Auto merge of #92686 - saethlin:unsafe-debug-asserts, r=Amanieu

Add debug assertions to some unsafe functions

As suggested by #51713

~~Some similar code calls `abort()` instead of `panic!()` but aborting doesn't work in a `const fn`, and the intrinsic for doing dispatch based on whether execution is in a const is unstable.~~

This picked up some invalid uses of `get_unchecked` in the compiler, and fixes them.

I can confirm that they do in fact pick up invalid uses of `get_unchecked` in the wild, though the user experience is less-than-awesome:
```
     Running unittests (target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50)

running 6 tests
error: test failed, to rerun pass '--lib'

Caused by:
  process didn't exit successfully: `/home/ben/rle-decode-helper/target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50` (signal: 4, SIGILL: illegal instruction)
```

~~As best I can tell these changes produce a 6% regression in the runtime of `./x.py test` when `[rust] debug = true` is set.~~
Latest commit (6894d55) brings the additional overhead from this PR down to 0.5%, while also adding a few more assertions. I think this actually covers all the places in `core` that it is reasonable to check for safety requirements at runtime.

Thoughts?

compiler/rustc_data_structures/src/map_in_place.rs

@@ -30,13 +30,13 @@ impl<T> MapInPlace<T> for Vec<T> {
             while read_i < old_len {
                 // move the read_i'th item out of the vector and map it
                 // to an iterator
-                let e = ptr::read(self.get_unchecked(read_i));
+                let e = ptr::read(self.as_ptr().add(read_i));
                 let iter = f(e).into_iter();
                 read_i += 1;
 
                 for e in iter {
                     if write_i < read_i {
-                        ptr::write(self.get_unchecked_mut(write_i), e);
+                        ptr::write(self.as_mut_ptr().add(write_i), e);
                         write_i += 1;
                     } else {
                         // If this is reached we ran out of space
@@ -76,13 +76,13 @@ impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
             while read_i < old_len {
                 // move the read_i'th item out of the vector and map it
                 // to an iterator
-                let e = ptr::read(self.get_unchecked(read_i));
+                let e = ptr::read(self.as_ptr().add(read_i));
                 let iter = f(e).into_iter();
                 read_i += 1;
 
                 for e in iter {
                     if write_i < read_i {
-                        ptr::write(self.get_unchecked_mut(write_i), e);
+                        ptr::write(self.as_mut_ptr().add(write_i), e);
                         write_i += 1;
                     } else {
                         // If this is reached we ran out of space

library/alloc/benches/vec.rs

@@ -627,10 +627,10 @@ fn bench_map_regular(b: &mut Bencher) {
 fn bench_map_fast(b: &mut Bencher) {
     let data = black_box([(0, 0); LEN]);
     b.iter(|| {
-        let mut result = Vec::with_capacity(data.len());
+        let mut result: Vec<u32> = Vec::with_capacity(data.len());
         for i in 0..data.len() {
             unsafe {
-                *result.get_unchecked_mut(i) = data[i].0;
+                *result.as_mut_ptr().add(i) = data[i].0;
                 result.set_len(i);
             }
         }

library/core/src/intrinsics.rs

@@ -1969,6 +1969,40 @@ extern "rust-intrinsic" {
 // (`transmute` also falls into this category, but it cannot be wrapped due to the
 // check that `T` and `U` have the same size.)
 
+/// Check that the preconditions of an unsafe function are followed, if debug_assertions are on,
+/// and only at runtime.
+///
+/// # Safety
+///
+/// Invoking this macro is only sound if the following code is already UB when the passed
+/// expression evaluates to false.
+///
+/// This macro expands to a check at runtime if debug_assertions is set. It has no effect at
+/// compile time, but the semantics of the contained `const_eval_select` must be the same at
+/// runtime and at compile time. Thus if the expression evaluates to false, this macro produces
+/// different behavior at compile time and at runtime, and invoking it is incorrect.
+///
+/// So in a sense it is UB if this macro is useful, but we expect callers of `unsafe fn` to make
+/// the occasional mistake, and this check should help them figure things out.
+#[allow_internal_unstable(const_eval_select)] // permit this to be called in stably-const fn
+macro_rules! assert_unsafe_precondition {
+    ($e:expr) => {
+        if cfg!(debug_assertions) {
+            // Use a closure so that we can capture arbitrary expressions from the invocation
+            let runtime = || {
+                if !$e {
+                    // abort instead of panicking to reduce impact on code size
+                    ::core::intrinsics::abort();
+                }
+            };
+            const fn comptime() {}
+
+            ::core::intrinsics::const_eval_select((), comptime, runtime);
+        }
+    };
+}
+pub(crate) use assert_unsafe_precondition;
+
 /// Checks whether `ptr` is properly aligned with respect to
 /// `align_of::<T>()`.
 pub(crate) fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
@@ -1977,7 +2011,6 @@ pub(crate) fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
 
