Deprecate the const_fn feature

Just unconditionally make the GDT methods const if we are using the
nightly compiler. This allows us to eliminate our `const_fn!` macro.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

Cargo.toml

@@ -25,6 +25,9 @@ repository = "https://github.com/rust-osdev/x86_64"
 version = "0.14.8"
 edition = "2018"
 
+[build-dependencies]
+rustversion = "1.0.5"
+
 [dependencies]
 bit_field = "0.10.1"
 bitflags = "1.0.4"
@@ -34,13 +37,13 @@ rustversion = "1.0.5"
 [features]
 default = [ "nightly", "instructions" ]
 instructions = []
-nightly = [ "const_fn", "step_trait", "abi_x86_interrupt", "doc_cfg" ]
+nightly = [ "step_trait", "abi_x86_interrupt", "doc_cfg" ]
 abi_x86_interrupt = []
-const_fn = []
 step_trait = []
 doc_cfg = []
 
 # These features are no longer used and only there for backwards compatibility.
+const_fn = []
 external_asm = []
 inline_asm = []
 

build.rs

@@ -0,0 +1,7 @@
+#[rustversion::nightly]
+fn main() {
+    println!("cargo:rustc-cfg=const_fn");
+}
+
+#[rustversion::not(nightly)]
+fn main() {}

src/lib.rs

@@ -2,7 +2,7 @@
 //! and access to various system registers.
 
 #![cfg_attr(not(test), no_std)]
-#![cfg_attr(feature = "const_fn", feature(const_mut_refs))] // GDT add_entry()
+#![cfg_attr(const_fn, feature(const_mut_refs))] // GDT add_entry()
 #![cfg_attr(feature = "abi_x86_interrupt", feature(abi_x86_interrupt))]
 #![cfg_attr(feature = "step_trait", feature(step_trait))]
 #![cfg_attr(feature = "doc_cfg", feature(doc_cfg))]
@@ -12,37 +12,6 @@
 
 pub use crate::addr::{align_down, align_up, PhysAddr, VirtAddr};
 
-/// Makes a function const only when `feature = "const_fn"` is enabled.
-///
-/// This is needed for const functions with bounds on their generic parameters,
-/// such as those in `Page` and `PhysFrame` and many more.
-macro_rules! const_fn {
-    (
-        $(#[$attr:meta])*
-        $sv:vis fn $($fn:tt)*
-    ) => {
-        $(#[$attr])*
-        #[cfg(feature = "const_fn")]
-        $sv const fn $($fn)*
-
-        $(#[$attr])*
-        #[cfg(not(feature = "const_fn"))]
-        $sv fn $($fn)*
-    };
-    (
-        $(#[$attr:meta])*
-        $sv:vis unsafe fn $($fn:tt)*
-    ) => {
-        $(#[$attr])*
-        #[cfg(feature = "const_fn")]
-        $sv const unsafe fn $($fn)*
-
-        $(#[$attr])*
-        #[cfg(not(feature = "const_fn"))]
-        $sv unsafe fn $($fn)*
-    };
-}
-
 pub mod addr;
 pub mod instructions;
 pub mod registers;

src/structures/gdt.rs

@@ -93,35 +93,34 @@ impl GlobalDescriptorTable {
         &self.table[..self.next_free]
     }
 
-    const_fn! {
-        /// Adds the given segment descriptor to the GDT, returning the segment selector.
-        ///
-        /// Panics if the GDT has no free entries left.
-        #[inline]
-        pub fn add_entry(&mut self, entry: Descriptor) -> SegmentSelector {
-            let index = match entry {
-                Descriptor::UserSegment(value) => self.push(value),
-                Descriptor::SystemSegment(value_low, value_high) => {
-                    let index = self.push(value_low);
-                    self.push(value_high);
-                    index
-                }
-            };
+    /// Adds the given segment descriptor to the GDT, returning the segment selector.
+    ///
+    /// Panics if the GDT has no free entries left.
+    #[inline]
+    #[rustversion::attr(nightly, const)]
+    pub fn add_entry(&mut self, entry: Descriptor) -> SegmentSelector {
+        let index = match entry {
+            Descriptor::UserSegment(value) => self.push(value),
+            Descriptor::SystemSegment(value_low, value_high) => {
+                let index = self.push(value_low);
+                self.push(value_high);
+                index
+            }
+        };
 
-            let rpl = match entry {
-                Descriptor::UserSegment(value) => {
-                    if DescriptorFlags::from_bits_truncate(value).contains(DescriptorFlags::DPL_RING_3)
-                    {
-                        PrivilegeLevel::Ring3
-                    } else {
-                        PrivilegeLevel::Ring0
-                    }
+        let rpl = match entry {
+            Descriptor::UserSegment(value) => {
+                if DescriptorFlags::from_bits_truncate(value).contains(DescriptorFlags::DPL_RING_3)
+                {
+                    PrivilegeLevel::Ring3
+                } else {
+                    PrivilegeLevel::Ring0
                 }
-                Descriptor::SystemSegment(_, _) => PrivilegeLevel::Ring0,
-            };
+            }
+            Descriptor::SystemSegment(_, _) => PrivilegeLevel::Ring0,
+        };
 
-            SegmentSelector::new(index as u16, rpl)
-        }
+        SegmentSelector::new(index as u16, rpl)
     }
 
     /// Loads the GDT in the CPU using the `lgdt` instruction. This does **not** alter any of the
@@ -155,17 +154,16 @@ impl GlobalDescriptorTable {
         }
     }
 
-    const_fn! {
-        #[inline]
-        fn push(&mut self, value: u64) -> usize {
-            if self.next_free < self.table.len() {
-                let index = self.next_free;
-                self.table[index] = value;
-                self.next_free += 1;
-                index
-            } else {
-                panic!("GDT full");
-            }
+    #[inline]
+    #[rustversion::attr(nightly, const)]
+    fn push(&mut self, value: u64) -> usize {
+        if self.next_free < self.table.len() {
+            let index = self.next_free;
+            self.table[index] = value;
+            self.next_free += 1;
+            index
+        } else {
+            panic!("GDT full");
         }
     }