clippy: prefer const fn where possible

This has the benefit that users clearly see that no side effects can happen.
However, the public API changes in the sense that once we remove const for a
function, user code might break.

Nevertheless, this is a code improvement, IMHO.

Regarding the device path code, which is generated: So far the simplest solution
is to deactivate this lint there. It is not trivial to decide which function can
be const and which not during code generation. It is future work to solve this,
if necessary.

Author: phip1611

uefi/src/data_types/mod.rs

@@ -39,7 +39,7 @@ impl Handle {
 
     /// Get the underlying raw pointer.
     #[must_use]
-    pub fn as_ptr(&self) -> *mut c_void {
+    pub const fn as_ptr(&self) -> *mut c_void {
         self.0.as_ptr()
     }
 
@@ -78,7 +78,7 @@ impl Event {
 
     /// Get the underlying raw pointer.
     #[must_use]
-    pub fn as_ptr(&self) -> *mut c_void {
+    pub const fn as_ptr(&self) -> *mut c_void {
         self.0.as_ptr()
     }
 }

uefi/src/data_types/owned_strs.rs

@@ -113,7 +113,7 @@ impl CString16 {
 
 impl Default for CString16 {
     fn default() -> Self {
-        CString16::new()
+        Self::new()
     }
 }
 
@@ -141,7 +141,7 @@ impl TryFrom<&str> for CString16 {
         // Add trailing nul.
         output.push(NUL_16);
 
-        Ok(CString16(output))
+        Ok(Self(output))
     }
 }
 
@@ -175,7 +175,7 @@ impl<'a> TryFrom<&UnalignedSlice<'a, u16>> for CString16 {
 
     fn try_from(input: &UnalignedSlice<u16>) -> Result<Self, Self::Error> {
         let v = input.to_vec();
-        CString16::try_from(v)
+        Self::try_from(v)
     }
 }
 
@@ -189,7 +189,7 @@ impl From<&CStr16> for CString16 {
 impl From<&CString16> for String {
     fn from(value: &CString16) -> Self {
         let slice: &CStr16 = value.as_ref();
-        String::from(slice)
+        Self::from(slice)
     }
 }
 

uefi/src/data_types/strs.rs

@@ -524,7 +524,7 @@ impl CStr16 {
 
     /// Returns if the string is empty. This ignores the null character.
     #[must_use]
-    pub fn is_empty(&self) -> bool {
+    pub const fn is_empty(&self) -> bool {
         self.num_chars() == 0
     }
 
@@ -566,7 +566,7 @@ impl CStr16 {
     /// Returns the underlying bytes as slice including the terminating null
     /// character.
     #[must_use]
-    pub fn as_bytes(&self) -> &[u8] {
+    pub const fn as_bytes(&self) -> &[u8] {
         unsafe { slice::from_raw_parts(self.0.as_ptr().cast(), self.num_bytes()) }
     }
 }
@@ -593,7 +593,7 @@ impl From<&CStr16> for alloc::string::String {
             .map(u16::from)
             .map(u32::from)
             .map(|int| char::from_u32(int).expect("Should be encodable as UTF-8"))
-            .collect::<alloc::string::String>()
+            .collect::<Self>()
     }
 }
 

uefi/src/data_types/unaligned_slice.rs

@@ -28,7 +28,7 @@ impl<'a, T: Copy> UnalignedSlice<'a, T> {
     /// The `data` pointer must point to a packed array of at least
     /// `len` elements of type `T`. The pointer must remain valid for as
     /// long as the `'a` lifetime.
-    pub unsafe fn new(data: *const T, len: usize) -> Self {
+    pub const unsafe fn new(data: *const T, len: usize) -> Self {
         Self {
             data,
             len,

uefi/src/fs/dir_entry_iter.rs

@@ -18,7 +18,7 @@ pub struct UefiDirectoryIter(UefiDirectoryHandle);
 impl UefiDirectoryIter {
     /// Constructor.
     #[must_use]
-    pub fn new(handle: UefiDirectoryHandle) -> Self {
+    pub const fn new(handle: UefiDirectoryHandle) -> Self {
         Self(handle)
     }
 }

