Add some targeted assume(bytes <= isize::MAX) around slices

Putting it on every single slice length was pretty expensive, but let's see if doing it just in `slice::from_raw_parts(_mut)` and `slice::Iter(Mut)::len` works better.

Author: scottmcm

library/core/src/mem/mod.rs

@@ -1235,6 +1235,17 @@ pub trait SizedTypeProperties: Sized {
     #[doc(hidden)]
     #[unstable(feature = "sized_type_properties", issue = "none")]
     const IS_ZST: bool = size_of::<Self>() == 0;
+
+    /// The largest safe length for a `[Self]`.
+    ///
+    /// Anything larger than this would make `size_of_val` overflow `isize::MAX`,
+    /// which is never allowed for a single object.
+    #[doc(hidden)]
+    #[unstable(feature = "sized_type_properties", issue = "none")]
+    const MAX_SLICE_LEN: usize = match size_of::<Self>() {
+        0 => usize::MAX,
+        n => (isize::MAX as usize) / n,
+    };
 }
 #[doc(hidden)]
 #[unstable(feature = "sized_type_properties", issue = "none")]

library/core/src/slice/raw.rs

@@ -1,7 +1,8 @@
 //! Free functions to create `&[T]` and `&mut [T]`.
 
 use crate::array;
-use crate::mem::{align_of, size_of};
+use crate::intrinsics;
+use crate::mem::{align_of, SizedTypeProperties};
 use crate::ops::Range;
 use crate::ptr;
 use crate::ub_checks;
@@ -98,13 +99,14 @@ pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T]
             "slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`",
             (
                 data: *mut () = data as *mut (),
-                size: usize = size_of::<T>(),
                 align: usize = align_of::<T>(),
                 len: usize = len,
+                max_len: usize = T::MAX_SLICE_LEN,
             ) =>
             ub_checks::is_aligned_and_not_null(data, align)
-                && ub_checks::is_valid_allocation_size(size, len)
+                && len <= max_len
         );
+        intrinsics::assume(len <= T::MAX_SLICE_LEN);
         &*ptr::slice_from_raw_parts(data, len)
     }
 }
@@ -152,13 +154,14 @@ pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a m
             "slice::from_raw_parts_mut requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`",
             (
                 data: *mut () = data as *mut (),
-                size: usize = size_of::<T>(),
                 align: usize = align_of::<T>(),
                 len: usize = len,
+                max_len: usize = T::MAX_SLICE_LEN,
             ) =>
             ub_checks::is_aligned_and_not_null(data, align)
-                && ub_checks::is_valid_allocation_size(size, len)
+                && len <= max_len
         );
+        intrinsics::assume(len <= T::MAX_SLICE_LEN);
         &mut *ptr::slice_from_raw_parts_mut(data, len)
     }
 }

library/core/src/ub_checks.rs

@@ -115,12 +115,6 @@ pub(crate) const fn is_aligned_and_not_null(ptr: *const (), align: usize) -> boo
     !ptr.is_null() && ptr.is_aligned_to(align)
 }
 
-#[inline]
-pub(crate) const fn is_valid_allocation_size(size: usize, len: usize) -> bool {
-    let max_len = if size == 0 { usize::MAX } else { isize::MAX as usize / size };
-    len <= max_len
-}
-
 /// Checks whether the regions of memory starting at `src` and `dst` of size
 /// `count * size` do *not* overlap.
 ///