Lower MSRV to 1.57

Makes progress on #554

Cargo.toml

@@ -20,7 +20,7 @@ authors = ["Joshua Liebow-Feeser <[email protected]>"]
 description = "Utilities for zero-copy parsing and serialization"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"
 repository = "https://github.com/google/zerocopy"
-rust-version = "1.60.0"
+rust-version = "1.57.0"
 
 exclude = [".*"]
 

src/macro_util.rs

@@ -179,7 +179,9 @@ pub const unsafe fn transmute_ref<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
     // - The caller has guaranteed that alignment is not increased.
     // - We know that the returned lifetime will not outlive the input lifetime
     //   thanks to the lifetime bounds on this function.
-    unsafe { &*dst }
+    //
+    // TODO(#67): Once our MSRV is 1.58, replace this `transmute` with `&*dst`.
+    unsafe { core::mem::transmute(dst) }
 }
 
 /// Transmutes a mutable reference of one type to a mutable reference of another

src/macros.rs

@@ -95,7 +95,7 @@ macro_rules! unsafe_impl {
         $($tyvar:ident $(: $(? $optbound:ident +)* + $($bound:ident +)* )?,)*
         => $trait:ident for $ty:ty
     ) => {
-        unsafe impl<$(const $constname: $constty,)* $($tyvar $(: $(? $optbound +)* $($bound +)*)?),*> $trait for $ty {
+        unsafe impl<$($tyvar $(: $(? $optbound +)* $($bound +)*)?),* $(, const $constname: $constty,)*> $trait for $ty {
             #[allow(clippy::missing_inline_in_public_items)]
             fn only_derive_is_allowed_to_implement_this_trait() {}
         }
@@ -212,7 +212,7 @@ macro_rules! impl_known_layout {
             use core::ptr::NonNull;
 
             // SAFETY: Delegates safety to `DstLayout::for_type`.
-            unsafe impl<$(const $constvar : $constty,)? $($tyvar $(: ?$optbound)?)?> KnownLayout for $ty {
+            unsafe impl<$($tyvar $(: ?$optbound)?)? $(, const $constvar : $constty)?> KnownLayout for $ty {
                 #[allow(clippy::missing_inline_in_public_items)]
                 fn only_derive_is_allowed_to_implement_this_trait() where Self: Sized {}
 

zerocopy-derive/Cargo.toml

@@ -14,7 +14,7 @@ authors = ["Joshua Liebow-Feeser <[email protected]>"]
 description = "Custom derive for traits from the zerocopy crate"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"
 repository = "https://github.com/google/zerocopy"
-rust-version = "1.60.0"
+rust-version = "1.57.0"
 
 exclude = [".*"]