Auto merge of #78818 - scottmcm:as_rchunks, r=KodrAus

Add `as_rchunks` (and friends) to slices

`@est31` mentioned (https://github.com/rust-lang/rust/issues/76354#issuecomment-717027175) that, for completeness, there needed to be an `as_chunks`-like method that chunks from the end (with the remainder at the beginning) like `rchunks` does.

So here's a PR for `as_rchunks: &[T] -> (&[T], &[[T; N]])` and `as_rchunks_mut: &mut [T] -> (&mut [T], &mut [[T; N]])`.

But as I was doing this and copy-pasting `from_raw_parts` calls, I thought that I should extract that into an unsafe method.  It started out a private helper, but it seemed like `as_chunks_unchecked` could be reasonable as a "real" method, so I added docs and made it public.  Let me know if you think it doesn't pull its weight.

1 files changed, 33 insertions, 0 deletions

diff --git a/src/test/codegen/slice-as_chunks.rs b/src/test/codegen/slice-as_chunks.rs
new file mode 100644
index 00000000000..48e3f73fc86
--- /dev/null
+++ b/src/test/codegen/slice-as_chunks.rs
@@ -0,0 +1,33 @@
+// no-system-llvm
+// 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"]
+#![feature(slice_as_chunks)]
+
+// CHECK-LABEL: @chunks4
+#[no_mangle]
+pub fn chunks4(x: &[u8]) -> &[[u8; 4]] {
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: lshr i64 %x.1, 2
+    // CHECK-NOT: shl
+    // CHECK-NOT: mul
+    // CHECK-NOT: udiv
+    // CHECK-NOT: urem
+    // CHECK: ret
+    x.as_chunks().0
+}
+
+// CHECK-LABEL: @chunks4_with_remainder
+#[no_mangle]
+pub fn chunks4_with_remainder(x: &[u8]) -> (&[[u8; 4]], &[u8]) {
+    // CHECK: and i64 %x.1, -4
+    // CHECK: and i64 %x.1, 3
+    // CHECK: lshr exact
+    // CHECK-NOT: mul
+    // CHECK-NOT: udiv
+    // CHECK-NOT: urem
+    // CHECK: ret
+    x.as_chunks()
+}