Rust values can be up to isize::MAX in size

rust-version

@@ -1 +1 @@
-df20355fa9fa5e9fb89be4e4bfee8a643bb7a23e
+297a8018b525c28ef10ee6a91d61954839b508b9

src/shims/intrinsics.rs

@@ -83,6 +83,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let val_byte = this.read_scalar(val_byte)?.to_u8()?;
                 let ptr = this.read_pointer(ptr)?;
                 let count = this.read_scalar(count)?.to_machine_usize(this)?;
+                // `checked_mul` enforces a too small bound (the correct one would probably be machine_isize_max),
+                // but no actual allocation can be big enough for the difference to be noticeable.
                 let byte_count = ty_layout.size.checked_mul(count, this).ok_or_else(|| {
                     err_ub_format!("overflow computing total size of `{}`", intrinsic_name)
                 })?;

tests/compile-fail/too-big-unsized.rs

@@ -6,13 +6,8 @@ struct MySlice {
     tail: [u8],
 }
 
-#[cfg(target_pointer_width = "64")]
-const TOO_BIG: usize = 1usize << 47;
-#[cfg(target_pointer_width = "32")]
-const TOO_BIG: usize = 1usize << 31;
-
 fn main() { unsafe {
     let ptr = Box::into_raw(Box::new(0u8));
     // The slice part is actually not "too big", but together with the `prefix` field it is.
-    let _x: &MySlice = mem::transmute((ptr, TOO_BIG-1)); //~ ERROR: invalid reference metadata: total size is bigger than largest supported object
+    let _x: &MySlice = mem::transmute((ptr, isize::MAX as usize)); //~ ERROR: invalid reference metadata: total size is bigger than largest supported object
 } }

tests/run-pass/slices.rs

@@ -2,6 +2,7 @@
 #![feature(new_uninit)]
 #![feature(slice_as_chunks)]
 #![feature(slice_partition_dedup)]
+#![feature(layout_for_ptr)]
 
 use std::slice;
 
@@ -250,9 +251,17 @@ fn test_for_invalidated_pointers() {
     buffer.copy_within(1.., 0);
 }
 
+fn large_raw_slice() {
+    let size = isize::MAX as usize;
+    // Creating a raw slice of size isize::MAX and asking for its size is okay.
+    let s = std::ptr::slice_from_raw_parts(1usize as *const u8, size);
+    assert_eq!(size, unsafe { std::mem::size_of_val_raw(s) });
+}
+
 fn main() {
     slice_of_zst();
     test_iter_ref_consistency();
     uninit_slice();
     test_for_invalidated_pointers();
+    large_raw_slice();
 }