Remove stabilized const_mut_refs feature

Make `GDT::append` and `GDT::push` `const` by default. The `const_fn` feature is now a no-op.

Cargo.toml

@@ -28,8 +28,9 @@ rustversion = "1.0.5"
 [features]
 default = ["nightly", "instructions"]
 instructions = []
-nightly = [ "const_fn", "step_trait", "abi_x86_interrupt", "asm_const" ]
+nightly = ["const_fn", "step_trait", "abi_x86_interrupt", "asm_const"]
 abi_x86_interrupt = []
+# deprecated, no longer needed
 const_fn = []
 asm_const = []
 step_trait = []

src/lib.rs

@@ -2,7 +2,6 @@
 //! and access to various system registers.
 
 #![cfg_attr(not(test), no_std)]
-#![cfg_attr(feature = "const_fn", feature(const_mut_refs))] // GDT::append()
 #![cfg_attr(feature = "abi_x86_interrupt", feature(abi_x86_interrupt))]
 #![cfg_attr(feature = "step_trait", feature(step_trait))]
 #![cfg_attr(feature = "doc_auto_cfg", feature(doc_auto_cfg))]

src/structures/gdt.rs

@@ -193,7 +193,6 @@ impl<const MAX: usize> GlobalDescriptorTable<MAX> {
     ///
     /// Panics if the GDT doesn't have enough free entries.
     #[inline]
-    #[cfg_attr(feature = "const_fn", rustversion::attr(all(), const))]
     pub fn append(&mut self, entry: Descriptor) -> SegmentSelector {
         let index = match entry {
             Descriptor::UserSegment(value) => {
@@ -246,7 +245,6 @@ impl<const MAX: usize> GlobalDescriptorTable<MAX> {
     }
 
     #[inline]
-    #[cfg_attr(feature = "const_fn", rustversion::attr(all(), const))]
     fn push(&mut self, value: u64) -> usize {
         let index = self.len;
         self.table[index] = Entry::new(value);