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#![no_std] use core::{mem, slice, str}; /// Error that can occur during [`concat`](fn.concat.html). #[derive(Debug, Clone, Copy, PartialEq, Eq)] pub enum Error { /// The passed strs are not adjacent. NotAdjacent, /// The first str is too long for concatenation. TooLong, } /// Concatenate two string slices if they are adjacent. /// /// If two strs are adjacent to each other in memory, this function /// concatenates both, creating a single str. /// /// # Errors /// /// Returns `Err` if the two slices aren't adjacent, `a` is after `b`, or if /// `a` is too long for proper concatenation (longer than `isize::MAX`). /// /// # Safety /// /// The provided slices must come from the same underlying allocation. The adjacency test can not /// reliably differentiate between the one-past-the-end pointer of one allocation and the start of /// another. However, all slices must be within a single allocation. /// /// # Examples /// /// Correct usage: /// /// ```rust /// # use str_concat::concat; /// let s = "0123456789"; /// unsafe { /// // SAFETY: slices from the same str originally. /// assert_eq!("0123456", concat(&s[..5], &s[5..7]).unwrap()); /// } /// ``` /// /// Non-adjacent string slices: /// /// ```rust /// # use str_concat::{concat, Error}; /// let s = "0123456789"; /// unsafe { /// // SAFETY: slices from the same str originally. /// assert_eq!(Err(Error::NotAdjacent), concat(&s[..5], &s[6..7])) /// } /// ``` pub unsafe fn concat<'a>(a: &'a str, b: &'a str) -> Result<&'a str, Error> { let slice = concat_slice(a.as_bytes(), b.as_bytes())?; // * concatenating two valid UTF8 strings will produce a valid UTF8 string // * a BOM in `b` is still valid: // > It is important to understand that the character U+FEFF appearing at // > any position other than the beginning of a stream MUST be interpreted // > with the semantics for the zero-width non-breaking space, and MUST // > NOT be interpreted as a signature. // * the grapheme *clusters* (and thus potentially the semantics of the string // might change if the first code point of `b` is a combining character, // a zero width joiner or similar. // This does not affect the correctness of UTF-8. Ok(str::from_utf8_unchecked(slice)) } /// Concatenate two slices if they are adjacent. /// /// If two slices are adjacent to each other in memory, this function /// concatenates both, creating a single longer slice. Note that slices of /// zero-sized types (ZST) are never considered adjacent. Otherwise it would be /// possible to concatenate a slice to itself. /// /// # Errors /// /// Returns `Err` if the two slices aren't adjacent, `a` is after `b`, or if the /// result is too long to be represented as a slice (size in bytes is larger /// than `isize::MAX`). /// /// When T is a zero-sized type (ZST) then always returns `Err(NotAdjacent)` otherwise. This is /// because ZST-slices are [extra weird][zst-str-concat] and [their safety][zst-unsafe-wg1] is not /// yet [fully determined][zst-unsafe-wg2]. /// /// [zst-str-concat]: https://github.com/oberien/str-concat/issues/5 /// [zst-unsafe-wg1]: https://github.com/rust-lang/unsafe-code-guidelines/issues/93 /// [zst-unsafe-wg2]: https://github.com/rust-lang/unsafe-code-guidelines/issues/168 /// /// # Safety /// /// The provided slices must come from the same underlying allocation. The adjacency test can not /// reliably differentiate between the one-past-the-end pointer of one allocation and the start of /// another. However, all slices must be within a single allocation. /// /// # Examples /// /// Correct usage: /// /// ```rust /// # use str_concat::concat_slice; /// let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; /// unsafe { /// // SAFETY: slices from the same bytes originally. /// assert_eq!( /// [0, 1, 2, 3, 4, 5, 6], /// concat_slice(&s[..5], &s[5..7]).unwrap()); /// } /// ``` /// /// Non-adjacent byte slices: /// /// ```rust /// # use str_concat::{concat_slice, Error}; /// let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; /// unsafe { /// // SAFETY: slices from the same bytes originally. /// assert_eq!