Rollup merge of rust-lang#63793 - oli-obk:🧹, r=dtolnay

Have tidy ensure that we document all `unsafe` blocks in libcore

cc @rust-lang/libs

I documented a few and added ignore flags on the other files. We can incrementally document the files, but won't regress any files this way.

src/libcore/alloc.rs

@@ -1,5 +1,7 @@
 //! Memory allocation APIs
 
+// ignore-tidy-undocumented-unsafe
+
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
 use crate::cmp;

src/libcore/any.rs

@@ -182,6 +182,7 @@ impl dyn Any {
     #[inline]
     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
         if self.is::<T>() {
+            // SAFETY: just checked whether we are pointing to the correct type
             unsafe {
                 Some(&*(self as *const dyn Any as *const T))
             }
@@ -217,6 +218,7 @@ impl dyn Any {
     #[inline]
     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
         if self.is::<T>() {
+            // SAFETY: just checked whether we are pointing to the correct type
             unsafe {
                 Some(&mut *(self as *mut dyn Any as *mut T))
             }
@@ -424,7 +426,11 @@ impl TypeId {
     #[rustc_const_unstable(feature="const_type_id")]
     pub const fn of<T: ?Sized + 'static>() -> TypeId {
         TypeId {
+            #[cfg(bootstrap)]
+            // SAFETY: going away soon
             t: unsafe { intrinsics::type_id::<T>() },
+            #[cfg(not(bootstrap))]
+            t: intrinsics::type_id::<T>(),
         }
     }
 }

src/libcore/array/iter.rs

@@ -51,7 +51,7 @@ where
     /// iterator (either via `IntoIterator` for arrays or via another way).
     #[unstable(feature = "array_value_iter", issue = "65798")]
     pub fn new(array: [T; N]) -> Self {
-        // The transmute here is actually safe. The docs of `MaybeUninit`
+        // SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
         // promise:
         //
         // > `MaybeUninit<T>` is guaranteed to have the same size and alignment
@@ -84,10 +84,10 @@ where
     /// Returns an immutable slice of all elements that have not been yielded
     /// yet.
     fn as_slice(&self) -> &[T] {
-        // This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
+        let slice = &self.data[self.alive.clone()];
+        // SAFETY: This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
         // the size and alignment of `T`. Furthermore, we know that all
         // elements within `alive` are properly initialized.
-        let slice = &self.data[self.alive.clone()];
         unsafe {
             mem::transmute::<&[MaybeUninit<T>], &[T]>(slice)
         }
@@ -117,7 +117,8 @@ where
         let idx = self.alive.start;
         self.alive.start += 1;
 
-        // Read the element from the array. This is safe: `idx` is an index
+        // Read the element from the array.
+        // SAFETY: This is safe: `idx` is an index
         // into the "alive" region of the array. Reading this element means
         // that `data[idx]` is regarded as dead now (i.e. do not touch). As
         // `idx` was the start of the alive-zone, the alive zone is now
@@ -163,7 +164,8 @@ where
         // + 1]`.
         self.alive.end -= 1;
 
-        // Read the element from the array. This is safe: `alive.end` is an
+        // Read the element from the array.
+        // SAFETY: This is safe: `alive.end` is an
         // index into the "alive" region of the array. Compare the previous
         // comment that states that the alive region is
         // `data[alive.start..alive.end + 1]`. Reading this element means that
@@ -226,6 +228,7 @@ where
     [T; N]: LengthAtMost32,
 {
     fn clone(&self) -> Self {
+        // SAFETY: each point of unsafety is documented inside the unsafe block
         unsafe {
             // This creates a new uninitialized array. Note that the `assume_init`
             // refers to the array, not the individual elements. And it is Ok if

src/libcore/array/mod.rs

@@ -156,6 +156,7 @@ where
     fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
         if slice.len() == N {
             let ptr = slice.as_ptr() as *const [T; N];
+            // SAFETY: ok because we just checked that the length fits
             unsafe { Ok(&*ptr) }
         } else {
             Err(TryFromSliceError(()))
@@ -173,6 +174,7 @@ where
     fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
         if slice.len() == N {
             let ptr = slice.as_mut_ptr() as *mut [T; N];
+            // SAFETY: ok because we just checked that the length fits
             unsafe { Ok(&mut *ptr) }
         } else {
             Err(TryFromSliceError(()))

src/libcore/ascii.rs

@@ -135,6 +135,7 @@ impl FusedIterator for EscapeDefault {}
 #[stable(feature = "ascii_escape_display", since = "1.39.0")]
 impl fmt::Display for EscapeDefault {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // SAFETY: ok because `escape_default` created only valid utf-8 data
         f.write_str(unsafe { from_utf8_unchecked(&self.data[self.range.clone()]) })
     }
 }

src/libcore/benches/ascii.rs

@@ -118,6 +118,7 @@ benches! {
     }
 
     fn case07_fake_simd_u32(bytes: &mut [u8]) {
+        // SAFETY: transmuting a sequence of `u8` to `u32` is always fine
         let (before, aligned, after) = unsafe {
             bytes.align_to_mut::<u32>()
         };
@@ -142,6 +143,7 @@ benches! {
     }
 
     fn case08_fake_simd_u64(bytes: &mut [u8]) {
+        // SAFETY: transmuting a sequence of `u8` to `u64` is always fine
         let (before, aligned, after) = unsafe {
             bytes.align_to_mut::<u64>()
         };

src/libcore/cell.rs

@@ -187,6 +187,8 @@
 //! ```
 //!
 
