Expand set_ptr_value / with_metadata_of docs
In preparation of a potential FCP, intends to clean up and expand the documentation of this operation.
Rewrite these blobs to explicitly mention the case of a sized operand. The previous made that seem wrong instead of emphasizing it is nothing but a simple cast. Instead, the explanation now emphasizes that the address portion of the argument, together with its provenance, is discarded which previously had to be inferred by the reader. Then an example demonstrates a simple line of incorrect usage based on this idea of provenance.
Tracking issue: https://github.com/rust-lang/rust/issues/75091
Update precondition tests (especially for zero-size access to null)
I don't much like the current way I've updated the precondition check helpers, but I couldn't come up with anything better. Ideas welcome.
I've organized `tests/ui/precondition-checks` mostly with one file per function that has `assert_unsafe_precondition` in it, with revisions that check each precondition. The important new test is `tests/ui/precondition-checks/zero-size-null.rs`.
Rewrite these blobs to explicitly mention the case of a sized operand.
The previous made that seem wrong instead of emphasizing it is nothing
but a simple cast. Instead, the explanation now emphasizes that the
address portion of the argument, together with its provenance, is
discarded which previously had to be inferred by the reader. Then an
example demonstrates a simple line of incorrect usage based on this
idea of provenance.
ptr::add/sub: do not claim equivalence with `offset(c as isize)`
In https://github.com/rust-lang/rust/pull/110837, the `offset` intrinsic got changed to also allow a `usize` offset parameter. The intention is that this will do an unsigned multiplication with the size, and we have UB if that overflows -- and we also have UB if the result is larger than `usize::MAX`, i.e., if a subsequent cast to `isize` would wrap. ~~The LLVM backend sets some attributes accordingly.~~
This updates the docs for `add`/`sub` to match that intent, in preparation for adjusting codegen to exploit this UB. We use this opportunity to clarify what the exact requirements are: we compute the offset using mathematical multiplication (so it's no problem to have an `isize * usize` multiplication, we just multiply integers), and the result must fit in an `isize`.
Cc `@rust-lang/opsem` `@nikic`
https://github.com/rust-lang/rust/pull/130239 updates Miri to detect this UB.
`sub` still has some cases of UB not reflected in the underlying intrinsic semantics (and Miri does not catch): when we subtract `usize::MAX`, then after casting to `isize` that's just `-1` so we end up adding one unit without noticing any UB, but actually the offset we gave does not fit in an `isize`. Miri will currently still not complain for such cases:
```rust
fn main() {
let x = &[0i32; 2];
let x = x.as_ptr();
// This should be UB, we are subtracting way too much.
unsafe { x.sub(usize::MAX).read() };
}
```
However, the LLVM IR we generate here also is UB-free. This is "just" library UB but not language UB.
Cc `@saethlin;` might be worth adding precondition checks against overflow on `offset`/`add`/`sub`?
Fixes https://github.com/rust-lang/rust/issues/130211
Since the stabilization in #127679 has reached stage0, 1.82-beta, we can
start using `&raw` freely, and even the soft-deprecated `ptr::addr_of!`
and `ptr::addr_of_mut!` can stop allowing the unstable feature.
I intentionally did not change any documentation or tests, but the rest
of those macro uses are all now using `&raw const` or `&raw mut` in the
standard library.
Take more advantage of the `isize::MAX` limit in `Layout`
Things like `padding_needed_for` are current implemented being super careful to handle things like `Layout::size` potentially being `usize::MAX`.
But now that #95295 has happened, that's no longer a concern. It's possible to add two `Layout::size`s together without risking overflow now.
So take advantage of that to remove a bunch of checked math that's not actually needed. For example, the round-up-and-add-next-size in `extend` doesn't need any overflow checks at all, just the final check for compatibility with the alignment.
(And while I was doing that I made it all unstably const, because there's nothing in `Layout` that's fundamentally runtime-only.)
Things like `padding_needed_for` are current implemented being super careful to handle things like `Layout::size` potentially being `usize::MAX`.
But now that 95295 has happened, that's no longer a concern. It's possible to add two `Layout::size`s together without risking overflow now.
So take advantage of that to remove a bunch of checked math that's not actually needed. For example, the round-up-and-add-next-size in `extend` doesn't need any overflow checks at all, just the final check for compatibility with the alignment.
(And while I was doing that I made it all unstably const, because there's nothing in `Layout` that's fundamentally runtime-only.)
some const cleanup: remove unnecessary attributes, add const-hack indications
I learned that we use `FIXME(const-hack)` on top of the "const-hack" label. That seems much better since it marks the right place in the code and moves around with the code. So I went through the PRs with that label and added appropriate FIXMEs in the code. IMO this means we can then remove the label -- Cc ``@rust-lang/wg-const-eval.``
I also noticed some const stability attributes that don't do anything useful, and removed them.
r? ``@fee1-dead``
move some const fn out of the const_ptr_as_ref feature
When a `const fn` is still `#[unstable]`, it should generally use the same feature to track its regular stability and const-stability. Then when that feature moves towards stabilization we can decide whether the const-ness can be stabilized as well, or whether it should be moved into a new feature.
Also, functions like `ptr::as_ref` (which returns an `Option<&mut T>`) require `is_null`, which is tricky and blocked on some design concerns (see #74939). So move those to the is_null feature gate, as they should be stabilized together with `ptr.is_null()`.
Affects #91822, #122034, #75402, https://github.com/rust-lang/rust/issues/74939
