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[package]
name = "with-checked-bytes"
description = "Extension trait for safely editing strings as byte slices"
version = "0.1.0"
edition = "2021"
authors = ["Thomas Karpiniec <tk@1.21jiggawatts.net>"]
license = "Apache-2.0"
[dependencies]

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# with-checked-bytes
A Rust extension trait for `&mut str` to simplify manipulating UTF-8 strings as if they were
plain ASCII bytes. The result is only applied to the original string if the end result of
the modification is valid UTF-8.
## Examples
```rust
use with_checked_bytes::WithCheckedBytes;
let mut my_string = String::from("hello");
my_string.with_checked_bytes_mut(|s| {
s[1] += 1;
}).unwrap();
assert_eq!(my_string, "hfllo");
```
```rust
let mut my_string = String::from("hello");
let old_value = my_string.with_checked_bytes_mut(|s| {
std::mem::replace(&mut s[3], b'z')
}).unwrap();
assert_eq!(old_value, b'l');
assert_eq!(my_string, "helzo");
```
```rust
let mut my_string = String::from("hello");
my_string.with_checked_bytes_mut(|s| {
s[1] = 0xff; // not valid UTF-8
}).unwrap(); // will panic - original string remains unmodified
```
## Licence
Apache 2.0. See `LICENSE`.

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//! An extension trait for `&mut str` to simplify manipulating UTF-8 strings as if they were
//! plain ASCII bytes. The result is only applied to the original string if the end result of
//! the modification is valid UTF-8.
//!
//! # Examples:
//!
//! ```
//! use with_checked_bytes::WithCheckedBytes;
//!
//! let mut my_string = String::from("hello");
//! my_string.with_checked_bytes_mut(|s| {
//! s[1] += 1;
//! }).unwrap();
//! assert_eq!(my_string, "hfllo");
//! ```
//!
//! ```
//! # use with_checked_bytes::WithCheckedBytes;
//! let mut my_string = String::from("hello");
//! let old_value = my_string.with_checked_bytes_mut(|s| {
//! std::mem::replace(&mut s[3], b'z')
//! }).unwrap();
//! assert_eq!(old_value, b'l');
//! assert_eq!(my_string, "helzo");
//! ```
//!
//! ```should_panic
//! # use with_checked_bytes::WithCheckedBytes;
//! let mut my_string = String::from("hello");
//! my_string.with_checked_bytes_mut(|s| {
//! s[1] = 0xff; // not valid UTF-8
//! }).unwrap(); // will panic - original string remains unmodified
//! ```
use std::ops::{Deref, DerefMut};
/// Extension trait for safely editing mutable UTF-8 strings as bytes
pub trait WithCheckedBytes {
/// Edit a mutable `String` or `&mut str` as if it were a byte array.
///
/// The provided closure will be executed with a mutable view of the String.
/// If the mutable buffer doesn't contain valid UTF-8 when the closure returns,
/// the original string will not be modified and an error will be returned.
///
/// If the buffer contains valid UTF-8, the original string will be overwritten
/// with the buffer's contents. Any value returned from the closure will be
/// passed back to the caller.
fn with_checked_bytes_mut<'a, R, F>(&'a mut self, f: F) -> Result<R, Error>
where
F: for<'b> FnOnce(&'b mut MutableStringBytes) -> R;
}
impl WithCheckedBytes for str {
fn with_checked_bytes_mut<'a, R, F>(&'a mut self, f: F) -> Result<R, Error>
where
F: for<'b> FnOnce(&'b mut MutableStringBytes) -> R,
{
let mut target = MutableStringBytes::Borrowed(self.as_bytes());
let res = f(&mut target);
match target {
MutableStringBytes::Borrowed(_) => (),
MutableStringBytes::Owned(v) => match std::str::from_utf8(&v) {
// SAFETY: We just proved that the new slice content is valid UTF-8
Ok(s) => unsafe { self.as_bytes_mut().copy_from_slice(s.as_bytes()) },
Err(_) => return Err(Error::InvalidUtf8),
},
}
Ok(res)
}
}
/// Mutable view into a string's content expressed as bytes
pub enum MutableStringBytes<'a> {
Borrowed(&'a [u8]),
Owned(Vec<u8>),
}
impl<'a> Deref for MutableStringBytes<'a> {
type Target = [u8];
fn deref(&self) -> &Self::Target {
match self {
Self::Borrowed(slice) => *slice,
Self::Owned(vec) => vec.as_slice(),
}
}
}
impl<'a> DerefMut for MutableStringBytes<'a> {
fn deref_mut(&mut self) -> &mut Self::Target {
if let Self::Borrowed(slice) = self {
let v = slice.to_vec();
let _ = std::mem::replace(self, Self::Owned(v));
}
match self {
Self::Borrowed(_) => unreachable!(),
Self::Owned(vec) => vec.as_mut_slice(),
}
}
}
/// Errors that can occur while mutating strings
#[derive(Debug)]
pub enum Error {
InvalidUtf8,
}
impl std::error::Error for Error {}
impl std::fmt::Display for Error {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "MutableStringBytes contains invalid UTF-8 after modifications")
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn twiddle_a_byte() {
let mut my_string = "Hello".to_owned();
my_string.with_checked_bytes_mut(|s| {
s[3] += 1;
}).unwrap();
assert_eq!(my_string, "Helmo");
}
#[test]
fn twiddle_a_byte_bad() {
let mut my_string = "Hello".to_owned();
my_string.with_checked_bytes_mut(|s| {
s[3] = 0xff;
}).unwrap_err();
assert_eq!(my_string, "Hello");
}
#[test]
fn return_a_byte() {
let mut my_string = "Hello".to_owned();
let ascii = my_string.with_checked_bytes_mut(|s| {
s[3]
}).unwrap();
assert_eq!(ascii, 108u8);
}
#[test]
fn slice_edit() {
let mut my_string = "Hello".to_owned();
let mut_slice = my_string.as_mut();
mut_slice.with_checked_bytes_mut(|s| {
s[0] += 1;
}).unwrap();
assert_eq!(my_string, "Iello");
}
}