uefi/src/fs/file_system/fs.rs

@@ -26,7 +26,7 @@ pub struct FileSystem<'a>(ScopedProtocol<'a, SimpleFileSystemProtocol>);
 impl<'a> FileSystem<'a> {
     /// Constructor.
     #[must_use]
-    pub fn new(proto: ScopedProtocol<'a, SimpleFileSystemProtocol>) -> Self {
+    pub const fn new(proto: ScopedProtocol<'a, SimpleFileSystemProtocol>) -> Self {
         Self(proto)
     }
 

uefi/src/fs/path/path.rs

@@ -21,7 +21,7 @@ impl Path {
 
     /// Returns the underlying string.
     #[must_use]
-    pub fn to_cstr16(&self) -> &CStr16 {
+    pub const fn to_cstr16(&self) -> &CStr16 {
         &self.0
     }
 
@@ -79,7 +79,7 @@ impl Path {
 
     /// Returns of the path is empty.
     #[must_use]
-    pub fn is_empty(&self) -> bool {
+    pub const fn is_empty(&self) -> bool {
         self.to_cstr16().is_empty()
     }
 }

uefi/src/helpers/mod.rs

@@ -54,6 +54,7 @@ pub fn system_table() -> SystemTable<Boot> {
 /// **PLEASE NOTE** that these helpers are meant for the pre exit boot service
 /// epoch. Limited functionality might work after exiting them, such as logging
 /// to the debugcon device.
+#[allow(clippy::missing_const_for_fn)]
 pub fn init() -> Result<()> {
     // Setup logging and memory allocation
 
@@ -72,6 +73,7 @@ pub fn init() -> Result<()> {
     Ok(())
 }
 
+#[allow(clippy::missing_const_for_fn)]
 pub(crate) fn exit() {
     #[cfg(feature = "logger")]
     logger::disable();

uefi/src/lib.rs

@@ -92,6 +92,7 @@
 #![deny(
     clippy::must_use_candidate,
     clippy::use_self,
+    clippy::missing_const_for_fn
 )]
 #![deny(missing_debug_implementations)]
 

uefi/src/mem/memory_map/impl_.rs

@@ -254,7 +254,7 @@ impl MemoryMapBackingMemory {
     /// takes into account that, as you go, more (small) allocations might
     /// happen.
     #[must_use]
-    fn safe_allocation_size_hint(mmm: MemoryMapMeta) -> usize {
+    const fn safe_allocation_size_hint(mmm: MemoryMapMeta) -> usize {
         // Allocate space for extra entries beyond the current size of the
         // memory map. The value of 8 matches the value in the Linux kernel:
         // https://github.com/torvalds/linux/blob/e544a07438/drivers/firmware/efi/libstub/efistub.h#L173
@@ -308,7 +308,7 @@ impl MemoryMapOwned {
     pub(crate) fn from_initialized_mem(buf: MemoryMapBackingMemory, meta: MemoryMapMeta) -> Self {
         assert!(meta.desc_size >= mem::size_of::<MemoryDescriptor>());
         let len = meta.entry_count();
-        MemoryMapOwned {
+        Self {
             key: MemoryMapKey(0),
             buf,
             meta,

uefi/src/proto/console/gop.rs

@@ -102,7 +102,7 @@ impl GraphicsOutput {
 
     /// Returns a [`ModeIter`].
     #[must_use]
-    pub fn modes<'a>(&'a self, bs: &'a BootServices) -> ModeIter {
+    pub const fn modes<'a>(&'a self, bs: &'a BootServices) -> ModeIter {
         ModeIter {
             gop: self,
             bs,

uefi/src/proto/device_path/device_path_gen.rs

@@ -4,6 +4,7 @@
 // `cargo xtask gen-code`
 //
 // See `/xtask/src/device_path/README.md` for more details.
+#![allow(clippy::missing_const_for_fn)]
 
 use crate::data_types::UnalignedSlice;
 use crate::mem::memory_map::MemoryType;

uefi/src/proto/device_path/mod.rs

@@ -171,7 +171,7 @@ impl DevicePathNode {
     /// remain valid for the lifetime `'a`, and cannot be mutated during
     /// that lifetime.
     #[must_use]
-    pub unsafe fn from_ffi_ptr<'a>(ptr: *const FfiDevicePath) -> &'a DevicePathNode {
+    pub unsafe fn from_ffi_ptr<'a>(ptr: *const FfiDevicePath) -> &'a Self {
         let header = *ptr.cast::<DevicePathHeader>();
 
         let data_len = usize::from(header.length) - mem::size_of::<DevicePathHeader>();
@@ -217,7 +217,7 @@ impl DevicePathNode {
 
