diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs index c69aafe687cf8..c5f059e6b11a0 100644 --- a/src/libcore/slice/mod.rs +++ b/src/libcore/slice/mod.rs @@ -1,5 +1,4 @@ // ignore-tidy-filelength -// ignore-tidy-undocumented-unsafe //! Slice management and manipulation. //! @@ -70,6 +69,8 @@ impl [T] { #[allow(unused_attributes)] #[allow_internal_unstable(const_fn_union)] pub const fn len(&self) -> usize { + // SAFETY: this is safe because `&[T]` and `FatPtr` have the same layout. + // Only `std` can make this guarantee. unsafe { crate::ptr::Repr { rust: self }.raw.len } } @@ -437,7 +438,8 @@ impl [T] { #[unstable(feature = "slice_ptr_range", issue = "65807")] #[inline] pub fn as_ptr_range(&self) -> Range<*const T> { - // The `add` here is safe, because: + let start = self.as_ptr(); + // SAFETY: The `add` here is safe, because: // // - Both pointers are part of the same object, as pointing directly // past the object also counts. @@ -454,7 +456,6 @@ impl [T] { // the end of the address space. // // See the documentation of pointer::add. - let start = self.as_ptr(); let end = unsafe { start.add(self.len()) }; start..end } @@ -478,8 +479,8 @@ impl [T] { #[unstable(feature = "slice_ptr_range", issue = "65807")] #[inline] pub fn as_mut_ptr_range(&mut self) -> Range<*mut T> { - // See as_ptr_range() above for why `add` here is safe. let start = self.as_mut_ptr(); + // SAFETY: See as_ptr_range() above for why `add` here is safe. let end = unsafe { start.add(self.len()) }; start..end } @@ -505,6 +506,8 @@ impl [T] { #[stable(feature = "rust1", since = "1.0.0")] #[inline] pub fn swap(&mut self, a: usize, b: usize) { + // SAFETY: `pa` and `pb` have been created from safe mutable references and refer + // to elements in the slice and therefore are guaranteed to be valid and aligned. unsafe { // Can't take two mutable loans from one vector, so instead just cast // them to their raw pointers to do the swap @@ -548,6 +551,10 @@ impl [T] { // Use the llvm.bswap intrinsic to reverse u8s in a usize let chunk = mem::size_of::(); while i + chunk - 1 < ln / 2 { + // SAFETY: the condition of the `while` guarantees that + // `i` and `ln - i - chunk` are inside the slice. + // The resulting pointers `pa` and `pb` are therefore valid, + // and can be read from and written to. unsafe { let pa: *mut T = self.get_unchecked_mut(i); let pb: *mut T = self.get_unchecked_mut(ln - i - chunk); @@ -564,6 +571,10 @@ impl [T] { // Use rotate-by-16 to reverse u16s in a u32 let chunk = mem::size_of::() / 2; while i + chunk - 1 < ln / 2 { + // SAFETY: the condition of the `while` guarantees that + // `i` and `ln - i - chunk` are inside the slice. + // The resulting pointers `pa` and `pb` are therefore valid, + // and can be read from and written to. unsafe { let pa: *mut T = self.get_unchecked_mut(i); let pb: *mut T = self.get_unchecked_mut(ln - i - chunk); @@ -577,8 +588,12 @@ impl [T] { } while i < ln / 2 { - // Unsafe swap to avoid the bounds check in safe swap. + // SAFETY: the condition of the `while` guarantees that `i` and `ln - i - 1` + // are inside the slice and refer to an element inside the slice. + // The resulting pointers `pa` and `pb` are therefore valid and aligned, + // and can be read from and written to. unsafe { + // Unsafe swap to avoid the bounds check in safe swap. let pa: *mut T = self.get_unchecked_mut(i); let pb: *mut T = self.get_unchecked_mut(ln - i - 1); ptr::swap(pa, pb); @@ -603,6 +618,9 @@ impl [T] { #[stable(feature = "rust1", since = "1.0.0")] #[inline] pub fn iter(&self) -> Iter<'_, T> { + // SAFETY: adding `self.len()` to the starting pointer gives a pointer + // at the end of `self`, which fulfills the expectations of `ptr.add()` + // and `NonNull::new_unchecked()`. unsafe { let ptr = self.as_ptr(); assume(!ptr.is_null()); @@ -631,6 +649,9 @@ impl [T] { #[stable(feature = "rust1", since = "1.0.0")] #[inline] pub fn iter_mut(&mut self) -> IterMut<'_, T> { + // SAFETY: adding `self.len()` to the starting pointer gives a pointer + // at the end of `self`, which fulfills the expectations of `ptr.add()` + // and `NonNull::new_unchecked()`. unsafe { let ptr = self.as_mut_ptr(); assume(!ptr.is_null()); @@ -1062,6 +1083,8 @@ impl [T] { let len = self.len(); let ptr = self.as_mut_ptr(); + // SAFETY: `[ptr;mid]` and `[mid;len]` are inside `self`, which fulfills the + // requirements of `from_raw_parts_mut`. unsafe { assert!(mid <= len); @@ -1548,14 +1571,14 @@ impl [T] { while size > 1 { let half = size / 2; let mid = base + half; - // mid is always in [0, size), that means mid is >= 0 and < size. + // SAFETY: mid is always in [0, size), that means mid is >= 0 and < size. // mid >= 0: by definition // mid < size: mid = size / 2 + size / 4 + size / 8 ... let cmp = f(unsafe { s.get_unchecked(mid) }); base = if cmp == Greater { base } else { mid }; size -= half; } - // base is always in [0, size) because base <= mid. + // SAFETY: base is always in [0, size) because base <= mid. let cmp = f(unsafe { s.get_unchecked(base) }); if cmp == Equal { Ok(base) } else { Err(base + (cmp == Less) as usize) } } @@ -2013,6 +2036,13 @@ impl [T] { let mut next_read: usize = 1; let mut next_write: usize = 1; + // SAFETY: the `while` condition guarantees `next_read` and `next_write` + // are less than `len`, thus are inside `self`. `prev_ptr_write` points to + // one element before `ptr_write`, but `next_write` starts at 1, so + // `prev_ptr_write` is never less than 0 and is inside the slice. + // This fulfils the requirements for dereferencing `ptr_read`, `prev_ptr_write` + // and `ptr_write`, and for using `ptr.add(next_read)`, `ptr.add(next_write - 1)` + // and `prev_ptr_write.offset(1)`. unsafe { // Avoid bounds checks by using raw pointers. while next_read < len { @@ -2097,6 +2127,8 @@ impl [T] { assert!(mid <= self.len()); let k = self.len() - mid; + // SAFETY: `[mid - mid;mid+k]` corresponds to the entire + // `self` slice, thus is valid for reads and writes. unsafe { let p = self.as_mut_ptr(); rotate::ptr_rotate(mid, p.add(mid), k); @@ -2138,6 +2170,8 @@ impl [T] { assert!(k <= self.len()); let mid = self.len() - k; + // SAFETY: `[mid - mid;mid+k]` corresponds to the entire + // `self` slice, thus is valid for reads and writes. unsafe { let p = self.as_mut_ptr(); rotate::ptr_rotate(mid, p.add(mid), k); @@ -2300,6 +2334,9 @@ impl [T] { T: Copy, { assert_eq!(self.len(), src.len(), "destination and source slices have different lengths"); + // SAFETY: `self` is valid for `self.len()` bytes by definition, and `src` was + // checked to have the same length. Both slices cannot be overlapping because + // Rust's mutable references are exclusive. unsafe { ptr::copy_nonoverlapping(src.as_ptr(), self.as_mut_ptr(), self.len()); } @@ -2353,6 +2390,7 @@ impl [T] { assert!(src_end <= self.len(), "src is out of bounds"); let count = src_end - src_start; assert!(dest <= self.len() - count, "dest is out of bounds"); + // SAFETY: the conditions for `ptr::copy` have been checked above. unsafe { ptr::copy(self.as_ptr().add(src_start), self.as_mut_ptr().add(dest), count); } @@ -2408,6 +2446,9 @@ impl [T] { #[stable(feature = "swap_with_slice", since = "1.27.0")] pub fn swap_with_slice(&mut self, other: &mut [T]) { assert!