Rollup merge of #120888 - saethlin:unsafe-precondition-cleanup, r=RalfJung

assert_unsafe_precondition cleanup

I moved the polymorphic `is_nonoverlapping` into the `Cell` function that uses it and renamed `intrinsics::is_nonoverlapping_mono` to just `intrinsics::is_nonoverlapping`.

We now also have some docs for `intrinsics::debug_assertions`.

r? RalfJung

Author: matthiaskrgr

library/core/src/cell.rs

@@ -237,9 +237,9 @@
 
 use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
-use crate::intrinsics::is_nonoverlapping;
+use crate::intrinsics;
 use crate::marker::{PhantomData, Unsize};
-use crate::mem;
+use crate::mem::{self, size_of};
 use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn};
 use crate::ptr::{self, NonNull};
 
@@ -435,11 +435,15 @@ impl<T> Cell<T> {
     #[inline]
     #[stable(feature = "move_cell", since = "1.17.0")]
     pub fn swap(&self, other: &Self) {
+        fn is_nonoverlapping<T>(src: *const T, dst: *const T) -> bool {
+            intrinsics::is_nonoverlapping(src.cast(), dst.cast(), size_of::<T>(), 1)
+        }
+
         if ptr::eq(self, other) {
             // Swapping wouldn't change anything.
             return;
         }
-        if !is_nonoverlapping(self, other, 1) {
+        if !is_nonoverlapping(self, other) {
             // See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
             panic!("`Cell::swap` on overlapping non-identical `Cell`s");
         }

library/core/src/intrinsics.rs

@@ -56,7 +56,7 @@
 
 use crate::marker::DiscriminantKind;
 use crate::marker::Tuple;
-use crate::mem::{self, align_of};
+use crate::mem::align_of;
 
 pub mod mir;
 pub mod simd;
@@ -1027,7 +1027,7 @@ extern "rust-intrinsic" {
 
     /// The size of the referenced value in bytes.
     ///
-    /// The stabilized version of this intrinsic is [`mem::size_of_val`].
+    /// The stabilized version of this intrinsic is [`crate::mem::size_of_val`].
     #[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
     #[rustc_nounwind]
     pub fn size_of_val<T: ?Sized>(_: *const T) -> usize;
@@ -1107,7 +1107,7 @@ extern "rust-intrinsic" {
 
     /// Moves a value out of scope without running drop glue.
     ///
-    /// This exists solely for [`mem::forget_unsized`]; normal `forget` uses
+    /// This exists solely for [`crate::mem::forget_unsized`]; normal `forget` uses
     /// `ManuallyDrop` instead.
     ///
     /// Note that, unlike most intrinsics, this is safe to call;
@@ -1233,7 +1233,7 @@ extern "rust-intrinsic" {
     /// Depending on what the code is doing, the following alternatives are preferable to
     /// pointer-to-integer transmutation:
     /// - If the code just wants to store data of arbitrary type in some buffer and needs to pick a
-    ///   type for that buffer, it can use [`MaybeUninit`][mem::MaybeUninit].
+    ///   type for that buffer, it can use [`MaybeUninit`][crate::mem::MaybeUninit].
     /// - If the code actually wants to work on the address the pointer points to, it can use `as`
     ///   casts or [`ptr.addr()`][pointer::addr].
     ///
@@ -2317,7 +2317,7 @@ extern "rust-intrinsic" {
     /// Therefore, implementations must not require the user to uphold
     /// any safety invariants.
     ///
-    /// The to-be-stabilized version of this intrinsic is [`mem::variant_count`].
+    /// The to-be-stabilized version of this intrinsic is [`crate::mem::variant_count`].
     #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
@@ -2569,6 +2569,19 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn is_val_statically_known<T: Copy>(arg: T) -> bool;
 
