Make ptr range iterable

Author: dtolnay

library/core/src/iter/range.rs

@@ -1,3 +1,5 @@
+mod ptr;
+
 use crate::char;
 use crate::convert::TryFrom;
 use crate::mem;

library/core/src/iter/range/ptr.rs

@@ -0,0 +1,274 @@
+use crate::cmp::Ordering;
+use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use crate::mem;
+use crate::ops::Range;
+
+macro_rules! impl_iterator_for_ptr_range {
+    ($mutability:ident /* const or mut */) => {
+        /// Iteration of a pointer range, as is common in code that interfaces
+        /// with C++ iterators.
+        ///
+        /// # Safety
+        ///
+        /// Traversing a pointer range is always safe, but **using the resulting
+        /// pointers** is not!
+        ///
+        /// The pointers between the start and end of a range "remember" the
+        /// [allocated object] that they refer into. Pointers resulting from
+        /// pointer arithmetic must not be used to read or write to any other
+        /// allocated object.
+        ///
+        /// As a consequence, pointers from a range traversal are only
+        /// dereferenceable if start and end of the original range both point
+        /// into the same allocated object. Dereferencing a pointer obtained via
+        /// iteration when this is not the case is Undefined Behavior.
+        ///
+        /// [allocated object]: crate::ptr#allocated-object
+        ///
+        /// # Example
+        ///
+        #[doc = example!($mutability)]
+        #[stable(feature = "iterate_ptr_range", since = "1.58.0")]
+        impl<T> Iterator for Range<*$mutability T> {
+            type Item = *$mutability T;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                if self.is_empty() {
+                    None
+                } else {
+                    let curr = self.start;
+                    let next = curr.wrapping_add(1);
+                    self.start = if (curr..self.end).contains(&next) {
+                        next
+                    } else {
+                        // Saturate to self.end if the wrapping_add wrapped or
+                        // landed beyond end.
+                        self.end
+                    };
+                    Some(curr)
+                }
+            }
+
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                if self.is_empty() {
+                    (0, Some(0))
+                } else if mem::size_of::<T>() == 0 {
+                    // T is zero sized so there are infinity of them in the
+                    // nonempty range.
+                    (usize::MAX, None)
+                } else {
+                    // In between self.start and self.end there are some number
+                    // of whole elements of type T, followed by possibly a
+                    // remainder element if T's size doesn't evenly divide the
+                    // byte distance between the endpoints. The remainder
+                    // element still counts as being part of this range, since
+                    // the pointer to it does lie between self.start and
+                    // self.end.
+                    let byte_offset = self.end as usize - self.start as usize;
+                    let number_of_whole_t = byte_offset / mem::size_of::<T>();
+                    let remainder_bytes = byte_offset % mem::size_of::<T>();
+                    let maybe_remainder_t = (remainder_bytes > 0) as usize;
+                    let hint = number_of_whole_t + maybe_remainder_t;
+                    (hint, Some(hint))
+                }
+            }
+
+            fn nth(&mut self, n: usize) -> Option<Self::Item> {
+                let _ = self.advance_by(n);
+                self.next()
+            }
+
+            fn last(mut self) -> Option<Self::Item> {
+                self.next_back()
+            }
+
+            fn min(mut self) -> Option<Self::Item> {
+                self.next()
+            }
+
+            fn max(mut self) -> Option<Self::Item> {
+                self.next_back()
+            }
+
+            fn is_sorted(self) -> bool {
+                true
+            }
+
+            fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+                match self.size_hint().1 {
+                    None => {
+                        // T is zero sized. Advancing does nothing.
+                        Ok(())
+                    }
+                    Some(len) => match n.cmp(&len) {
+                        Ordering::Less => {
+                            // Advance past n number of whole elements.
+                            self.start = self.start.wrapping_add(n);
+                            Ok(())
+                        }
+                        Ordering::Equal => {
+                            // Advance past every single element in the
+                            // iterator, including perhaps the remainder
+                            // element, leaving an empty iterator.
+                            self.start = self.end;
+                            Ok(())
+                        }
+                        Ordering::Greater => {
+                            // Advance too far.
+                            self.start = self.end;
+                            Err(len)
+                        }
+                    }
+                }
+            }
+
+            #[doc(hidden)]
+            unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
