[RFC PATCH v3 05/22] rust: hex: add encode_hex_iter and encode_hex_upper_iter methods

[Ariel Miculas <amiculas@cisco.com>]

Add encode_hex_iter and encode_hex_upper_iter which return an iterator,
so it can be used in conjunction with the Vec::from_iter_fallible
function to (try to) create a hex string.

Replace BytesToHexChars with BytesToHexByteSequence because String is
not available. We need to collect the values in a Vec<u8>, append a NUL
character and the create a CStr on top of it.

Also remove the ToHex trait because it uses the FromIterator trait which
cannot be implemented in the kernel due to the prohibition of infallible
allocations.

Signed-off-by: Ariel Miculas <amiculas@cisco.com>
---
 rust/hex/lib.rs | 60 ++++++++++++-------------------------------------
 1 file changed, 14 insertions(+), 46 deletions(-)

diff --git a/rust/hex/lib.rs b/rust/hex/lib.rs
index a23def3f80db..3abf61ccbfc3 100644
--- a/rust/hex/lib.rs
+++ b/rust/hex/lib.rs
@@ -57,43 +57,18 @@
 #[cfg(all(feature = "alloc", feature = "serde"))]
 pub use crate::serde::{serialize, serialize_upper};
 
-/// Encoding values as hex string.
-///
-/// This trait is implemented for all `T` which implement `AsRef<[u8]>`. This
-/// includes `String`, `str`, `Vec<u8>` and `[u8]`.
-///
-/// # Example
-///
-/// ```
-/// use hex::ToHex;
-///
-/// println!("{}", "Hello world!".encode_hex::<String>());
-/// # assert_eq!("Hello world!".encode_hex::<String>(), "48656c6c6f20776f726c6421".to_string());
-/// ```
-///
-/// *Note*: instead of using this trait, you might want to use [`encode()`].
-pub trait ToHex {
-    /// Encode the hex strict representing `self` into the result. Lower case
-    /// letters are used (e.g. `f9b4ca`)
-    fn encode_hex<T: iter::FromIterator<char>>(&self) -> T;
-
-    /// Encode the hex strict representing `self` into the result. Upper case
-    /// letters are used (e.g. `F9B4CA`)
-    fn encode_hex_upper<T: iter::FromIterator<char>>(&self) -> T;
-}
-
 const HEX_CHARS_LOWER: &[u8; 16] = b"0123456789abcdef";
 const HEX_CHARS_UPPER: &[u8; 16] = b"0123456789ABCDEF";
 
-struct BytesToHexChars<'a> {
+struct BytesToHexByteSequence<'a> {
     inner: ::core::slice::Iter<'a, u8>,
     table: &'static [u8; 16],
-    next: Option<char>,
+    next: Option<u8>,
 }
 
-impl<'a> BytesToHexChars<'a> {
-    fn new(inner: &'a [u8], table: &'static [u8; 16]) -> BytesToHexChars<'a> {
-        BytesToHexChars {
+impl<'a> BytesToHexByteSequence<'a> {
+    fn new(inner: &'a [u8], table: &'static [u8; 16]) -> BytesToHexByteSequence<'a> {
+        BytesToHexByteSequence {
             inner: inner.iter(),
             table,
             next: None,
@@ -101,15 +76,15 @@ impl<'a> BytesToHexChars<'a> {
     }
 }
 
-impl<'a> Iterator for BytesToHexChars<'a> {
-    type Item = char;
+impl<'a> Iterator for BytesToHexByteSequence<'a> {
+    type Item = u8;
 
     fn next(&mut self) -> Option<Self::Item> {
         match self.next.take() {
             Some(current) => Some(current),
             None => self.inner.next().map(|byte| {
-                let current = self.table[(byte >> 4) as usize] as char;
-                self.next = Some(self.table[(byte & 0x0F) as usize] as char);
+                let current = self.table[(byte >> 4) as usize];
+                self.next = Some(self.table[(byte & 0x0F) as usize]);
                 current
             }),
         }
@@ -121,7 +96,7 @@ fn size_hint(&self) -> (usize, Option<usize>) {
     }
 }
 
-impl<'a> iter::ExactSizeIterator for BytesToHexChars<'a> {
+impl<'a> iter::ExactSizeIterator for BytesToHexByteSequence<'a> {
     fn len(&self) -> usize {
         let mut length = self.inner.len() * 2;
         if self.next.is_some() {
@@ -131,19 +106,12 @@ fn len(&self) -> usize {
     }
 }
 
-#[inline]
-fn encode_to_iter<T: iter::FromIterator<char>>(table: &'static [u8; 16], source: &[u8]) -> T {
-    BytesToHexChars::new(source, table).collect()
+pub fn encode_hex_iter<'a>(source: &'a [u8]) -> impl iter::Iterator<Item = u8> + 'a {
+    BytesToHexByteSequence::new(source, HEX_CHARS_LOWER).into_iter()
 }
 
-impl<T: AsRef<[u8]>> ToHex for T {
-    fn encode_hex<U: iter::FromIterator<char>>(&self) -> U {
-        encode_to_iter(HEX_CHARS_LOWER, self.as_ref())
-    }
-
-    fn encode_hex_upper<U: iter::FromIterator<char>>(&self) -> U {
-        encode_to_iter(HEX_CHARS_UPPER, self.as_ref())
-    }
+pub fn encode_hex_upper_iter<'a>(source: &'a [u8]) -> impl iter::Iterator<Item = u8> + 'a {
+    BytesToHexByteSequence::new(source, HEX_CHARS_UPPER).into_iter()
 }
 
 /// Types that can be decoded from a hex string.
-- 
2.34.1