(Err(Error::NotAdjacent), concat_slice(&s[..5], &s[6..7])) /// } /// ``` /// pub unsafe fn concat_slice<'a, T>(a: &'a [T], b: &'a [T]) -> Result<&'a [T], Error> { let a_ptr = a.as_ptr(); let b_ptr = b.as_ptr(); let a_len = a.len(); let b_len = b.len(); if mem::size_of::<T>() == 0 { // NOTE(HeroicKatora) // Never consider ZST slices adjacent through this function. You could // infinitely duplicate a non-zero length slice by concatenating it to // itself as opposed to non-ZST slice types. That would just be weird. // // It is however safe. // See: https://github.com/rust-lang/unsafe-code-guidelines/issues/93 // and https://github.com/rust-lang/unsafe-code-guidelines/issues/168 // Issue: https://github.com/oberien/str-concat/issues/5 return Err(Error::NotAdjacent) } // `max_len <= isize::max_value()` let max_len = isize::max_value() as usize / mem::size_of::<T>(); // These should be guaranteed for the slices. assert!(a_len <= max_len as usize); assert!(b_len <= max_len as usize); // https://doc.rust-lang.org/std/primitive.pointer.html#safety-1 // * starting pointer in-bounds obviously // * ending pointer one byte past the end of an allocated object // * explicit isize overflow check above // * no wraparound required // why: this is the one byte past the end pointer for the input slice `a` if a_ptr.offset(a_len as isize) != b_ptr { return Err(Error::NotAdjacent); } // UNWRAP: both smaller than isize, can't wrap in usize. // This is because in rust `usize` and `isize` are both guaranteed to have // the same number of bits as a pointer [1]. As `isize` is signed, a `usize` // can always store the sum of two positive `isize`. // [1]: https://doc.rust-lang.org/reference/types/numeric.html#machine-dependent-integer-types let new_len = a_len.checked_add(b_len).unwrap(); // Ensure the length is bounded. The bound is strict from the definition of `max_len` // `new_len <= max_len` <=> `new_len * mem::size_of::<T>() <= isize::max_value()` if !(new_len <= max_len) { return Err(Error::TooLong); } // https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html#safety // * slices are adjacent (checked above) // * no double-free / leak because we work on borrowed data // * no use-after-free because `a` and `b` have same lifetime // * the total size is smaller than `isize::MAX` bytes, as max_len is rounded down Ok(slice::from_raw_parts(a_ptr, new_len)) } /// Concatenate two adjacent string slices no matter their order. /// /// This is the same as [`concat`] except that it also concatenates /// `b` to `a` if `b` is in front of `a` (in which case [`concat`] errors). /// /// # Safety /// /// The provided slices must come from the same underlying allocation. The adjacency test can not /// reliably differentiate between the one-past-the-end pointer of one allocation and the start of /// another. However, all slices must be within a single allocation. /// /// # Examples /// /// Reversed order: /// /// ```rust /// # use str_concat::concat_unordered; /// let s = "0123456789"; /// unsafe { /// // SAFETY: slices from the same str originally. /// assert_eq!("0123456", concat_unordered(&s[5..7], &s[..5]).unwrap()); /// } /// ``` /// /// Normal order: /// /// ```rust /// # use str_concat::{concat_unordered, Error}; /// let s = "0123456789"; /// unsafe { /// // SAFETY: slices from the same str originally. /// assert_eq!("0123456", concat_unordered(&s[..5], &s[5..7]).unwrap()) /// } /// ``` /// /// [`concat`]: fn.concat.html pub unsafe fn concat_unordered<'a>(a: &'a str, b: &'a str) -> Result<&'a str, Error> { // add lengths to handle empty-string cases correctly let a_ptr = a.as_bytes().as_ptr() as usize; let a_end_ptr = a_ptr + a.len(); let b_ptr = b.as_bytes().as_ptr() as usize; // make the order of `a` and `b` not matter let (a, b) = if a_ptr <= b_ptr && a_end_ptr <= b_ptr { (a, b) } else { (b, a) }; concat(a, b) } /// Concatenate two adjacent slices no matter their order. /// /// This is the same as [`concat_slice`] except that it also concatenates `b` to /// `a` if `b` is in front of `a` (in which case of [`concat_slice`] errors). /// Keep in mind that slices of zero-sized types (ZST) will still not be concatenated. /// /// # Safety /// /// The provided slices must come from the same underlying allocation. The adjacency test can not /// reliably differentiate between the one-past-the-end pointer of one allocation and the start of /// another. However, all slices must be within a single allocation. /// /// # Examples /// /// Reversed order: /// /// ```rust /// # use str_concat::concat_slice_unordered; /// let s = [0, 1, 2, 3, 4, 5, 6]; /// unsafe { /// // SAFETY: slices from the same bytes originally. /// assert_eq!