 /// Checks whether the regions of memory starting at `src` and `dst` of size
 /// `count * size_of::<T>()` do *not* overlap.
-#[cfg(debug_assertions)]
 pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -> bool {
     let src_usize = src.addr();
     let dst_usize = dst.addr();
@@ -2079,28 +2112,16 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
         pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
     }
 
-    #[cfg(debug_assertions)]
-    fn runtime_check<T>(src: *const T, dst: *mut T, count: usize) {
-        if !is_aligned_and_not_null(src)
-            || !is_aligned_and_not_null(dst)
-            || !is_nonoverlapping(src, dst, count)
-        {
-            // Not panicking to keep codegen impact smaller.
-            abort();
-        }
-    }
-    #[cfg(debug_assertions)]
-    const fn compiletime_check<T>(_src: *const T, _dst: *mut T, _count: usize) {}
-    #[cfg(debug_assertions)]
-    // SAFETY: As per our safety precondition, we may assume that the `abort` above is never reached.
-    // Therefore, compiletime_check and runtime_check are observably equivalent.
-    unsafe {
-        const_eval_select((src, dst, count), compiletime_check, runtime_check);
-    }
-
     // SAFETY: the safety contract for `copy_nonoverlapping` must be
     // upheld by the caller.
-    unsafe { copy_nonoverlapping(src, dst, count) }
+    unsafe {
+        assert_unsafe_precondition!(
+            is_aligned_and_not_null(src)
+                && is_aligned_and_not_null(dst)
+                && is_nonoverlapping(src, dst, count)
+        );
+        copy_nonoverlapping(src, dst, count)
+    }
 }
 
 /// Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
@@ -2173,24 +2194,11 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
         fn copy<T>(src: *const T, dst: *mut T, count: usize);
     }
 
-    #[cfg(debug_assertions)]
-    fn runtime_check<T>(src: *const T, dst: *mut T) {
-        if !is_aligned_and_not_null(src) || !is_aligned_and_not_null(dst) {
-            // Not panicking to keep codegen impact smaller.
-            abort();
-        }
-    }
-    #[cfg(debug_assertions)]
-    const fn compiletime_check<T>(_src: *const T, _dst: *mut T) {}
-    #[cfg(debug_assertions)]
-    // SAFETY: As per our safety precondition, we may assume that the `abort` above is never reached.
-    // Therefore, compiletime_check and runtime_check are observably equivalent.
+    // SAFETY: the safety contract for `copy` must be upheld by the caller.
     unsafe {
-        const_eval_select((src, dst), compiletime_check, runtime_check);
+        assert_unsafe_precondition!(is_aligned_and_not_null(src) && is_aligned_and_not_null(dst));
+        copy(src, dst, count)
     }
-
-    // SAFETY: the safety contract for `copy` must be upheld by the caller.
-    unsafe { copy(src, dst, count) }
 }
 
 /// Sets `count * size_of::<T>()` bytes of memory starting at `dst` to
@@ -2274,24 +2282,11 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
         fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
     }
 
-    #[cfg(debug_assertions)]
-    fn runtime_check<T>(ptr: *mut T) {
-        debug_assert!(
-            is_aligned_and_not_null(ptr),
-            "attempt to write to unaligned or null pointer"
-        );
-    }
-    #[cfg(debug_assertions)]
-    const fn compiletime_check<T>(_ptr: *mut T) {}
-    #[cfg(debug_assertions)]
-    // SAFETY: runtime debug-assertions are a best-effort basis; it's fine to
-    // not do them during compile time
+    // SAFETY: the safety contract for `write_bytes` must be upheld by the caller.
     unsafe {
-        const_eval_select((dst,), compiletime_check, runtime_check);
+        assert_unsafe_precondition!(is_aligned_and_not_null(dst));
+        write_bytes(dst, val, count)
     }
-
-    // SAFETY: the safety contract for `write_bytes` must be upheld by the caller.
-    unsafe { write_bytes(dst, val, count) }
 }
 
