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

@@ -206,7 +206,7 @@ impl IndexMut<usize> for MemoryMapRefMut<'_> {
 /// [`BootServices::get_memory_map`]: crate::table::boot::BootServices::get_memory_map
 #[derive(Debug)]
 #[allow(clippy::len_without_is_empty)] // this type is never empty
-pub(crate) struct MemoryMapBackingMemory(NonNull<[u8]>);
+pub struct MemoryMapBackingMemory(NonNull<[u8]>);
 
 impl MemoryMapBackingMemory {
     /// Constructs a new [`MemoryMapBackingMemory`].
@@ -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,