( /// [0, 1, 2, 3, 4, 5, 6], /// concat_slice_unordered(&s[5..7], &s[..5]).unwrap()); /// } /// ``` /// /// Normal order: /// /// ```rust /// # use str_concat::{concat_slice_unordered, Error}; /// let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; /// unsafe { /// // SAFETY: slices from the same bytes originally. /// assert_eq!( /// [0, 1, 2, 3, 4, 5, 6], /// concat_slice_unordered(&s[..5], &s[5..7]).unwrap()) /// } /// ``` /// /// [`concat_slice`]: fn.concat_slice.html pub unsafe fn concat_slice_unordered<'a, T>(a: &'a [T], b: &'a [T]) -> Result<&'a [T], Error> { // add lengths to handle empty cases correctly let a_ptr = a.as_ptr() as usize; let a_end_ptr = a_ptr + a.len() * mem::size_of::<T>(); let b_ptr = b.as_ptr() as usize; // make the order of `a` and `b` not matter let (a, b) = if a_ptr <= b_ptr && a_end_ptr <= b_ptr { (a, b) } else { (b, a) }; concat_slice(a, b) } #[cfg(test)] mod tests { use super::{concat, concat_unordered, concat_slice, concat_slice_unordered, Error}; #[test] fn simple_success() { let s = "0123456789"; unsafe { assert_eq!(Ok("0123456"), concat(&s[..5], &s[5..7])); assert_eq!(Ok("0123456"), concat_unordered(&s[..5], &s[5..7])); } } #[test] fn unordered() { let s = "0123456789"; unsafe { assert_eq!(Err(Error::NotAdjacent), concat(&s[5..7], &s[..5])); assert_eq!(Ok("0123456"), concat_unordered(&s[5..7], &s[..5])); } } #[test] fn simple_fail() { let s = "0123456789"; unsafe { assert_eq!(Err(Error::NotAdjacent), concat(&s[..5], &s[6..7])) } } #[test] fn zero_width_joiner() { let s = "0\u{200d}1"; unsafe { assert_eq!(Ok("0\u{200d}1"), concat(&s[..1], &s[1..5])); } } #[test] fn zero_width_joiner_combining_grave() { let s = "0\u{200d}̀1"; unsafe { assert_eq!(Ok("0\u{200d}\u{300}1"), concat(&s[..1], &s[1..7])); } } #[test] fn bom() { let s = "0\u{FEFF}1"; unsafe { assert_eq!(Ok("0\u{FEFF}1"), concat(&s[..1], &s[1..5])); } } #[test] fn empty_str() { let s = "0123"; unsafe { assert_eq!(Ok("0123"), concat(&s[..0], s)); assert_eq!(Ok("0123"), concat_unordered(&s[..0], s)); assert_eq!(Ok("0123"), concat_unordered(s, &s[..0])); assert_eq!(Ok("0123"), concat(s, &s[4..])); assert_eq!(Ok("0123"), concat_unordered(s, &s[4..])); assert_eq!(Ok("0123"), concat_unordered(&s[4..], s)); } } #[test] fn typed_slices() { #[derive(Debug, PartialEq)] struct T(usize); let s: &[T] = &[T(0), T(1), T(2), T(3)][..]; unsafe { assert_eq!(Ok(s), concat_slice(&s[..2], &s[2..])); assert_eq!(Ok(s), concat_slice_unordered(&s[..2], &s[2..])); assert_eq!(Ok(s), concat_slice_unordered(&s[2..], &s[..2])); // One slice empty assert_eq!(Ok(s), concat_slice(&s[..0], s)); assert_eq!(Ok(s), concat_slice_unordered(&s[..0], s)); assert_eq!(Ok(s), concat_slice_unordered(s, &s[..0])); assert_eq!(Ok(s), concat_slice(s, &s[4..])); assert_eq!(Ok(s), concat_slice_unordered(s, &s[4..])); assert_eq!(Ok(s), concat_slice_unordered(&s[4..], s)); } } #[test] fn typed_fail() { #[derive(Debug, PartialEq)] struct T(usize); let s: &[T] = &[T(0), T(1), T(2), T(3)][..]; unsafe { assert_eq!(Err(Error::NotAdjacent), concat_slice(&s[..1], &s[2..])); assert_eq!(Err(Error::NotAdjacent), concat_slice_unordered(&s[..1], &s[2..])); assert_eq!(Err(Error::NotAdjacent), concat_slice(&s[2..], &s[..2])); } } #[test] fn zst_fail() { #[derive(Clone, Copy, Debug, PartialEq)] struct Zst; let s: &[Zst] = &[Zst; 4]; unsafe { assert_eq!(Err(Error::NotAdjacent), concat_slice(&s[..1], &s[1..])); assert_eq!(Err(Error::NotAdjacent), concat_slice_unordered(&s[..1], &s[1..])); } } }