Asserts the maximum value that can be returned from Vec::len

alloc/src/vec/mod.rs

@@ -56,7 +56,6 @@
 #[cfg(not(no_global_oom_handling))]
 use core::cmp;
 use core::cmp::Ordering;
-use core::fmt;
 use core::hash::{Hash, Hasher};
 #[cfg(not(no_global_oom_handling))]
 use core::iter;
@@ -65,6 +64,7 @@ use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::{self, Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice::{self, SliceIndex};
+use core::{fmt, intrinsics};
 
 #[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
 pub use self::extract_if::ExtractIf;
@@ -2680,7 +2680,14 @@ impl<T, A: Allocator> Vec<T, A> {
     #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
     #[rustc_confusables("length", "size")]
     pub const fn len(&self) -> usize {
-        self.len
+        let len = self.len;
+
+        // SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so the maximum value can
+        // be returned is `usize::checked_div(mem::size_of::<T>()).unwrap_or(usize::MAX)`, which
+        // matches the definition of `T::MAX_SLICE_LEN`.
+        unsafe { intrinsics::assume(len <= T::MAX_SLICE_LEN) };
+
+        len
     }
 
     /// Returns `true` if the vector contains no elements.

core/src/mem/mod.rs

@@ -1241,6 +1241,17 @@ pub trait SizedTypeProperties: Sized {
     #[doc(hidden)]
     #[unstable(feature = "sized_type_properties", issue = "none")]
     const LAYOUT: Layout = Layout::new::<Self>();
+
+    /// 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")]