Attach min_const_generics to a crate feature (#63)

* Attach min_const_generics to a crate feature

* Remove `min_const_generics` detection logic

Cargo.toml

@@ -17,6 +17,7 @@ extern_crate_alloc = []
 extern_crate_std = ["extern_crate_alloc"]
 zeroable_maybe_uninit = []
 derive = ["bytemuck_derive"]
+min_const_generics = []
 
 [dependencies]
 # use the upper line for testing against bytemuck_derive changes, if any

build.rs

@@ -1,45 +0,0 @@
-use std::{
-  env,
-  process::Command,
-  str::{self, FromStr},
-};
-
-fn main() {
-  let minor = match rustc_minor_version() {
-    Some(minor) => minor,
-    None => return,
-  };
-
-  if minor >= 51 {
-    println!("cargo:rustc-cfg=min_const_generics");
-  }
-}
-
-fn rustc_minor_version() -> Option<u32> {
-  let rustc = match env::var_os("RUSTC") {
-    Some(rustc) => rustc,
-    None => return None,
-  };
-
-  let output = match Command::new(rustc).arg("--version").output() {
-    Ok(output) => output,
-    Err(_) => return None,
-  };
-
-  let version = match str::from_utf8(&output.stdout) {
-    Ok(version) => version,
-    Err(_) => return None,
-  };
-
-  let mut pieces = version.split('.');
-  if pieces.next() != Some("rustc 1") {
-    return None;
-  }
-
-  let next = match pieces.next() {
-    Some(next) => next,
-    None => return None,
-  };
-
-  u32::from_str(next).ok()
-}

src/lib.rs

@@ -50,7 +50,7 @@ use core::{marker::*, mem::*, num::*, ptr::*};
 #[doc(hidden)]
 pub use ::core as __core;
 
-#[cfg(not(generic_const))]
+#[cfg(not(feature = "min_const_generics"))]
 macro_rules! impl_unsafe_marker_for_array {
   ( $marker:ident , $( $n:expr ),* ) => {
     $(unsafe impl<T> $marker for [T; $n] where T: $marker {})*

src/pod.rs

@@ -66,10 +66,10 @@ unsafe impl<T: Pod> Pod for ManuallyDrop<T> {}
 
 // Note(Lokathor): MaybeUninit can NEVER be Pod.
 
-#[cfg(min_const_generics)]
+#[cfg(feature = "min_const_generics")]
 unsafe impl<T, const N: usize> Pod for [T; N] where T: Pod {}
 
-#[cfg(not(min_const_generics))]
+#[cfg(not(feature = "min_const_generics"))]
 impl_unsafe_marker_for_array!(
   Pod, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
   20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 48, 64, 96, 128, 256,

src/zeroable.rs

@@ -109,10 +109,10 @@ unsafe impl<
 {
 }
 
-#[cfg(min_const_generics)]
+#[cfg(feature = "min_const_generics")]
 unsafe impl<T, const N: usize> Zeroable for [T; N] where T: Zeroable {}
 
-#[cfg(not(min_const_generics))]
+#[cfg(not(feature = "min_const_generics"))]
 impl_unsafe_marker_for_array!(
   Zeroable, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
   19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 48, 64, 96, 128, 256,

tests/array_tests.rs

@@ -0,0 +1,12 @@
+#[test]
+pub fn test_cast_array() {
+  let x = [0u32, 1u32, 2u32];
+  let _: [u16; 6] = bytemuck::cast(x);
+}
+
+#[cfg(feature = "min_const_generics")]
+#[test]
+pub fn test_cast_long_array() {
+  let x = [0u32; 65];
+  let _: [u16; 130] = bytemuck::cast(x);
+}