(self.len() == other.len(), "destination and source slices have different lengths"); + // SAFETY: `self` is valid for `self.len()` bytes by definition, and `src` was + // checked to have the same length. Both slices cannot be overlapping because + // Rust's mutable references are exclusive. unsafe { ptr::swap_nonoverlapping(self.as_mut_ptr(), other.as_mut_ptr(), self.len()); } @@ -2439,6 +2480,8 @@ impl [T] { // iterative stein’s algorithm // We should still make this `const fn` (and revert to recursive algorithm if we do) // because relying on llvm to consteval all this is… well, it makes me uncomfortable. + + // SAFETY: `a` and `b` are checked to be non-zero values. let (ctz_a, mut ctz_b) = unsafe { if a == 0 { return b; @@ -2458,6 +2501,7 @@ impl [T] { mem::swap(&mut a, &mut b); } b = b - a; + // SAFETY: `b` is checked to be non-zero. unsafe { if b == 0 { break; @@ -2848,11 +2892,13 @@ impl SliceIndex<[T]> for usize { #[inline] fn get(self, slice: &[T]) -> Option<&T> { + // SAFETY: `self` is checked to be in bounds. if self < slice.len() { unsafe { Some(self.get_unchecked(slice)) } } else { None } } #[inline] fn get_mut(self, slice: &mut [T]) -> Option<&mut T> { + // SAFETY: `self` is checked to be in bounds. if self < slice.len() { unsafe { Some(self.get_unchecked_mut(slice)) } } else { None } } @@ -2888,6 +2934,7 @@ impl SliceIndex<[T]> for ops::Range { if self.start > self.end || self.end > slice.len() { None } else { + // SAFETY: `self` is checked to be valid and in bounds above. unsafe { Some(self.get_unchecked(slice)) } } } @@ -2897,6 +2944,7 @@ impl SliceIndex<[T]> for ops::Range { if self.start > self.end || self.end > slice.len() { None } else { + // SAFETY: `self` is checked to be valid and in bounds above. unsafe { Some(self.get_unchecked_mut(slice)) } } } @@ -2918,6 +2966,7 @@ impl SliceIndex<[T]> for ops::Range { } else if self.end > slice.len() { slice_index_len_fail(self.end, slice.len()); } + // SAFETY: `self` is checked to be valid and in bounds above. unsafe { self.get_unchecked(slice) } } @@ -2928,6 +2977,7 @@ impl SliceIndex<[T]> for ops::Range { } else if self.end > slice.len() { slice_index_len_fail(self.end, slice.len()); } + // SAFETY: `self` is checked to be valid and in bounds above. unsafe { self.get_unchecked_mut(slice) } } } @@ -3239,6 +3289,8 @@ macro_rules! iterator { // 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)) } } @@ -3292,6 +3344,10 @@ macro_rules! iterator { #[inline] fn next(&mut self) -> Option<$elem> { // could be implemented with slices, but this avoids bounds checks + + // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null, + // and slices over non-ZSTs must also have a non-null end pointer. + // The call to `next_unchecked!` is safe since we check if the iterator is empty first. unsafe { assume(!self.ptr.as_ptr().is_null()); if mem::size_of::() != 0 { @@ -3325,14 +3381,14 @@ macro_rules! iterator { // could be (due to wrapping). self.end = self.ptr.as_ptr(); } else { + // SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr unsafe { - // End can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr self.ptr = NonNull::new_unchecked(self.end as *mut T); } } return None; } - // We are in bounds. `post_inc_start` does the right thing even for ZSTs. + // SAFETY: we are in bounds. `post_inc_start` does the right thing even for ZSTs. unsafe { self.post_inc_start(n as isize); Some(next_unchecked!(self)) @@ -3439,6 +3495,8 @@ macro_rules! iterator { let mut i = 0; while let Some(x) = self.next() { if predicate(x) { + // SAFETY: we are guaranteed to be in bounds by the loop invariant: + // when `i >= n`, `self.next()` returns `None` and the loop breaks. unsafe { assume(i < n) }; return Some(i); } @@ -3460,6 +3518,8 @@ macro_rules! iterator { while let Some(x) = self.next_back() { i -= 1; if predicate(x) { + // SAFETY: `i` must be lower than `n` since it starts at `n` + // and is only decreasing. unsafe { assume(i < n) }; return Some(i); } @@ -3475,6 +3535,10 @@ macro_rules! iterator { #[inline] fn next_back(&mut self) -> Option<$elem> { // could be implemented with slices, but this avoids bounds checks + + // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null, + // and slices over non-ZSTs must also have a non-null end pointer. + // The call to `next_back_unchecked!