Automatically enable const evaluation

When running on a compiler newer than 1.65,
automatically enable usage of the macro in const contexts.

Closes #4

README.md

@@ -45,12 +45,18 @@ fn main() {
 }
 ```
 
-## Feature flags ##
+## Usage in constants ##
+`memoffset` has support for compile-time `offset_of!` on rust>=1.65, or on older nightly compilers.
 
-### Usage in constants ###
-`memoffset` has **experimental** support for compile-time `offset_of!` on a nightly compiler.
+Stable support for const evaluation is not as complete as unstable due to dependence on [`#![feature(const_refs_to_cell)]`](https://github.com/rust-lang/rust/issues/80384).
+This means that if need to get the offset of a cell, you'll have to remain on nightly for now.
 
-In order to use it, you must enable the `unstable_const` crate feature and several compiler features.
+### How to enable on stable ###
+If you're running on any compiler newer than 1.65, there's no need to enable anything.
+
+### How to enable on an older nightly ###
+In order to use it on an older nightly compiler, you must enable the `unstable_const` crate feature and several compiler features.
+Do note that `unstable_const` is an unstable feature that is set to be removed in a future version of `memoffset`.
 
 Cargo.toml:
 ```toml

build.rs

@@ -19,4 +19,7 @@ fn main() {
     if ac.probe_rustc_version(1, 51) {
         println!("cargo:rustc-cfg=raw_ref_macros");
     }
+    if ac.probe_rustc_version(1, 65) {
+        println!("cargo:rustc-cfg=stable_const");
+    }
 }

src/lib.rs

@@ -57,9 +57,10 @@
 
 #![no_std]
 #![cfg_attr(
-    feature = "unstable_const",
-    feature(const_ptr_offset_from, const_refs_to_cell)
+    all(feature = "unstable_const", not(stable_const)),
+    feature(const_ptr_offset_from)
 )]
+#![cfg_attr(feature = "unstable_const", feature(const_refs_to_cell))]
 
 #[macro_use]
 #[cfg(doctests)]

src/offset_of.rs

@@ -46,7 +46,7 @@ macro_rules! _memoffset__let_base_ptr {
 }
 
 /// Macro to compute the distance between two pointers.
-#[cfg(feature = "unstable_const")]
+#[cfg(any(feature = "unstable_const", stable_const))]
 #[macro_export]
 #[doc(hidden)]
 macro_rules! _memoffset_offset_from_unsafe {
@@ -58,7 +58,7 @@ macro_rules! _memoffset_offset_from_unsafe {
         unsafe { (field as *const u8).offset_from(base as *const u8) as usize }
     }};
 }
-#[cfg(not(feature = "unstable_const"))]
+#[cfg(not(any(feature = "unstable_const", stable_const)))]
 #[macro_export]
 #[doc(hidden)]
 macro_rules! _memoffset_offset_from_unsafe {
@@ -312,7 +312,7 @@ mod tests {
         assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, c) as usize);
     }
 
-    #[cfg(feature = "unstable_const")]
+    #[cfg(any(feature = "unstable_const", stable_const))]
     #[test]
     fn const_offset() {
         #[repr(C)]
@@ -337,7 +337,7 @@ mod tests {
         assert_eq!([0; offset_of!(Foo, b)].len(), 4);
     }
 
-    #[cfg(feature = "unstable_const")]
+    #[cfg(any(feature = "unstable_const", stable_const))]
     #[test]
     fn const_fn_offset() {
         const fn test_fn() -> usize {