Auto merge of #108763 - scottmcm:indexing-nuw-lengths, r=cuviper

Use `nuw` when calculating slice lengths from `Range`s

An `assume` would definitely not be worth it, but since the flag is almost free we might as well tell LLVM this, especially on `_unchecked` calls where there's no obvious way for it to deduce it.

(Today neither safe nor unsafe indexing gets it: <https://rust.godbolt.org/z/G1jYT548s>)

1 files changed, 35 insertions, 0 deletions

diff --git a/tests/codegen/slice-indexing.rs b/tests/codegen/slice-indexing.rs
new file mode 100644
index 00000000000..c40d59fb0cf
--- /dev/null
+++ b/tests/codegen/slice-indexing.rs
@@ -0,0 +1,35 @@
+// compile-flags: -O
+// only-64bit (because the LLVM type of i64 for usize shows up)
+// ignore-debug: the debug assertions get in the way
+
+#![crate_type = "lib"]
+
+use std::ops::Range;
+
+// CHECK-LABEL: @index_by_range(
+#[no_mangle]
+pub fn index_by_range(x: &[u16], r: Range<usize>) -> &[u16] {
+    // CHECK: sub nuw i64
+    &x[r]
+}
+
+// CHECK-LABEL: @get_unchecked_by_range(
+#[no_mangle]
+pub unsafe fn get_unchecked_by_range(x: &[u16], r: Range<usize>) -> &[u16] {
+    // CHECK: sub nuw i64
+    x.get_unchecked(r)
+}
+
+// CHECK-LABEL: @index_mut_by_range(
+#[no_mangle]
+pub fn index_mut_by_range(x: &mut [i32], r: Range<usize>) -> &mut [i32] {
+    // CHECK: sub nuw i64
+    &mut x[r]
+}
+
+// CHECK-LABEL: @get_unchecked_mut_by_range(
+#[no_mangle]
+pub unsafe fn get_unchecked_mut_by_range(x: &mut [i32], r: Range<usize>) -> &mut [i32] {
+    // CHECK: sub nuw i64
+    x.get_unchecked_mut(r)
+}