Test that bounds checks are elided when slice len is checked up-front

src/test/codegen/issue-69101-bounds-check.rs

@@ -0,0 +1,26 @@
+// no-system-llvm
+// compile-flags: -O
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @already_sliced_no_bounds_check
+#[no_mangle]
+pub fn already_sliced_no_bounds_check(a: &[u8], b: &[u8], c: &mut [u8]) {
+    // CHECK: slice_index_len_fail
+    // CHECK-NOT: panic_bounds_check
+    let _ = (&a[..2048], &b[..2048], &mut c[..2048]);
+    for i in 0..1024 {
+        c[i] = a[i] ^ b[i];
+    }
+}
+
+// make sure we're checking for the right thing: there can be a panic if the slice is too small
+// CHECK-LABEL: @already_sliced_bounds_check
+#[no_mangle]
+pub fn already_sliced_bounds_check(a: &[u8], b: &[u8], c: &mut [u8]) {
+    // CHECK: slice_index_len_fail
+    // CHECK: panic_bounds_check
+    let _ = (&a[..1023], &b[..2048], &mut c[..2048]);
+    for i in 0..1024 {
+        c[i] = a[i] ^ b[i];
+    }
+}