+                self.start.wrapping_add(idx)
+            }
+        }
+
+        #[stable(feature = "iterate_ptr_range", since = "1.58.0")]
+        impl<T> DoubleEndedIterator for Range<*$mutability T> {
+            fn next_back(&mut self) -> Option<Self::Item> {
+                match self.size_hint().1 {
+                    None => {
+                        // T is zero sized so the iterator never progresses past
+                        // start, even if going backwards.
+                        Some(self.start)
+                    }
+                    Some(0) => {
+                        None
+                    }
+                    Some(len) => {
+                        self.end = self.start.wrapping_add(len - 1);
+                        Some(self.end)
+                    }
+                }
+            }
+
+            fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+                match self.size_hint().1 {
+                    None => {
+                        // T is zero sized.
+                        Some(self.start)
+                    }
+                    Some(len) => {
+                        if n < len {
+                            self.end = self.start.wrapping_add(len - n - 1);
+                            Some(self.end)
+                        } else {
+                            self.end = self.start;
+                            None
+                        }
+                    }
+                }
+            }
+
+            fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+                match self.size_hint().1 {
+                    None => {
+                        // T is zero sized. Advancing does nothing.
+                        Ok(())
+                    }
+                    Some(len) => match n.cmp(&len) {
+                        Ordering::Less => {
+                            // Advance leaving `len - n` elements in the
+                            // iterator. Careful to preserve the remainder
+                            // element if told to advance by 0.
+                            if n > 0 {
+                                self.end = self.start.wrapping_add(len - n);
+                            }
+                            Ok(())
+                        }
+                        Ordering::Equal => {
+                            // Advance past every single element in the
+                            // iterator, leaving an empty iterator.
+                            self.end = self.start;
+                            Ok(())
+                        }
+                        Ordering::Greater => {
+                            // Advance too far.
+                            self.end = self.start;
+                            Err(len)
+                        }
+                    }
+                }
+            }
+        }
+
+        #[stable(feature = "iterate_ptr_range", since = "1.58.0")]
+        impl<T> FusedIterator for Range<*$mutability T> {}
+
+        #[unstable(feature = "trusted_len", issue = "37572")]
+        unsafe impl<T> TrustedLen for Range<*$mutability T> {}
+
+        #[doc(hidden)]
+        #[unstable(feature = "trusted_random_access", issue = "none")]
+        unsafe impl<T> TrustedRandomAccess for Range<*$mutability T> {}
+
+        #[doc(hidden)]
+        #[unstable(feature = "trusted_random_access", issue = "none")]
+        unsafe impl<T> TrustedRandomAccessNoCoerce for Range<*$mutability T> {
+            const MAY_HAVE_SIDE_EFFECT: bool = false;
+        }
+    };
+}
+
+macro_rules! example {
+    (const) => {
+        doc_comment_to_literal! {
+            /// ```
+            /// // Designed to be called from C++ or C.
+            /// #[no_mangle]
+            /// unsafe extern "C" fn demo(start: *const u16, end: *const u16) {
+            ///     for ptr in start..end {
+            ///         println!("{}", *ptr);
+            ///     }
+            /// }
+            ///
+            /// fn main() {
+            ///     let slice = &[1u16, 2, 3];
+            ///     let range = slice.as_ptr_range();
+            ///     unsafe { demo(range.start, range.end); }
+            /// }
+            /// ```
+        }
+    };
+
+    (mut) => {
+        doc_comment_to_literal! {
+            /// ```
+            /// #![feature(vec_spare_capacity)]
+            ///
+            /// use core::ptr;
+            ///
+            /// // Designed to be called from C++ or C.
+            /// #[no_mangle]
+            /// unsafe extern "C" fn demo(start: *mut u16, end: *mut u16) {
+            ///     for (i, ptr) in (start..end).enumerate() {
+            ///         ptr::write(ptr, i as u16);
+            ///     }
+            /// }
+            ///
+            /// fn main() {
+            ///     let mut vec: Vec<u16> = Vec::with_capacity(100);
+            ///     let range = vec.spare_capacity_mut().as_mut_ptr_range();
+            ///     unsafe {
+            ///         demo(range.start.cast::<u16>(), range.end.cast::<u16>());
+            ///         vec.set_len(100);
+            ///     }
+            /// }
+            /// ```
+        }
+    };
+}
+
+macro_rules! doc_comment_to_literal {
+    ($(#[doc = $example:literal])*) => {
+        concat!($($example, '\n'),*)
+    };
+}
+
+impl_iterator_for_ptr_range!(const);
+impl_iterator_for_ptr_range!(mut);