Rollup merge of rust-lang#145411 - the8472:cows-have-no-branches, r=Mark-Simulacrum

regression test for Cow<[u8]> layout

requested by ralf in rust-lang#117763 (comment)

Author: Zalathar

tests/codegen-llvm/issues/cows-dont-have-branches-117763.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -Copt-level=3
+//@ needs-deterministic-layouts
+
+// Currently Vec<T> and &[T] have layouts that start with (pointer, len)
+// which makes the conversion branchless.
+// A nice-to-have property, not guaranteed.
+#![crate_type = "cdylib"]
+
+// CHECK-LABEL: @branchless_cow_slices
+#[no_mangle]
+pub fn branchless_cow_slices<'a>(cow: &'a std::borrow::Cow<'a, [u8]>) -> &'a [u8] {
+    // CHECK-NOT: br
+    // CHECK-NOT: select
+    // CHECK-NOT: icmp
+    // CHECK: ret { ptr, {{i32|i64}} }
+    &*cow
+}