core/slice/iter/macros.rs
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//! Macros used by iterators of slice.
/// Convenience & performance macro for consuming the `end_or_len` field, by
/// giving a `(&mut) usize` or `(&mut) NonNull<T>` depending whether `T` is
/// or is not a ZST respectively.
///
/// Internally, this reads the `end` through a pointer-to-`NonNull` so that
/// it'll get the appropriate non-null metadata in the backend without needing
/// to call `assume` manually.
macro_rules! if_zst {
(mut $this:ident, $len:ident => $zst_body:expr, $end:ident => $other_body:expr,) => {{
#![allow(unused_unsafe)] // we're sometimes used within an unsafe block
if T::IS_ZST {
// SAFETY: for ZSTs, the pointer is storing a provenance-free length,
// so consuming and updating it as a `usize` is fine.
let $len = unsafe { &mut *(&raw mut $this.end_or_len).cast::<usize>() };
$zst_body
} else {
// SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
let $end = unsafe { &mut *(&raw mut $this.end_or_len).cast::<NonNull<T>>() };
$other_body
}
}};
($this:ident, $len:ident => $zst_body:expr, $end:ident => $other_body:expr,) => {{
#![allow(unused_unsafe)] // we're sometimes used within an unsafe block
if T::IS_ZST {
let $len = $this.end_or_len.addr();
$zst_body
} else {
// SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
let $end = unsafe { *(&raw const $this.end_or_len).cast::<NonNull<T>>() };
$other_body
}
}};
}
// Inlining is_empty and len makes a huge performance difference
macro_rules! is_empty {
($self: ident) => {
if_zst!($self,
len => len == 0,
end => $self.ptr == end,
)
};
}
macro_rules! len {
($self: ident) => {{
if_zst!($self,
len => len,
end => {
// To get rid of some bounds checks (see `position`), we use ptr_sub instead of
// offset_from (Tested by `codegen/slice-position-bounds-check`.)
// SAFETY: by the type invariant pointers are aligned and `start <= end`
unsafe { end.sub_ptr($self.ptr) }
},
)
}};
}
// The shared definition of the `Iter` and `IterMut` iterators
macro_rules! iterator {
(
struct $name:ident -> $ptr:ty,
$elem:ty,
$raw_mut:tt,
{$( $mut_:tt )?},
$into_ref:ident,
{$($extra:tt)*}
) => {
impl<'a, T> $name<'a, T> {
/// Returns the last element and moves the end of the iterator backwards by 1.
///
/// # Safety
///
/// The iterator must not be empty
#[inline]
unsafe fn next_back_unchecked(&mut self) -> $elem {
// SAFETY: the caller promised it's not empty, so
// the offsetting is in-bounds and there's an element to return.
unsafe { self.pre_dec_end(1).$into_ref() }
}
// Helper function for creating a slice from the iterator.
#[inline(always)]
fn make_slice(&self) -> &'a [T] {
// SAFETY: the iterator was created from a slice with pointer
// `self.ptr` and length `len!(self)`. This guarantees that all
// the prerequisites for `from_raw_parts` are fulfilled.
unsafe { from_raw_parts(self.ptr.as_ptr(), len!(self)) }
}
// Helper function for moving the start of the iterator forwards by `offset` elements,
// returning the old start.
// Unsafe because the offset must not exceed `self.len()`.
#[inline(always)]
unsafe fn post_inc_start(&mut self, offset: usize) -> NonNull<T> {
let old = self.ptr;
// SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
// so this new pointer is inside `self` and thus guaranteed to be non-null.
unsafe {
if_zst!(mut self,
// Using the intrinsic directly avoids emitting a UbCheck
len => *len = crate::intrinsics::unchecked_sub(*len, offset),
_end => self.ptr = self.ptr.add(offset),
);
}
old
}
// Helper function for moving the end of the iterator backwards by `offset` elements,
// returning the new end.
// Unsafe because the offset must not exceed `self.len()`.