 /// Selects which function to call depending on the context.

library/core/src/num/nonzero.rs

@@ -52,9 +52,13 @@ macro_rules! nonzero_integers {
                 #[$const_new_unchecked_stability]
                 #[must_use]
                 #[inline]
+                #[rustc_allow_const_fn_unstable(const_fn_fn_ptr_basics)] // required by assert_unsafe_precondition
                 pub const unsafe fn new_unchecked(n: $Int) -> Self {
                     // SAFETY: this is guaranteed to be safe by the caller.
-                    unsafe { Self(n) }
+                    unsafe {
+                        core::intrinsics::assert_unsafe_precondition!(n != 0);
+                        Self(n)
+                    }
                 }
 
                 /// Creates a non-zero if the given value is not zero.

library/core/src/ptr/mod.rs

@@ -332,7 +332,10 @@
 use crate::cmp::Ordering;
 use crate::fmt;
 use crate::hash;
-use crate::intrinsics::{self, abort, is_aligned_and_not_null};
+use crate::intrinsics::{
+    self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping,
+};
+
 use crate::mem::{self, MaybeUninit};
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -749,6 +752,16 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
         };
     }
 
+    // SAFETY: the caller must guarantee that `x` and `y` are
+    // valid for writes and properly aligned.
+    unsafe {
+        assert_unsafe_precondition!(
+            is_aligned_and_not_null(x)
+                && is_aligned_and_not_null(y)
+                && is_nonoverlapping(x, y, count)
+        );
+    }
+
     // NOTE(scottmcm) MIRI is disabled here as reading in smaller units is a
     // pessimization for it.  Also, if the type contains any unaligned pointers,
     // copying those over multiple reads is difficult to support.
@@ -839,6 +852,7 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
     // and cannot overlap `src` since `dst` must point to a distinct
     // allocated object.
     unsafe {
+        assert_unsafe_precondition!(is_aligned_and_not_null(dst));
         mem::swap(&mut *dst, &mut src); // cannot overlap
     }
     src
@@ -1318,12 +1332,11 @@ pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 #[inline]
 #[stable(feature = "volatile", since = "1.9.0")]
 pub unsafe fn read_volatile<T>(src: *const T) -> T {
-    if cfg!(debug_assertions) && !is_aligned_and_not_null(src) {
-        // Not panicking to keep codegen impact smaller.
-        abort();
-    }
     // SAFETY: the caller must uphold the safety contract for `volatile_load`.
-    unsafe { intrinsics::volatile_load(src) }
+    unsafe {
+        assert_unsafe_precondition!(is_aligned_and_not_null(src));
+        intrinsics::volatile_load(src)
+    }
 }
 
 /// Performs a volatile write of a memory location with the given value without
@@ -1389,12 +1402,9 @@ pub unsafe fn read_volatile<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "volatile", since = "1.9.0")]
 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
-    if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
-        // Not panicking to keep codegen impact smaller.
-        abort();
-    }
     // SAFETY: the caller must uphold the safety contract for `volatile_store`.
     unsafe {
+        assert_unsafe_precondition!(is_aligned_and_not_null(dst));
         intrinsics::volatile_store(dst, src);
     }
 }

library/core/src/slice/index.rs

@@ -1,5 +1,6 @@
 //! Indexing implementations for `[T]`.
 
