Add elem_offset and related methods

Author: wr7

library/core/src/slice/mod.rs

@@ -4540,6 +4540,121 @@ impl<T> [T] {
         // are disjunct and in bounds.
         unsafe { Ok(self.get_many_unchecked_mut(indices)) }
     }
+
+    /// Returns the index that an element reference points to.
+    ///
+    /// Returns `None` if `element` does not point within the slice or if it points between elements.
+    ///
+    /// This method is useful for extending slice iterators like [`slice::split`].
+    ///
+    /// Note that this uses pointer arithmetic and **does not compare elements**.
+    /// To find the index of an element via comparison, use
+    /// [`.iter().position()`](crate::iter::Iterator::position) instead.
+    ///
+    /// # Panics
+    /// Panics if `T` is zero-sized.
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// #![feature(substr_range)]
+    ///
+    /// let nums: &[u32] = &[1, 7, 1, 1];
+    /// let num = &nums[2];
+    ///
+    /// assert_eq!(num, &1);
+    /// assert_eq!(nums.elem_offset(num), Some(2));
+    /// ```
+    /// Returning `None` with an in-between element:
+    /// ```
+    /// #![feature(substr_range)]
+    ///
+    /// let arr: &[[u32; 2]] = &[[0, 1], [2, 3]];
+    /// let flat_arr: &[u32] = arr.as_flattened();
+    ///
+    /// let ok_elm: &[u32; 2] = flat_arr[0..2].try_into().unwrap();
+    /// let weird_elm: &[u32; 2] = flat_arr[1..3].try_into().unwrap();
+    ///
+    /// assert_eq!(ok_elm, &[0, 1]);
+    /// assert_eq!(weird_elm, &[1, 2]);
+    ///
+    /// assert_eq!(arr.elem_offset(ok_elm), Some(0)); // Points to element 0
+    /// assert_eq!(arr.elem_offset(weird_elm), None); // Points between element 0 and 1
+    /// ```
+    #[must_use]
+    #[unstable(feature = "substr_range", issue = "126769")]
+    pub fn elem_offset(&self, element: &T) -> Option<usize> {
+        if T::IS_ZST {
+            panic!("elements are zero-sized");
+        }
+
+        let self_start = self.as_ptr() as usize;
+        let elem_start = element as *const T as usize;
+
+        let byte_offset = elem_start.wrapping_sub(self_start);
+
+        if byte_offset % mem::size_of::<T>() != 0 {
+            return None;
+        }
+
+        let offset = byte_offset / mem::size_of::<T>();
+
+        if offset < self.len() { Some(offset) } else { None }
+    }
+
+    /// Returns the range of indices that a subslice points to.
+    ///
+    /// Returns `None` if `subslice` does not point within the slice or if it points between elements.
+    ///
+    /// This method **does not compare elements**. Instead, this method finds the location in the slice that
+    /// `subslice` was obtained from. To find the index of a subslice via comparison, instead use
+    /// [`.windows()`](slice::windows)[`.position()`](crate::iter::Iterator::position).
+    ///
+    /// This method is useful for extending slice iterators like [`slice::split`].
+    ///
+    /// Note that this may return a false positive (either `Some(0..0)` or `Some(self.len()..self.len())`)
+    /// if `subslice` has a length of zero and points to the beginning or end of another, separate, slice.
+    ///
+    /// # Panics
+    /// Panics if `T` is zero-sized.
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// #![feature(substr_range)]
+    ///
+    /// let nums = &[0, 5, 10, 0, 0, 5];
+    ///
+    /// let mut iter = nums
+    ///     .split(|t| *t == 0)
+    ///     .map(|n| nums.subslice_range(n).unwrap());
+    ///
+    /// assert_eq!(iter.next(), Some(0..0));
+    /// assert_eq!(iter.next(), Some(1..3));
+    /// assert_eq!(iter.next(), Some(4..4));
+    /// assert_eq!(iter.next(), Some(5..6));
+    /// ```
+    #[must_use]
+    #[unstable(feature = "substr_range", issue = "126769")]
+    pub fn subslice_range(&self, subslice: &[T]) -> Option<Range<usize>> {
+        if T::IS_ZST {
+            panic!("elements are zero-sized");
+        }
+
+        let self_start = self.as_ptr() as usize;
+        let subslice_start = subslice.as_ptr() as usize;
+
+        let byte_start = subslice_start.wrapping_sub(self_start);
+
+        if byte_start % core::mem::size_of::<T>() != 0 {
+            return None;
+        }
+
+        let start = byte_start / core::mem::size_of::<T>();
+        let end = start.wrapping_add(subslice.len());
+
+        if start <= self.len() && end <= self.len() { Some(start..end) } else { None }
+    }
 }
 
 impl<T, const N: usize> [[T; N]] {

library/core/src/str/mod.rs

@@ -19,6 +19,7 @@ use self::pattern::{DoubleEndedSearcher, ReverseSearcher, Searcher};
 use crate::ascii;
 use crate::char::{self, EscapeDebugExtArgs};
 use crate::mem;
+use crate::ops::Range;
 use crate::slice::{self, SliceIndex};
 
 pub mod pattern;
@@ -2721,6 +2722,39 @@ impl str {
     pub fn escape_unicode(&self) -> EscapeUnicode<'_> {
         EscapeUnicode { inner: self.chars().flat_map(CharEscapeUnicode) }
     }
+
+    /// Returns the range that a substring points to.
+    ///
+    /// Returns `None` if `substr` does not point within `self`.
+    ///
+    /// Unlike [`str::find`], **this does not search through the string**.
+    /// Instead, it uses pointer arithmetic to find where in the string
+    /// `substr` is derived from.
+    ///
+    /// This is useful for extending [`str::split`] and similar methods.
+    ///
+    /// Note that this method may return false positives (typically either
+    /// `Some(0..0)` or `Some(self.len()..self.len())`) if `substr` is a
+    /// zero-length `str` that points at the beginning or end of another,
+    /// independent, `str`.
+    ///
+    /// # Examples
+    /// ```
+    /// #![feature(substr_range)]
+    ///
+    /// let data = "a, b, b, a";
+    /// let mut iter = data.split(", ").map(|s| data.substr_range(s).unwrap());
+    ///
+    /// assert_eq!(iter.next(), Some(0..1));
+    /// assert_eq!(iter.next(), Some(3..4));
+    /// assert_eq!(iter.next(), Some(6..7));
+    /// assert_eq!(iter.next(), Some(9..10));
+    /// ```
+    #[must_use]
+    #[unstable(feature = "substr_range", issue = "126769")]
+    pub fn substr_range(&self, substr: &str) -> Option<Range<usize>> {
+        self.as_bytes().subslice_range(substr.as_bytes())
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]