+// ignore-tidy-undocumented-unsafe
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cmp::Ordering;

src/libcore/char/convert.rs

@@ -224,6 +224,7 @@ impl TryFrom<u32> for char {
         if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
             Err(CharTryFromError(()))
         } else {
+            // SAFETY: checked that it's a legal unicode value
             Ok(unsafe { from_u32_unchecked(i) })
         }
     }

src/libcore/char/decode.rs

@@ -87,7 +87,7 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
         };
 
         if u < 0xD800 || 0xDFFF < u {
-            // not a surrogate
+            // SAFETY: not a surrogate
             Some(Ok(unsafe { from_u32_unchecked(u as u32) }))
         } else if u >= 0xDC00 {
             // a trailing surrogate
@@ -107,6 +107,7 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
 
             // all ok, so lets decode it.
             let c = (((u - 0xD800) as u32) << 10 | (u2 - 0xDC00) as u32) + 0x1_0000;
+            // SAFETY: we checked that it's a legal unicode value
             Some(Ok(unsafe { from_u32_unchecked(c) }))
         }
     }

src/libcore/char/methods.rs

@@ -438,6 +438,7 @@ impl char {
     #[inline]
     pub fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
         let code = self as u32;
+        // SAFETY: each arm checks the size of the slice and only uses `get_unchecked` unsafe ops
         unsafe {
             let len = if code < MAX_ONE_B && !dst.is_empty() {
                 *dst.get_unchecked_mut(0) = code as u8;
@@ -507,6 +508,7 @@ impl char {
     #[inline]
     pub fn encode_utf16(self, dst: &mut [u16]) -> &mut [u16] {
         let mut code = self as u32;
+        // SAFETY: each arm checks whether there are enough bits to write into
         unsafe {
             if (code & 0xFFFF) == code && !dst.is_empty() {
                 // The BMP falls through (assuming non-surrogate, as it should)

src/libcore/ffi.rs

@@ -315,6 +315,7 @@ impl<'f> Clone for VaListImpl<'f> {
     #[inline]
     fn clone(&self) -> Self {
         let mut dest = crate::mem::MaybeUninit::uninit();
+        // SAFETY: we write to the `MaybeUninit`, thus it is initialized and `assume_init` is legal
         unsafe {
             va_copy(dest.as_mut_ptr(), self);
             dest.assume_init()

src/libcore/fmt/float.rs

@@ -2,6 +2,8 @@ use crate::fmt::{Formatter, Result, LowerExp, UpperExp, Display, Debug};
 use crate::mem::MaybeUninit;
 use crate::num::flt2dec;
 
+// ignore-tidy-undocumented-unsafe
+
 // Don't inline this so callers don't use the stack space this function
 // requires unless they have to.
 #[inline(never)]

src/libcore/fmt/mod.rs

@@ -1,5 +1,7 @@
 //! Utilities for formatting and printing strings.
 
+// ignore-tidy-undocumented-unsafe
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cell::{UnsafeCell, Cell, RefCell, Ref, RefMut};

src/libcore/fmt/num.rs

@@ -1,5 +1,7 @@
 //! Integer and floating-point number formatting
 
+// ignore-tidy-undocumented-unsafe
+
 
 use crate::fmt;
 use crate::ops::{Div, Rem, Sub};

src/libcore/hash/mod.rs

@@ -79,6 +79,8 @@
 //! }
 //! ```
 
+// ignore-tidy-undocumented-unsafe
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::fmt;

src/libcore/hash/sip.rs

@@ -1,5 +1,7 @@
 //! An implementation of SipHash.
 
+// ignore-tidy-undocumented-unsafe
+
 #![allow(deprecated)] // the types in this module are deprecated
 
 use crate::marker::PhantomData;

src/libcore/hint.rs

@@ -2,6 +2,8 @@
 
 //! Hints to compiler that affects how code should be emitted or optimized.
 
+// ignore-tidy-undocumented-unsafe
+
 use crate::intrinsics;
 
 /// Informs the compiler that this point in the code is not reachable, enabling

src/libcore/iter/adapters/mod.rs

@@ -517,8 +517,18 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
         // overflow handling
         loop {
             let mul = n.checked_mul(step);
-            if unsafe { intrinsics::likely(mul.is_some()) } {
-                return self.iter.nth(mul.unwrap() - 1);
+            #[cfg(bootstrap)]
+            {
+                // SAFETY: going away soon
+                if unsafe { intrinsics::likely(mul.is_some()) } {
+                    return self.iter.nth(mul.unwrap() - 1);
+                }
+            }
+            #[cfg(not(bootstrap))]
+            {
+                if intrinsics::likely(mul.is_some()) {
+                    return self.iter.nth(mul.unwrap() - 1);
+                }
             }
             let div_n = usize::MAX / n;
             let div_step = usize::MAX / step;

src/libcore/iter/adapters/zip.rs

@@ -1,3 +1,5 @@
+// ignore-tidy-undocumented-unsafe
+
 use crate::cmp;
 
 use super::super::{Iterator, DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen};

src/libcore/mem/maybe_uninit.rs

@@ -1,6 +1,8 @@
 use crate::intrinsics;
 use crate::mem::ManuallyDrop;
 
