Auto merge of #390 - RalfJung:strict-provenance, r=Amanieu

use strict provenance APIs on nightly

This makes hashbrown compatible with strict provenance, which helps Miri in ensuring pointers are used correctly.

Fixes #384

ci/miri.sh

@@ -9,4 +9,4 @@ rustup toolchain install nightly --component miri
 rustup override set nightly
 cargo miri setup
 
-MIRIFLAGS='-Zmiri-retag-fields' cargo miri test
+MIRIFLAGS='-Zmiri-strict-provenance' cargo miri test --features nightly

src/lib.rs

@@ -22,7 +22,8 @@
         slice_ptr_get,
         nonnull_slice_from_raw_parts,
         maybe_uninit_array_assume_init,
-        build_hasher_simple_hash_one
+        build_hasher_simple_hash_one,
+        strict_provenance
     )
 )]
 #![allow(
@@ -37,6 +38,7 @@
 )]
 #![warn(missing_docs)]
 #![warn(rust_2018_idioms)]
+#![cfg_attr(feature = "nightly", warn(fuzzy_provenance_casts))]
 
 #[cfg(test)]
 #[macro_use]

src/raw/mod.rs

@@ -69,6 +69,16 @@ fn unlikely(b: bool) -> bool {
     b
 }
 
+// Use strict provenance functions if available.
+#[cfg(feature = "nightly")]
+use core::ptr::invalid_mut;
+// Implement it with a cast otherwise.
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+fn invalid_mut<T>(addr: usize) -> *mut T {
+    addr as *mut T
+}
+
 #[inline]
 unsafe fn offset_from<T>(to: *const T, from: *const T) -> usize {
     to.offset_from(from) as usize
@@ -371,7 +381,7 @@ impl<T> Bucket<T> {
             // won't overflow because index must be less than length (bucket_mask)
             // and bucket_mask is guaranteed to be less than `isize::MAX`
             // (see TableLayout::calculate_layout_for method)
-            (index + 1) as *mut T
+            invalid_mut(index + 1)
         } else {
             base.as_ptr().sub(index)
         };
@@ -507,7 +517,8 @@ impl<T> Bucket<T> {
     pub fn as_ptr(&self) -> *mut T {
         if Self::IS_ZERO_SIZED_TYPE {
             // Just return an arbitrary ZST pointer which is properly aligned
-            mem::align_of::<T>() as *mut T
+            // invalid pointer is good enough for ZST
+            invalid_mut(mem::align_of::<T>())
         } else {
             unsafe { self.ptr.as_ptr().sub(1) }
         }
@@ -553,7 +564,8 @@ impl<T> Bucket<T> {
     #[inline]
     unsafe fn next_n(&self, offset: usize) -> Self {
         let ptr = if Self::IS_ZERO_SIZED_TYPE {
-            (self.ptr.as_ptr() as usize + offset) as *mut T
+            // invalid pointer is good enough for ZST
+            invalid_mut(self.ptr.as_ptr() as usize + offset)
         } else {
             self.ptr.as_ptr().sub(offset)
         };

src/set.rs

@@ -2907,4 +2907,11 @@ mod test_set {
             set.insert(i);
         }
     }
+
+    #[test]
+    fn collect() {
+        // At the time of writing, this hits the ZST case in from_base_index
+        // (and without the `map`, it does not).
+        let mut _set: HashSet<_> = (0..3).map(|_| ()).collect();
+    }
 }