+    /// Returns the value of `cfg!(debug_assertions)`, but after monomorphization instead of in
+    /// macro expansion.
+    ///
+    /// This always returns `false` in const eval and Miri. The interpreter provides better
+    /// diagnostics than the checks that this is used to implement. However, this means
+    /// you should only be using this intrinsic to guard requirements that, if violated,
+    /// immediately lead to UB. Otherwise, const-eval and Miri will miss out on those
+    /// checks entirely.
+    ///
+    /// Since this is evaluated after monomorphization, branching on this value can be used to
+    /// implement debug assertions that are included in the precompiled standard library, but can
+    /// be optimized out by builds that monomorphize the standard library code with debug
+    /// assertions disabled. This intrinsic is primarily used by [`assert_unsafe_precondition`].
     #[rustc_const_unstable(feature = "delayed_debug_assertions", issue = "none")]
     #[rustc_safe_intrinsic]
     #[cfg(not(bootstrap))]
@@ -2597,7 +2610,7 @@ pub(crate) const fn debug_assertions() -> bool {
 /// These checks are behind a condition which is evaluated at codegen time, not expansion time like
 /// [`debug_assert`]. This means that a standard library built with optimizations and debug
 /// assertions disabled will have these checks optimized out of its monomorphizations, but if a
-/// a caller of the standard library has debug assertions enabled and monomorphizes an expansion of
+/// caller of the standard library has debug assertions enabled and monomorphizes an expansion of
 /// this macro, that monomorphization will contain the check.
 ///
 /// Since these checks cannot be optimized out in MIR, some care must be taken in both call and
@@ -2606,8 +2619,8 @@ pub(crate) const fn debug_assertions() -> bool {
 /// combination of properties ensures that the code for the checks is only compiled once, and has a
 /// minimal impact on the caller's code size.
 ///
-/// Caller should also introducing any other `let` bindings or any code outside this macro in order
-/// to call it. Since the precompiled standard library is built with full debuginfo and these
+/// Callers should also avoid introducing any other `let` bindings or any code outside this macro in
+/// order to call it. Since the precompiled standard library is built with full debuginfo and these
 /// variables cannot be optimized out in MIR, an innocent-looking `let` can produce enough
 /// debuginfo to have a measurable compile-time impact on debug builds.
 ///
@@ -2659,33 +2672,12 @@ pub(crate) fn is_valid_allocation_size(size: usize, len: usize) -> bool {
     len <= max_len
 }
 
-pub(crate) fn is_nonoverlapping_mono(
-    src: *const (),
-    dst: *const (),
-    size: usize,
-    count: usize,
-) -> bool {
-    let src_usize = src.addr();
-    let dst_usize = dst.addr();
-    let Some(size) = size.checked_mul(count) else {
-        crate::panicking::panic_nounwind(
-            "is_nonoverlapping: `size_of::<T>() * count` overflows a usize",
-        )
-    };
-    let diff = src_usize.abs_diff(dst_usize);
-    // If the absolute distance between the ptrs is at least as big as the size of the buffer,
-    // they do not overlap.
-    diff >= size
-}
-
 /// Checks whether the regions of memory starting at `src` and `dst` of size
-/// `count * size_of::<T>()` do *not* overlap.
-#[inline]
-pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -> bool {
+/// `count * size` do *not* overlap.
+pub(crate) fn is_nonoverlapping(src: *const (), dst: *const (), size: usize, count: usize) -> bool {
     let src_usize = src.addr();
     let dst_usize = dst.addr();
-    let Some(size) = mem::size_of::<T>().checked_mul(count) else {
-        // Use panic_nounwind instead of Option::expect, so that this function is nounwind.
+    let Some(size) = size.checked_mul(count) else {
         crate::panicking::panic_nounwind(
             "is_nonoverlapping: `size_of::<T>() * count` overflows a usize",
         )
@@ -2809,7 +2801,7 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
             ) =>
             is_aligned_and_not_null(src, align)
                 && is_aligned_and_not_null(dst, align)
-                && is_nonoverlapping_mono(src, dst, size, count)
+                && is_nonoverlapping(src, dst, size, count)
         );
         copy_nonoverlapping(src, dst, count)
     }

library/core/src/ptr/mod.rs

@@ -383,7 +383,7 @@ use crate::cmp::Ordering;
 use crate::fmt;
 use crate::hash;
 use crate::intrinsics::{
-    self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping_mono,
+    self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping,
 };
 use crate::marker::FnPtr;
 
@@ -978,7 +978,7 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
             ) =>
             is_aligned_and_not_null(x, align)
                 && is_aligned_and_not_null(y, align)
-                && is_nonoverlapping_mono(x, y, size, count)
+                && is_nonoverlapping(x, y, size, count)
         );
     }