_range_end($max)]
         $v struct $type {
             private: u32
         }

src/librustc_mir/lib.rs

@@ -30,6 +30,8 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(exhaustive_patterns)]
 #![feature(range_contains)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(rustc_attrs)]
+#![cfg_attr(stage0, feature(attr_literals))]
 #![feature(never_type)]
 #![feature(specialization)]
 #![feature(try_trait)]

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);
+}