+use crate::intrinsics::assert_unsafe_precondition;
 use crate::intrinsics::const_eval_select;
 use crate::ops;
 use crate::ptr;
@@ -219,13 +220,19 @@ unsafe impl<T> const SliceIndex<[T]> for usize {
         // cannot be longer than `isize::MAX`. They also guarantee that
         // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
         // so the call to `add` is safe.
-        unsafe { slice.as_ptr().add(self) }
+        unsafe {
+            assert_unsafe_precondition!(self < slice.len());
+            slice.as_ptr().add(self)
+        }
     }
 
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut T {
         // SAFETY: see comments for `get_unchecked` above.
-        unsafe { slice.as_mut_ptr().add(self) }
+        unsafe {
+            assert_unsafe_precondition!(self < slice.len());
+            slice.as_mut_ptr().add(self)
+        }
     }
 
     #[inline]
@@ -272,13 +279,18 @@ unsafe impl<T> const SliceIndex<[T]> for ops::Range<usize> {
         // cannot be longer than `isize::MAX`. They also guarantee that
         // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
         // so the call to `add` is safe.
-        unsafe { ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), self.end - self.start) }
+
+        unsafe {
+            assert_unsafe_precondition!(self.end >= self.start && self.end <= slice.len());
+            ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), self.end - self.start)
+        }
     }
 
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
         // SAFETY: see comments for `get_unchecked` above.
         unsafe {
+            assert_unsafe_precondition!(self.end >= self.start && self.end <= slice.len());
             ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), self.end - self.start)
         }
     }

library/core/src/slice/mod.rs

@@ -7,6 +7,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cmp::Ordering::{self, Greater, Less};
+use crate::intrinsics::{assert_unsafe_precondition, exact_div};
 use crate::marker::Copy;
 use crate::mem;
 use crate::num::NonZeroUsize;
@@ -656,15 +657,10 @@ impl<T> [T] {
     #[unstable(feature = "slice_swap_unchecked", issue = "88539")]
     #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
     pub const unsafe fn swap_unchecked(&mut self, a: usize, b: usize) {
-        #[cfg(debug_assertions)]
-        {
-            let _ = &self[a];
-            let _ = &self[b];
-        }
-
         let ptr = self.as_mut_ptr();
         // SAFETY: caller has to guarantee that `a < self.len()` and `b < self.len()`
         unsafe {
+            assert_unsafe_precondition!(a < self.len() && b < self.len());
             ptr::swap(ptr.add(a), ptr.add(b));
         }
     }
@@ -970,11 +966,11 @@ impl<T> [T] {
     #[inline]
     #[must_use]
     pub unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]] {
-        debug_assert_ne!(N, 0);
-        debug_assert_eq!(self.len() % N, 0);
-        let new_len =
-            // SAFETY: Our precondition is exactly what's needed to call this
-            unsafe { crate::intrinsics::exact_div(self.len(), N) };
+        // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
+        let new_len = unsafe {
+            assert_unsafe_precondition!(N != 0 && self.len() % N == 0);
+            exact_div(self.len(), N)
+        };
         // SAFETY: We cast a slice of `new_len * N` elements into
         // a slice of `new_len` many `N` elements chunks.
         unsafe { from_raw_parts(self.as_ptr().cast(), new_len) }
@@ -1109,11 +1105,11 @@ impl<T> [T] {
     #[inline]
     #[must_use]
     pub unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]] {
-        debug_assert_ne!(N, 0);
-        debug_assert_eq!(self.len() % N, 0);
-        let new_len =
-            // SAFETY: Our precondition is exactly what's needed to call this
-            unsafe { crate::intrinsics::exact_div(self.len(), N) };
+        // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
+        let new_len = unsafe {
+            assert_unsafe_precondition!(N != 0 && self.len() % N == 0);
+            exact_div(self.len(), N)
+        };
         // SAFETY: We cast a slice of `new_len * N` elements into
         // a slice of `new_len` many `N` elements chunks.
         unsafe { from_raw_parts_mut(self.as_mut_ptr().cast(), new_len) }
@@ -1675,7 +1671,10 @@ impl<T> [T] {
         //
         // `[ptr; mid]` and `[mid; len]` are not overlapping, so returning a mutable reference
         // is fine.
-        unsafe { (from_raw_parts_mut(ptr, mid), from_raw_parts_mut(ptr.add(mid), len - mid)) }
+        unsafe {
+            assert_unsafe_precondition!(mid <= len);
+            (from_raw_parts_mut(ptr, mid), from_raw_parts_mut(ptr.add(mid), len - mid))
+        }
     }
 
     /// Divides one slice into an array and a remainder slice at an index.