#[inline(always)]
unsafe fn pre_dec_end(&mut self, offset: usize) -> NonNull<T> {
if_zst!(mut self,
// SAFETY: By our precondition, `offset` can be at most the
// current length, so the subtraction can never overflow.
len => unsafe {
// Using the intrinsic directly avoids emitting a UbCheck
*len = crate::intrinsics::unchecked_sub(*len, offset);
self.ptr
},
// SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
// which is guaranteed to not overflow an `isize`. Also, the resulting pointer
// is in bounds of `slice`, which fulfills the other requirements for `offset`.
end => unsafe {
*end = end.sub(offset);
*end
},
)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for $name<'_, T> {
#[inline(always)]
fn len(&self) -> usize {
len!(self)
}
#[inline(always)]
fn is_empty(&self) -> bool {
is_empty!(self)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for $name<'a, T> {
type Item = $elem;
#[inline]
fn next(&mut self) -> Option<$elem> {
// could be implemented with slices, but this avoids bounds checks
// SAFETY: The call to `next_unchecked` is
// safe since we check if the iterator is empty first.
unsafe {
if is_empty!(self) {
None
} else {
Some(self.next_unchecked())
}
}
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let exact = len!(self);
(exact, Some(exact))
}
#[inline]
fn count(self) -> usize {
len!(self)
}
#[inline]
fn nth(&mut self, n: usize) -> Option<$elem> {
if n >= len!(self) {
// This iterator is now empty.
if_zst!(mut self,
len => *len = 0,
end => self.ptr = *end,
);
return None;
}
// SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
unsafe {
self.post_inc_start(n);
Some(self.next_unchecked())
}
}
#[inline]
fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
let advance = cmp::min(len!(self), n);
// SAFETY: By construction, `advance` does not exceed `self.len()`.
unsafe { self.post_inc_start(advance) };
NonZero::new(n - advance).map_or(Ok(()), Err)
}
#[inline]
fn last(mut self) -> Option<$elem> {
self.next_back()
}
#[inline]
fn fold<B, F>(self, init: B, mut f: F) -> B
where
F: FnMut(B, Self::Item) -> B,
{
// this implementation consists of the following optimizations compared to the
// default implementation:
// - do-while loop, as is llvm's preferred loop shape,
// see https://releases.llvm.org/16.0.0/docs/LoopTerminology.html#more-canonical-loops
// - bumps an index instead of a pointer since the latter case inhibits
// some optimizations, see #111603
// - avoids Option wrapping/matching
if is_empty!(self) {
return init;
}
let mut acc = init;
let mut i = 0;
let len = len!(self);
loop {
// SAFETY: the loop iterates `i in 0..len`, which always is in bounds of
// the slice allocation
acc = f(acc, unsafe { & $( $mut_ )? *self.ptr.add(i).as_ptr() });
// SAFETY: `i` can't overflow since it'll only reach usize::MAX if the
// slice had that length, in which case we'll break out of the loop
// after the increment
i = unsafe { i.unchecked_add(1) };
if i == len {
break;
}
}
acc
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile.
#[inline]
fn for_each<F>(mut self, mut f: F)
where
Self: Sized,
F: FnMut(Self::Item),
{
while let Some(x) = self.next() {
f(x);
}
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile.
#[inline]
fn all<F>(&mut self, mut f: F) -> bool
where
Self: Sized,
F: FnMut(Self::Item) -> bool,
{
while let Some(x) = self.next() {
if !f(x) {
return false;
}
}
true
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile.
#[inline]
fn any<F>(&mut self, mut f: F) -> bool
where
Self: Sized,
F: FnMut(Self::Item) -> bool,
{
while let Some(x) = self.next() {
if f(x) {
return true;
}
}
false
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile.
#[inline]
fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{
while let Some(x) = self.next() {
if predicate(&x) {
return Some(x);
}
}
None
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile.
#[inline]
fn find_map<B, F>(&mut self, mut f: F) -> Option<B>
where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,
{
while let Some(x) = self.next() {
if let Some(y) = f(x) {
return Some(y);
}
}
None
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile. Also, the `assume` avoids a bounds check.
