Introduce Cursor and CursorMut traits

src/buf/chain.rs

@@ -1,5 +1,5 @@
 use crate::buf::{IntoIter, UninitSlice};
-use crate::{Buf, BufMut};
+use crate::{Buf, BufMut, Cursor, CursorMut};
 
 #[cfg(feature = "std")]
 use std::io::IoSlice;
@@ -226,6 +226,33 @@ where
     }
 }
 
+impl<T: Cursor, U: Cursor> Cursor for Chain<T, U> {
+    type Cursor<'a>
+        = Chain<T::Cursor<'a>, U::Cursor<'a>>
+    where
+        Self: 'a;
+
+    fn cursor(&self, index: usize) -> Self::Cursor<'_> {
+        let mut chain = self.a.cursor(0).chain(self.b.cursor(0));
+        chain.advance(index);
+        chain
+    }
+}
+
+unsafe impl<T: CursorMut, U: CursorMut> CursorMut for Chain<T, U> {
+    type CursorMut<'a>
+        = Chain<T::CursorMut<'a>, U::CursorMut<'a>>
+    where
+        Self: 'a;
+
+    fn cursor_mut(&mut self, index: usize) -> Self::CursorMut<'_> {
+        let mut chain = self.a.cursor_mut(0).chain_mut(self.b.cursor_mut(0));
+        // SAFETY: advance_mut must be safe on CursorMut::CursorMut
+        unsafe { chain.advance_mut(index) };
+        chain
+    }
+}
+
 impl<T, U> IntoIterator for Chain<T, U>
 where
     T: Buf,

src/buf/cursor.rs

@@ -0,0 +1,35 @@
+use crate::{buf::Chain, Buf};
+use std::collections::VecDeque;
+
+pub trait Cursor: Buf {
+    type Cursor<'a>: Buf
+    where
+        Self: 'a;
+
+    fn cursor(&self, index: usize) -> Self::Cursor<'_>;
+}
+
+impl Cursor for &[u8] {
+    type Cursor<'a>
+        = &'a [u8]
+    where
+        Self: 'a;
+
+    fn cursor(&self, index: usize) -> Self::Cursor<'_> {
+        &self[index..]
+    }
+}
+
+impl Cursor for VecDeque<u8> {
+    type Cursor<'a>
+        = Chain<&'a [u8], &'a [u8]>
+    where
+        Self: 'a;
+
+    fn cursor(&self, index: usize) -> Self::Cursor<'_> {
+        let (s1, s2) = self.as_slices();
+        let mut chain = s1.chain(s2);
+        chain.advance(index);
+        chain
+    }
+}

src/buf/cursor_mut.rs

@@ -0,0 +1,26 @@
+use crate::BufMut;
+
+/// # Safety
+///
+/// It must be safe to call Self::CursorMut::advance_mut. This should be true
+/// if the type Self::CursorMut does not allow to access bytes writen to it.
+/// For exemple, Vec<u8> is not ok, as the content written to it is accesible.
+/// On the other hand &mut [u8] is ok, as the content written cant't be accessed.
+pub unsafe trait CursorMut: BufMut {
+    type CursorMut<'a>: BufMut
+    where
+        Self: 'a;
+
+    fn cursor_mut(&mut self, index: usize) -> Self::CursorMut<'_>;
+}
+
+unsafe impl CursorMut for &mut [u8] {
+    type CursorMut<'a>
+        = &'a mut [u8]
+    where
+        Self: 'a;
+
+    fn cursor_mut(&mut self, index: usize) -> Self::CursorMut<'_> {
+        &mut self[index..]
+    }
+}

src/buf/limit.rs

@@ -1,5 +1,6 @@
 use crate::buf::UninitSlice;
 use crate::BufMut;
+use crate::CursorMut;
 
 use core::cmp;
 
@@ -73,3 +74,14 @@ unsafe impl<T: BufMut> BufMut for Limit<T> {
         self.limit -= cnt;
     }
 }
+
+unsafe impl<T: CursorMut> CursorMut for Limit<T> {
+    type CursorMut<'a>
+        = Limit<T::CursorMut<'a>>
+    where
+        Self: 'a;
+
+    fn cursor_mut(&mut self, index: usize) -> Self::CursorMut<'_> {
+        self.inner.cursor_mut(index).limit(self.limit - index)
+    }
+}

src/buf/mod.rs

@@ -17,6 +17,8 @@
 mod buf_impl;
 mod buf_mut;
 mod chain;
+mod cursor;
+mod cursor_mut;
 mod iter;
 mod limit;
 #[cfg(feature = "std")]
@@ -30,6 +32,8 @@ mod writer;
 pub use self::buf_impl::Buf;
 pub use self::buf_mut::BufMut;
 pub use self::chain::Chain;
+pub use self::cursor::Cursor;
+pub use self::cursor_mut::CursorMut;
 pub use self::iter::IntoIter;
 pub use self::limit::Limit;
 pub use self::take::Take;

src/buf/take.rs

@@ -1,4 +1,4 @@
-use crate::Buf;
+use crate::{Buf, Cursor};
 
 use core::cmp;
 
@@ -185,3 +185,14 @@ impl<T: Buf> Buf for Take<T> {
         cnt
     }
 }
+
+impl<T: Cursor> Cursor for Take<T> {
+    type Cursor<'a>
+        = Take<T::Cursor<'a>>
+    where
+        Self: 'a;
+
+    fn cursor(&self, index: usize) -> Self::Cursor<'_> {
+        self.inner.cursor(index).take(self.limit - index)
+    }
+}

src/lib.rs

@@ -77,7 +77,7 @@ extern crate alloc;
 extern crate std;
 
 pub mod buf;
-pub use crate::buf::{Buf, BufMut};
+pub use crate::buf::{Buf, BufMut, Cursor, CursorMut};
 
 mod bytes;
 mod bytes_mut;