Create align_offset feature so that we can continue to work on 1.34

Cargo.toml

@@ -18,12 +18,19 @@ extern_crate_alloc = []
 extern_crate_std = ["extern_crate_alloc"]
 zeroable_maybe_uninit = []
 zeroable_atomics = []
-min_const_generics = []
+
+# Causes MSRV 1.36, use `align_offset` method instead of casting to `usize` to
+# check alignment of pointers, this *may* improve codegen in some cases (but it
+# has never been formally benchmarked!)
+align_offset = []
+
+min_const_generics = [] # Causes MSRV 1.51
+
 wasm_simd = []    # Until >= 1.54.0 is MSRV this is an off-by-default feature.
 must_cast = []    # Until >= 1.57.0 is MSRV this is an off-by-default feature.
 aarch64_simd = [] # Until >= 1.59.0 is MSRV this is an off-by-default feature.
 
-# Do not use if you can avoid it, because this is unsound.
+# Do not use if you can avoid it, because this is **unsound**!!!!
 unsound_ptr_pod_impl = []
 
 # NOT SEMVER SUPPORTED! TEMPORARY ONLY!

src/internal.rs

@@ -136,11 +136,18 @@ pub(crate) unsafe fn pod_read_unaligned<T: Copy>(bytes: &[u8]) -> T {
 /// * If `align` is not a power of two. This includes when `align` is zero.
 #[inline]
 pub(crate) fn is_aligned_to(ptr: *const (), align: usize) -> bool {
+  #[cfg(feature = "align_offset")]
+  {
     // This is in a way better than `ptr as usize % align == 0`,
-  // because casting a pointer to an integer has the side effect that it exposes
-  // the pointer's provenance, which may theoretically inhibit some compiler
-  // optimizations.
+    // because casting a pointer to an integer has the side effect that it
+    // exposes the pointer's provenance, which may theoretically inhibit
+    // some compiler optimizations.
     ptr.align_offset(align) == 0
+  }
+  #[cfg(not(feature = "align_offset"))]
+  {
+    ((ptr as usize) % align) == 0
+  }
 }
 
 /// Re-interprets `&[u8]` as `&T`.