sl-std: add SubsliceOffset trait

See the relevant documentation on the trait for more info

sl-std/src/lib.rs

@@ -16,4 +16,5 @@ pub mod chars;
 pub mod iter;
 pub mod punycode;
 pub mod rand;
+pub mod slice;
 pub mod time;

sl-std/src/slice.rs

@@ -0,0 +1,27 @@
+pub trait SubsliceOffset {
+    /// Returns the byte offset of an inner slice relative to an enclosing outer slice.
+    ///
+    /// Examples
+    ///
+    /// ```
+    /// let string = "a\nb\nc";
+    /// let lines: Vec<&str> = string.lines().collect();
+    /// assert!(string.subslice_offset(lines[0]) == Some(0)); // &"a"
+    /// assert!(string.subslice_offset(lines[1]) == Some(2)); // &"b"
+    /// assert!(string.subslice_offset(lines[2]) == Some(4)); // &"c"
+    /// assert!(string.subslice_offset("other!") == None);
+    /// ```
+    fn subslice_offset(&self, inner: &Self) -> Option<usize>;
+}
+
+impl SubsliceOffset for str {
+    fn subslice_offset(&self, inner: &str) -> Option<usize> {
+        let outer = self.as_ptr() as usize;
+        let inner = inner.as_ptr() as usize;
+        if (outer..outer + self.len()).contains(&inner) {
+            Some(inner.wrapping_sub(outer))
+        } else {
+            None
+        }
+    }
+}