Unclear is offsetting one element past array length is allowed

[Rosdf]

Location

https://doc.rust-lang.org/std/primitive.pointer.html#method.offset

Summary

Offset doc states that

If the computed offset is non-zero, then self must be derived from a pointer to some allocated object, and the entire memory range between self and the result must be in bounds of that allocated object.

allocated object doc states that

For all addresses a in addresses, a is in the range base .. (base + size) (note that this requires a < base + size, not a <= base + size)

so as i understand offsetting pointer to the head of an array one past it's length should not be allowed, but in practice it is allowed

fn main() {
    static A: [u8; 8] = [1, 2, 3, 4, 5, 6, 7, 8];
    static mut B: *const u8 = unsafe {A.as_ptr().offset(8)};
}

this code compiles just fine, but this one produces an error

fn main() {
    static A: [u8; 8] = [1, 2, 3, 4, 5, 6, 7, 8];
    static mut B: *const u8 = unsafe {A.as_ptr().offset(9)};
}

error[E0080]: could not evaluate static initializer
   --> /home/daria/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/const_ptr.rs:455:18
    |
455 |         unsafe { intrinsics::offset(self, count) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer arithmetic: expected a pointer to 9 bytes of memory, but got alloc1 which is only 8 bytes from the end of the allocation
    |
note: inside `std::ptr::const_ptr::<impl *const u8>::offset`
   --> /home/daria/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/const_ptr.rs:455:18
    |
455 |         unsafe { intrinsics::offset(self, count) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `B`
   --> src/main.rs:3:39
    |
3   |     static mut B: *const u8 = unsafe {A.as_ptr().offset(9)};
    |                                       ^^^^^^^^^^^^^^^^^^^^

[xizheyin]

As far as I know, while the documentation states that pointers must be within the bounds of the allocated object (a < base + size), there is indeed a special case: "one-past-the-end" pointers are explicitly allowed. "One-past-the-end" pointers are crucial for iteration, allowing representation of the end of a valid range. It enables more natural range checking and pointer arithmetic implementations. Maybe the documentation should be improved.

[xizheyin]

@rustbot claim

[moxian]

@rustbot label: +T-opsem +T-libs-api +A-raw-pointers

[RalfJung]

so as i understand offsetting pointer to the head of an array one past it's length should not be allowed,

No, you misunderstood. The "offset" docs are very explicit about this:
"the entire memory range between self and the result must be in bounds of that allocated object."

If you do A.as_ptr().offset(8), the range between the old and new pointer is the range covering the indices 0..8. That entire range is within the bounds of the array.

[xizheyin]

#138976 is merged, so this issue need close.

@rustbot close