#[inline]
#[rustc_inherit_overflow_checks]
fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
Self: Sized,
P: FnMut(Self::Item) -> bool,
{
let n = len!(self);
let mut i = 0;
while let Some(x) = self.next() {
if predicate(x) {
// SAFETY: we are guaranteed to be in bounds by the loop invariant:
// when `i >= n`, `self.next()` returns `None` and the loop breaks.
unsafe { assert_unchecked(i < n) };
return Some(i);
}
i += 1;
}
None
}
// We override the default implementation, which uses `try_fold`,
// because this simple implementation generates less LLVM IR and is
// faster to compile. Also, the `assume` avoids a bounds check.
#[inline]
fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,
Self: Sized + ExactSizeIterator + DoubleEndedIterator
{
let n = len!(self);
let mut i = n;
while let Some(x) = self.next_back() {
i -= 1;
if predicate(x) {
// SAFETY: `i` must be lower than `n` since it starts at `n`
// and is only decreasing.
unsafe { assert_unchecked(i < n) };
return Some(i);
}
}
None
}
#[inline]
unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
// SAFETY: the caller must guarantee that `i` is in bounds of
// the underlying slice, so `i` cannot overflow an `isize`, and
// the returned references is guaranteed to refer to an element
// of the slice and thus guaranteed to be valid.
//
// Also note that the caller also guarantees that we're never
// called with the same index again, and that no other methods
// that will access this subslice are called, so it is valid
// for the returned reference to be mutable in the case of
// `IterMut`
unsafe { & $( $mut_ )? * self.ptr.as_ptr().add(idx) }
}
$($extra)*
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for $name<'a, T> {
#[inline]
fn next_back(&mut self) -> Option<$elem> {
// could be implemented with slices, but this avoids bounds checks
// SAFETY: The call to `next_back_unchecked`
// is safe since we check if the iterator is empty first.
unsafe {
if is_empty!(self) {
None
} else {
Some(self.next_back_unchecked())
}
}
}
#[inline]
fn nth_back(&mut self, n: usize) -> Option<$elem> {
if n >= len!(self) {
// This iterator is now empty.
if_zst!(mut self,
len => *len = 0,
end => *end = self.ptr,
);
return None;
}
// SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
unsafe {
self.pre_dec_end(n);
Some(self.next_back_unchecked())
}
}
#[inline]
fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
let advance = cmp::min(len!(self), n);
// SAFETY: By construction, `advance` does not exceed `self.len()`.
unsafe { self.pre_dec_end(advance) };
NonZero::new(n - advance).map_or(Ok(()), Err)
}
}
#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for $name<'_, T> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for $name<'_, T> {}
impl<'a, T> UncheckedIterator for $name<'a, T> {
#[inline]
unsafe fn next_unchecked(&mut self) -> $elem {
// SAFETY: The caller promised there's at least one more item.
unsafe {
self.post_inc_start(1).$into_ref()
}
}
}
#[stable(feature = "default_iters", since = "1.70.0")]
impl<T> Default for $name<'_, T> {
/// Creates an empty slice iterator.
///
/// ```
#[doc = concat!("# use core::slice::", stringify!($name), ";")]
#[doc = concat!("let iter: ", stringify!($name<'_, u8>), " = Default::default();")]
/// assert_eq!(iter.len(), 0);
/// ```
fn default() -> Self {
(& $( $mut_ )? []).into_iter()
}
}
}
}
macro_rules! forward_iterator {
($name:ident: $elem:ident, $iter_of:ty) => {
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, $elem, P> Iterator for $name<'a, $elem, P>
where
P: FnMut(&T) -> bool,
{
type Item = $iter_of;
#[inline]
fn next(&mut self) -> Option<$iter_of> {
self.inner.next()
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
self.inner.size_hint()
}
}
#[stable(feature = "fused", since = "1.26.0")]
impl<'a, $elem, P> FusedIterator for $name<'a, $elem, P> where P: FnMut(&T) -> bool {}
};
}