+// ignore-tidy-undocumented-unsafe
+
 /// A wrapper type to construct uninitialized instances of `T`.
 ///
 /// # Initialization invariant

src/libcore/mem/mod.rs

@@ -93,6 +93,8 @@ pub fn forget<T>(t: T) {
 #[inline]
 #[unstable(feature = "forget_unsized", issue = "0")]
 pub fn forget_unsized<T: ?Sized>(t: T) {
+    // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since
+    // we'll be implementing this function soon via `ManuallyDrop`
     unsafe { intrinsics::forget(t) }
 }
 
@@ -266,7 +268,11 @@ pub const fn size_of<T>() -> usize {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
+    #[cfg(bootstrap)]
+    // SAFETY: going away soon
     unsafe { intrinsics::size_of_val(val) }
+    #[cfg(not(bootstrap))]
+    intrinsics::size_of_val(val)
 }
 
 /// Returns the [ABI]-required minimum alignment of a type.
@@ -310,7 +316,11 @@ pub fn min_align_of<T>() -> usize {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
+    #[cfg(bootstrap)]
+    // SAFETY: going away soon
     unsafe { intrinsics::min_align_of_val(val) }
+    #[cfg(not(bootstrap))]
+    intrinsics::min_align_of_val(val)
 }
 
 /// Returns the [ABI]-required minimum alignment of a type.
@@ -350,8 +360,9 @@ pub const fn align_of<T>() -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[allow(deprecated)]
 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
-    unsafe { intrinsics::min_align_of_val(val) }
+    min_align_of_val(val)
 }
 
 /// Returns `true` if dropping values of type `T` matters.
@@ -508,6 +519,8 @@ pub unsafe fn uninitialized<T>() -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn swap<T>(x: &mut T, y: &mut T) {
+    // SAFETY: the raw pointers have been created from safe mutable references satisfying all the
+    // constraints on `ptr::swap_nonoverlapping_one`
     unsafe {
         ptr::swap_nonoverlapping_one(x, y);
     }
@@ -822,7 +835,11 @@ impl<T> fmt::Debug for Discriminant<T> {
 /// ```
 #[stable(feature = "discriminant_value", since = "1.21.0")]
 pub fn discriminant<T>(v: &T) -> Discriminant<T> {
+    #[cfg(bootstrap)]
+    // SAFETY: going away soon
     unsafe {
         Discriminant(intrinsics::discriminant_value(v), PhantomData)
     }
+    #[cfg(not(bootstrap))]
+    Discriminant(intrinsics::discriminant_value(v), PhantomData)
 }

src/libcore/num/dec2flt/algorithm.rs

@@ -58,6 +58,8 @@ mod fpu_precision {
     pub struct FPUControlWord(u16);
 
     fn set_cw(cw: u16) {
+        // SAFETY: the `fldcw` instruction has been audited to be able to work correctly with
+        // any `u16`
         unsafe { asm!("fldcw $0" :: "m" (cw) :: "volatile") }
     }
 
@@ -74,6 +76,8 @@ mod fpu_precision {
 
         // Get the original value of the control word to restore it later, when the
         // `FPUControlWord` structure is dropped
+        // SAFETY: the `fnstcw` instruction has been audited to be able to work correctly with
+        // any `u16`
         unsafe { asm!("fnstcw $0" : "=*m" (&cw) ::: "volatile") }
 
         // Set the control word to the desired precision. This is achieved by masking away the old

src/libcore/num/f32.rs

@@ -414,6 +414,7 @@ impl f32 {
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[inline]
     pub fn to_bits(self) -> u32 {
+        // SAFETY: `u32` is a plain old datatype so we can always transmute to it
         unsafe { mem::transmute(self) }
     }
 
@@ -456,6 +457,7 @@ impl f32 {
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[inline]
     pub fn from_bits(v: u32) -> Self {
+        // SAFETY: `u32` is a plain old datatype so we can always transmute from it
         // It turns out the safety issues with sNaN were overblown! Hooray!
         unsafe { mem::transmute(v) }
     }

src/libcore/num/f64.rs

@@ -427,6 +427,7 @@ impl f64 {
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[inline]
     pub fn to_bits(self) -> u64 {
+        // SAFETY: `u64` is a plain old datatype so we can always transmute to it
         unsafe { mem::transmute(self) }
     }
 
@@ -469,6 +470,7 @@ impl f64 {
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[inline]
     pub fn from_bits(v: u64) -> Self {
+        // SAFETY: `u64` is a plain old datatype so we can always transmute from it
         // It turns out the safety issues with sNaN were overblown! Hooray!
         unsafe { mem::transmute(v) }
     }