rust-lang#66219 documented unsafe in core::{alloc, hash}

src/libcore/alloc.rs

@@ -1,7 +1,5 @@
 //! Memory allocation APIs
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
 use crate::cmp;
@@ -88,6 +86,7 @@ impl Layout {
             return Err(LayoutErr { private: () });
         }
 
+        // SAFETY: performed checks above
         unsafe {
             Ok(Layout::from_size_align_unchecked(size, align))
         }
@@ -120,11 +119,11 @@ impl Layout {
     #[inline]
     pub fn new<T>() -> Self {
         let (size, align) = size_align::<T>();
-        // Note that the align is guaranteed by rustc to be a power of two and
+        debug_assert!(Layout::from_size_align(size, align).is_ok());
+        // SAFETY: Note that the align is guaranteed by rustc to be a power of two and
         // the size+align combo is guaranteed to fit in our address space. As a
         // result use the unchecked constructor here to avoid inserting code
         // that panics if it isn't optimized well enough.
-        debug_assert!(Layout::from_size_align(size, align).is_ok());
         unsafe {
             Layout::from_size_align_unchecked(size, align)
         }
@@ -137,8 +136,8 @@ impl Layout {
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
-        // See rationale in `new` for why this us using an unsafe variant below
         debug_assert!(Layout::from_size_align(size, align).is_ok());
+        // SAFETY: See rationale in `new` for why this us using an unsafe variant below
         unsafe {
             Layout::from_size_align_unchecked(size, align)
         }
@@ -243,9 +242,9 @@ impl Layout {
         let alloc_size = padded_size.checked_mul(n)
             .ok_or(LayoutErr { private: () })?;
 
+        // SAFETY: self.align is already known to be valid and alloc_size has been
+        // padded already.
         unsafe {
-            // self.align is already known to be valid and alloc_size has been
-            // padded already.
             Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
         }
     }
@@ -1074,6 +1073,7 @@ pub unsafe trait Alloc {
     {
         let k = Layout::new::<T>();
         if k.size() > 0 {
+            // SAFETY: layout has nonzero size
             unsafe { self.alloc(k).map(|p| p.cast()) }
         } else {
             Err(AllocErr)
@@ -1143,6 +1143,7 @@ pub unsafe trait Alloc {
     {
         match Layout::array::<T>(n) {
             Ok(ref layout) if layout.size() > 0 => {
+                // SAFETY: layout has nonzero size
                 unsafe {
                     self.alloc(layout.clone()).map(|p| p.cast())
                 }

src/libcore/hash/mod.rs

@@ -79,8 +79,6 @@
 //! }
 //! ```
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::fmt;
@@ -569,6 +567,8 @@ mod impls {
                 fn hash_slice<H: Hasher>(data: &[$ty], state: &mut H) {
                     let newlen = data.len() * mem::size_of::<$ty>();
                     let ptr = data.as_ptr() as *const u8;
+                    // SAFETY: all of the requirements for from_raw_parts are guaranteed since
+                    // data is a slice
                     state.write(unsafe { slice::from_raw_parts(ptr, newlen) })
                 }
             }
@@ -688,7 +688,7 @@ mod impls {
                 // Thin pointer
                 state.write_usize(*self as *const () as usize);
             } else {
-                // Fat pointer
+                // SAFETY: since it's not a thin pointer, it's a fat pointer
                 let (a, b) = unsafe {
                     *(self as *const Self as *const (usize, usize))
                 };
@@ -705,7 +705,7 @@ mod impls {
                 // Thin pointer
                 state.write_usize(*self as *const () as usize);
             } else {
-                // Fat pointer
+                // SAFETY: since it's not a thin pointer, it's a fat pointer
                 let (a, b) = unsafe {
                     *(self as *const Self as *const (usize, usize))
                 };

src/libcore/hash/sip.rs

@@ -1,7 +1,5 @@
 //! An implementation of SipHash.
 
-// ignore-tidy-undocumented-unsafe
-
 #![allow(deprecated)] // the types in this module are deprecated
 
 use crate::marker::PhantomData;
@@ -222,8 +220,10 @@ impl<S: Sip> Hasher<S> {
         let needed = 8 - self.ntail;
         let fill = cmp::min(length, needed);
         if fill == 8 {
+            // SAFETY: msg has exactly sizeof(u64) == 8
             self.tail = unsafe { load_int_le!(msg, 0, u64) };
         } else {
+            // SAFETY: fill < 7
             self.tail |= unsafe { u8to64_le(msg, 0, fill) } << (8 * self.ntail);
             if length < needed {
                 self.ntail += length;
@@ -236,6 +236,7 @@ impl<S: Sip> Hasher<S> {
 
         // Buffered tail is now flushed, process new input.
         self.ntail = length - needed;
+        // SAFETY: self.ntail + needed - 1 = length - 1 < 8
         self.tail = unsafe { u8to64_le(msg, needed, self.ntail) };
     }
 }
@@ -270,6 +271,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
     // see short_write comment for explanation
     #[inline]
     fn write_usize(&mut self, i: usize) {
+        // SAFETY: bytes leaves scope as i does
         let bytes = unsafe {
             crate::slice::from_raw_parts(&i as *const usize as *const u8, mem::size_of::<usize>())
         };
@@ -291,6 +293,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
 
         if self.ntail != 0 {
             needed = 8 - self.ntail;
+            // SAFETY: needed < 8 since self.ntail != 0
             self.tail |= unsafe { u8to64_le(msg, 0, cmp::min(length, needed)) } << 8 * self.ntail;
             if length < needed {
                 self.ntail += length;
@@ -309,6 +312,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
 
         let mut i = needed;
         while i < len - left {
+            // SAFETY: i + 8 <= length
             let mi = unsafe { load_int_le!(msg, i, u64) };
 
             self.state.v3 ^= mi;
@@ -318,6 +322,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
             i += 8;
         }
 
+        // SAFETY: left < 8
         self.tail = unsafe { u8to64_le(msg, i, left) };
         self.ntail = left;
     }