Optimize `array::IntoIter`

`.into_iter()` on arrays was slower than it needed to be (especially compared to slice iterator) since it uses `Range<usize>`, which needs to handle degenerate ranges like `10..4`.

This PR adds an internal `IndexRange` type that's like `Range<usize>` but with a safety invariant that means it doesn't need to worry about those cases -- it only handles `start <= end` -- and thus can give LLVM more information to optimize better.

I added one simple demonstration of the improvement as a codegen test.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/test/codegen/slice-iter-len-eq-zero.rs b/src/test/codegen/slice-iter-len-eq-zero.rs
index fd19e624cdd..1124028253d 100644
--- a/src/test/codegen/slice-iter-len-eq-zero.rs
+++ b/src/test/codegen/slice-iter-len-eq-zero.rs
@@ -1,5 +1,6 @@
 // no-system-llvm
 // compile-flags: -O
+// ignore-debug: the debug assertions add extra comparisons
 #![crate_type = "lib"]
 
 type Demo = [u8; 3];
@@ -12,3 +13,16 @@ pub fn slice_iter_len_eq_zero(y: std::slice::Iter<'_, Demo>) -> bool {
     // CHECK: ret i1 %2
     y.len() == 0
 }
+
+// CHECK-LABEL: @array_into_iter_len_eq_zero
+#[no_mangle]
+pub fn array_into_iter_len_eq_zero(y: std::array::IntoIter<Demo, 123>) -> bool {
+    // This should be able to just check that the indexes are equal, and not
+    // need any subtractions or comparisons to handle `start > end`.
+
+    // CHECK-NOT: icmp
+    // CHECK-NOT: sub
+    // CHECK: %1 = icmp eq {{i16|i32|i64}}
+    // CHECK: ret i1 %1
+    y.len() == 0
+}