` is safe since we check if the iterator is empty first. unsafe { assume(!self.ptr.as_ptr().is_null()); if mem::size_of::() != 0 { @@ -3495,7 +3559,7 @@ macro_rules! iterator { self.end = self.ptr.as_ptr(); return None; } - // We are in bounds. `pre_dec_end` does the right thing even for ZSTs. + // SAFETY: we are in bounds. `pre_dec_end` does the right thing even for ZSTs. unsafe { self.pre_dec_end(n as isize); Some(next_back_unchecked!(self)) @@ -3690,6 +3754,8 @@ impl<'a, T> IterMut<'a, T> { /// ``` #[stable(feature = "iter_to_slice", since = "1.4.0")] pub fn into_slice(self) -> &'a mut [T] { + // SAFETY: the iterator was created from a mutable slice with pointer `self.ptr` and length `len!(self)`. + // This guarantees that all the prerequisites for `from_raw_parts_mut` are fulfilled. unsafe { from_raw_parts_mut(self.ptr.as_ptr(), len!(self)) } } @@ -5855,12 +5921,20 @@ pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] /// Converts a reference to T into a slice of length 1 (without copying). #[stable(feature = "from_ref", since = "1.28.0")] pub fn from_ref(s: &T) -> &[T] { + // SAFETY: a reference is guaranteed to be valid for reads. The returned + // reference cannot be mutated as it is an immutable reference. + // `mem::size_of::()` cannot be larger than `isize::MAX`. + // Thus the call to `from_raw_parts` is safe. unsafe { from_raw_parts(s, 1) } } /// Converts a reference to T into a slice of length 1 (without copying). #[stable(feature = "from_ref", since = "1.28.0")] pub fn from_mut(s: &mut T) -> &mut [T] { + // SAFETY: a mutable reference is guaranteed to be valid for writes. + // The reference cannot be accessed by another pointer as it is an mutable reference. + // `mem::size_of::()` cannot be larger than `isize::MAX`. + // Thus the call to `from_raw_parts_mut` is safe. unsafe { from_raw_parts_mut(s, 1) } } @@ -5993,6 +6067,9 @@ where if self.as_ptr().guaranteed_eq(other.as_ptr()) { return true; } + + // SAFETY: `self` and `other` are references and are thus guaranteed to be valid. + // The two slices have been checked to have the same size above. unsafe { let size = mem::size_of_val(self); memcmp(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0 @@ -6095,6 +6172,9 @@ impl SliceOrd for u8 { #[inline] fn compare(left: &[Self], right: &[Self]) -> Ordering { let order = + // SAFETY: `left` and `right` are references and are thus guaranteed to be valid. + // We use the minimum of both lengths which guarantees that both regions are + // valid for reads in that interval. unsafe { memcmp(left.as_ptr(), right.as_ptr(), cmp::min(left.len(), right.len())) }; if order == 0 { left.len().cmp(&right.len()) @@ -6164,6 +6244,10 @@ impl SliceContains for u8 { impl SliceContains for i8 { fn slice_contains(&self, x: &[Self]) -> bool { let byte = *self as u8; + // SAFETY: `i8` and `u8` have the same memory layout, thus casting `x.as_ptr()` + // as `*const u8` is safe. The `x.as_ptr()` comes from a reference and is thus guaranteed + // to be valid for reads for the length of the slice `x.len()`, which cannot be larger + // than `isize::MAX`. The returned slice is never mutated. let bytes: &[u8] = unsafe { from_raw_parts(x.as_ptr() as *const u8, x.len()) }; memchr::memchr(byte, bytes).is_some() }