Cleanup const_fn!

Fixes #222

The following methods can be made `const` on stable:
  - `GlobalDescriptorTable::from_raw_slice`
  - `MappedFrame::{start_address, size}`
  - `Page<Size4KiB>::p1_index`

The remaining functions still need `const_fn` because:
  - Some GDT methods use `&mut self`
  - The IDT uses function pointers as a type argument
  - The PageSize marker trait is used all over

Comments were updated where appropriate.

Signed-off-by: Joe Richey <[email protected]>

src/lib.rs

@@ -2,10 +2,10 @@
 //! and access to various system registers.
 
 #![cfg_attr(not(test), no_std)]
-#![cfg_attr(feature = "const_fn", feature(const_panic))]
-#![cfg_attr(feature = "const_fn", feature(const_mut_refs))]
-#![cfg_attr(feature = "const_fn", feature(const_fn_fn_ptr_basics))]
-#![cfg_attr(feature = "const_fn", feature(const_fn_trait_bound))]
+#![cfg_attr(feature = "const_fn", feature(const_panic))] // Better panic messages
+#![cfg_attr(feature = "const_fn", feature(const_mut_refs))] // GDT add_entry()
+#![cfg_attr(feature = "const_fn", feature(const_fn_fn_ptr_basics))] // IDT new()
+#![cfg_attr(feature = "const_fn", feature(const_fn_trait_bound))] // PageSize marker trait
 #![cfg_attr(feature = "inline_asm", feature(asm))]
 #![cfg_attr(feature = "abi_x86_interrupt", feature(abi_x86_interrupt))]
 #![cfg_attr(docsrs, feature(doc_cfg))]

src/structures/gdt.rs

@@ -112,12 +112,15 @@ impl GlobalDescriptorTable {
     /// * The user must make sure that the entries are well formed
     /// * The provided slice **must not be larger than 8 items** (only up to the first 8 will be observed.)
     #[inline]
-    #[cfg(feature = "const_fn")]
     pub const unsafe fn from_raw_slice(slice: &[u64]) -> GlobalDescriptorTable {
+        #[cfg(feature = "const_fn")]
         assert!(
             slice.len() <= 8,
             "initializing a GDT from a slice requires it to be **at most** 8 elements."
         );
+        #[cfg(not(feature = "const_fn"))]
+        slice[7]; // Will fail if len > 8
+
         let next_free = slice.len();
 
         let mut table = [0; 8];

src/structures/paging/frame.rs

@@ -11,7 +11,7 @@ use core::ops::{Add, AddAssign, Sub, SubAssign};
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 #[repr(C)]
 pub struct PhysFrame<S: PageSize = Size4KiB> {
-    start_address: PhysAddr,
+    pub(crate) start_address: PhysAddr, // TODO: remove when start_address() is const
     size: PhantomData<S>,
 }
 

src/structures/paging/mapper/mod.rs

@@ -85,25 +85,21 @@ pub enum MappedFrame {
 }
 
 impl MappedFrame {
-    const_fn! {
-        /// Returns the start address of the frame.
-        pub fn start_address(&self) -> PhysAddr {
-            match self {
-                MappedFrame::Size4KiB(frame) => frame.start_address(),
-                MappedFrame::Size2MiB(frame) => frame.start_address(),
-                MappedFrame::Size1GiB(frame) => frame.start_address(),
-            }
+    /// Returns the start address of the frame.
+    pub const fn start_address(&self) -> PhysAddr {
+        match self {
+            MappedFrame::Size4KiB(frame) => frame.start_address,
+            MappedFrame::Size2MiB(frame) => frame.start_address,
+            MappedFrame::Size1GiB(frame) => frame.start_address,
         }
     }
 
-    const_fn! {
-        /// Returns the size the frame (4KB, 2MB or 1GB).
-        pub fn size(&self) -> u64 {
-            match self {
-                MappedFrame::Size4KiB(frame) => frame.size(),
-                MappedFrame::Size2MiB(frame) => frame.size(),
-                MappedFrame::Size1GiB(frame) => frame.size(),
-            }
+    /// Returns the size the frame (4KB, 2MB or 1GB).
+    pub const fn size(&self) -> u64 {
+        match self {
+            MappedFrame::Size4KiB(_) => Size4KiB::SIZE,
+            MappedFrame::Size2MiB(_) => Size2MiB::SIZE,
+            MappedFrame::Size1GiB(_) => Size1GiB::SIZE,
         }
     }
 }

src/structures/paging/page.rs

@@ -210,12 +210,10 @@ impl Page<Size4KiB> {
         Page::containing_address(VirtAddr::new(addr))
     }
 
-    const_fn! {
-        /// Returns the level 1 page table index of this page.
-        #[inline]
-        pub fn p1_index(self) -> PageTableIndex {
-            self.start_address().p1_index()
-        }
+    /// Returns the level 1 page table index of this page.
+    #[inline]
+    pub const fn p1_index(self) -> PageTableIndex {
+        self.start_address.p1_index()
     }
 }
 

src/structures/paging/page_table.rs

@@ -189,7 +189,6 @@ pub struct PageTable {
 
 impl PageTable {
     /// Creates an empty page table.
-    #[cfg(feature = "const_fn")]
     #[inline]
     pub const fn new() -> Self {
         const EMPTY: PageTableEntry = PageTableEntry::new();
@@ -198,16 +197,6 @@ impl PageTable {
         }
     }
 
-    /// Creates an empty page table.
-    #[cfg(not(feature = "const_fn"))]
-    #[inline]
-    pub fn new() -> Self {
-        const EMPTY: PageTableEntry = PageTableEntry::new();
-        PageTable {
-            entries: [EMPTY; ENTRY_COUNT],
-        }
-    }
-
     /// Clears all entries.
     #[inline]
     pub fn zero(&mut self) {