     /// Returns the payload data of this node.
     #[must_use]
-    pub fn data(&self) -> &[u8] {
+    pub const fn data(&self) -> &[u8] {
         &self.data
     }
 
@@ -339,7 +339,7 @@ impl PartialEq for DevicePathInstance {
 
 #[cfg(feature = "alloc")]
 impl ToOwned for DevicePathInstance {
-    type Owned = Box<DevicePathInstance>;
+    type Owned = Box<Self>;
 
     fn to_owned(&self) -> Self::Owned {
         self.to_boxed()
@@ -433,7 +433,7 @@ impl DevicePath {
     /// remain valid for the lifetime `'a`, and cannot be mutated during
     /// that lifetime.
     #[must_use]
-    pub unsafe fn from_ffi_ptr<'a>(ptr: *const FfiDevicePath) -> &'a DevicePath {
+    pub unsafe fn from_ffi_ptr<'a>(ptr: *const FfiDevicePath) -> &'a Self {
         &*Self::ptr_from_ffi(ptr.cast::<c_void>())
     }
 
@@ -527,7 +527,7 @@ impl<'a> TryFrom<&[u8]> for &'a DevicePath {
 
 #[cfg(feature = "alloc")]
 impl ToOwned for DevicePath {
-    type Owned = Box<DevicePath>;
+    type Owned = Box<Self>;
 
     fn to_owned(&self) -> Self::Owned {
         self.to_boxed()

uefi/src/proto/loaded_image.rs

@@ -185,7 +185,7 @@ impl LoadedImage {
     ///  - `MemoryType::BOOT_SERVICES_CODE` for UEFI boot drivers
     ///  - `MemoryType::RUNTIME_SERVICES_CODE` for UEFI runtime drivers
     #[must_use]
-    pub fn code_type(&self) -> MemoryType {
+    pub const fn code_type(&self) -> MemoryType {
         self.0.image_code_type
     }
 
@@ -196,7 +196,7 @@ impl LoadedImage {
     ///  - `MemoryType::BOOT_SERVICES_DATA` for UEFI boot drivers
     ///  - `MemoryType::RUNTIME_SERVICES_DATA` for UEFI runtime drivers
     #[must_use]
-    pub fn data_type(&self) -> MemoryType {
+    pub const fn data_type(&self) -> MemoryType {
         self.0.image_data_type
     }
 }

uefi/src/proto/media/file/dir.rs

@@ -22,7 +22,7 @@ impl Directory {
     /// This function should only be called on files which ARE directories,
     /// doing otherwise is unsafe.
     #[must_use]
-    pub unsafe fn new(handle: FileHandle) -> Self {
+    pub const unsafe fn new(handle: FileHandle) -> Self {
         Self(RegularFile::new(handle))
     }
 

uefi/src/proto/media/file/info.rs

@@ -248,13 +248,13 @@ impl FileInfo {
 
     /// Returns if the file is a directory.
     #[must_use]
-    pub fn is_directory(&self) -> bool {
+    pub const fn is_directory(&self) -> bool {
         self.attribute.contains(FileAttribute::DIRECTORY)
     }
 
     /// Returns if the file is a regular file.
     #[must_use]
-    pub fn is_regular_file(&self) -> bool {
+    pub const fn is_regular_file(&self) -> bool {
         !self.is_directory()
     }
 }

uefi/src/proto/media/file/regular.rs

@@ -19,7 +19,7 @@ impl RegularFile {
     /// This function should only be called on handles which ARE NOT directories,
     /// doing otherwise is unsafe.
     #[must_use]
-    pub unsafe fn new(handle: FileHandle) -> Self {
+    pub const unsafe fn new(handle: FileHandle) -> Self {
         Self(handle)
     }