pread, pwrite: do not error out if {p,g}{read,write}_with if the offs…

…et is equal to slice len (#94)

We now allow reading zero-sized types (like possibly an empty array) from empty slice, or the exact end of a byte array,
and similarly, one can write a empty string or byte array into the tail end of a byte array (i.e., write nothing to nothing).

Co-authored-by: Nickie S <DCNick3@users.noreply.github.com>

src/lib.rs

@@ -377,6 +377,11 @@ mod tests {
         assert!(error.is_ok());
     }
 
+    /// In this test, we are testing preading
+    /// at length boundaries.
+    /// In the past, this test was supposed to test failures for `hello_world`.
+    /// Since PR#94, this test is unwrapping as we exploit
+    /// the fact that if you do &x[x.len()..] you get an empty slice.
     #[test]
     fn pread_str_weird() {
         use super::ctx::*;
@@ -385,7 +390,7 @@ mod tests {
         let hello_world = bytes.pread_with::<&str>(0, StrCtx::Delimiter(NULL));
         #[cfg(feature = "std")]
         println!("1 {hello_world:?}");
-        assert!(hello_world.is_err());
+        assert!(hello_world.unwrap().is_empty());
         let error = bytes.pread_with::<&str>(7, StrCtx::Delimiter(SPACE));
         #[cfg(feature = "std")]
         println!("2 {error:?}");

src/pread.rs

@@ -20,6 +20,11 @@ use crate::error;
 /// over chunks of memory or any other indexable type — but scroll does come with a set of powerful
 /// blanket implementations for data being a continous block of byte-addressable memory.
 ///
+/// Note that in the particular case of the implementation of `Pread` for `[u8]`,
+/// reading it at the length boundary of that slice will cause to read from an empty slice.
+/// i.e. we make use of the fact that `&bytes[bytes.len()..]` will return an empty slice, rather
+/// than returning an error. In the past, scroll returned an offset error.
+///
 /// Pread provides two main groups of functions: pread and gread.
 ///
 /// `pread` is the basic function that simply extracts a given type from a given data store - either
@@ -167,7 +172,7 @@ impl<Ctx: Copy, E: From<error::Error>> Pread<Ctx, E> for [u8] {
         ctx: Ctx,
     ) -> result::Result<N, E> {
         let start = *offset;
-        if start >= self.len() {
+        if start > self.len() {
             return Err(error::Error::BadOffset(start).into());
         }
         N::try_from_ctx(&self[start..], ctx).map(|(n, size)| {

src/pwrite.rs

@@ -19,6 +19,13 @@ use crate::error;
 /// with 'read' switched for 'write' and 'From' switched with 'Into' so if you haven't yet you
 /// should read the documentation of `Pread` first.
 ///
+/// As with `Pread`, note that in the particular case of the implementation of `Pwrite` for `[u8]`,
+/// writing it at the length boundary of that slice will cause to write in an empty slice.
+/// i.e. we make use of the fact that `&bytes[bytes.len()..]` will return an empty slice, rather
+/// than returning an error. In the past, scroll returned an offset error.
+/// In this case, this is relevant if you are writing an empty slice inside an empty slice and
+/// expected this to work.
+///
 /// Unless you need to implement your own data store — that is either can't convert to `&[u8]` or
 /// have a data that does not expose a `&mut [u8]` — you will probably want to implement
 /// [TryIntoCtx](ctx/trait.TryIntoCtx.html) on your Rust types to be written.
@@ -87,7 +94,7 @@ impl<Ctx: Copy, E: From<error::Error>> Pwrite<Ctx, E> for [u8] {
         offset: usize,
         ctx: Ctx,
     ) -> result::Result<usize, E> {
-        if offset >= self.len() {
+        if offset > self.len() {
             return Err(error::Error::BadOffset(offset).into());
         }
         let dst = &mut self[offset..];