Unsafe fields

[jhpratt]

Fixes #381

Rendered

Tracking:

• Tracking issue for RFC 3458: Unsafe fields rust#132922

[juntyr]

I’m not sure I like that the entire struct expression is now inside an unsafe block (though I’m not sure what a better syntax would be). If the safety invariant requires the entire struct, this makes sense. However, if it is more specific, a larger unsafe block is too broad as it also allows unsafe code usage to initialise the safe fields, which should get their own unsafe blocks. Though perhaps this could just be linted against, e.g. don’t use unsafe expressions in a struct initialiser without putting them inside nested unsafe blocks.

[jhpratt]

The alternative is to do this:

let mut foo = Foo {
    safe_field: 0,
    unsafe_field: unsafe { 1 },
};

That feels worse to me.

Presumably the safety invariant being promised is an invariant within the struct, even if it is not strictly required that that be the case.

Ideally we'd also have partial initialization, such that it would be possible to do

let mut foo = Foo { safe_field: 0 };
unsafe { foo.unsafe_field = 1; }

but I expect that's quite a bit farther away.

[juntyr]

I agree that just unsafe in one field initialiser expression is insufficient as it means something very different. I do like however that it clearly shows which field the unsafety applies to.

It reminds me a bit of unsafe blocks in unsafe functions (#2585). Ideally, the fact that struct init is unsafe would not allow unsafe field init expressions. I doubt that special-casing struct inits to not propagate outer unsafe blocks would be backwards compatible, so a new lint would be the only avenue in this direction.

Some new syntax like this

let mut foo = unsafe Foo {
    safe_field: 0,
    unsafe unsafe_field: 1,
};

would communicate intent better but looks quite unnatural to me.

Perhaps in the future there might be explicit safe blocks to reassert a safe context within an unsafe block, so that it could be written as follows:

let mut foo = unsafe {
    Foo {
        safe_field: safe { /* some more complex expr here */ },
        unsafe_field: safe { /* some more complex expr here */ },
    }
};

which could be encouraged with clippy lints. Though for now just moving safe non-trivial struct field initialisers into variables that are initialised outside the unsafe block.

Overall, I think going with the original syntax of

let mut foo = unsafe {
    Foo {
        safe_field: 2,
        unsafe_field: 1,
    }
};

looks like a good and intuitive solution, though I still think that a lint against using unsafe expressions inside the struct expression without a nested unsafe block would still help.

[jhpratt]

Maybe this?

let mut foo = unsafe Foo {
    safe_field: 0,
    unsafe_field: 1,
};

unsafe would be followed immediately by a struct expression, and doing so would only indicate that unsafe fields may be initialized. It would not introduce an unsafe context.

[juntyr]

I like it - the syntax is still close enough to the unsafe block syntax (except when initialising tuple structs) that it's relatively intuitive what the unsafety applies to, but coupled strongly to the struct name so that it also makes sense why no unsafe context is introduced.

[scottmcm]

I think there's something nice about

let mut foo = unsafe Foo {
    safe_field: 0,
    unsafe_field: 1,
};

but to me that would go with unsafe struct Foo, like how unsafe trait Bar leads to unsafe impl Bar.

Spitballing: if per-field safety is really needed, then maybe

let mut foo = Foo {
    safe_field: 0,
    unsafe unsafe_field: 1,
};

though maybe the answer is the same as for unsafe { unsafe_fn_call(complicated_expr) }: if you don't want the expr in the block, use a let.

After all, there's always field shorthand available, so you can do

let safe_field = unsafe { complex to construct };
let unsafe_field = easy and safe to construct;
unsafe { Foo { safe_field, unsafe_field } }

Perhaps the more important factor would be the workflow for the errors the programmer gets when changing a (internal and thus not semver break) field to unsafe. Anything at the struct level wouldn't give new "hey, you need unsafe here" if you already had another field that was unsafe.

Of course, if the safety is at the struct level (not the field level) then that doesn't come up.

---

Hmm, the "you added an additional requirement to something that's already in an unsafe block and there's no way to help you make sure you handed that" is a pre-existing problem that needs tooling for that everywhere, so maybe it's not something this RFC needs to think about.

If we had unsafe(aligned(a), nonzero(b)) { ... } so that tooling could help ensure that people thought about everything declared in the safety section, then we'd just have unsafe(initialized(ptr, len)) { ... } so that it acknowledged the discharge of the obligation for the type invariant, and the "I need 100 unsafe blocks" problem goes away.

[algesten]

By introducing unsafe fields, Rust can improve the situation where a field that is otherwise safe is used as a safety invariant.

I think the motivation could point out who would benefit from this.

I assume it's library authors a making unsafe fields a "reminder to self" about upholding some invariant as opposed to say expecting a unsafe field in an API surface. I.e we still expect unsafe set_len rather than pub unsafe len, right?

[programmerjake]

this seems closely related to mut(self) fields #3323 which should probably be mentioned.

[jhpratt]

I assume it's library authors a making unsafe fields a "reminder to self" about upholding some invariant as opposed to say expecting a unsafe field in an API surface. I.e we still expect unsafe set_len rather than pub unsafe len, right?

Not necessarily. It is entirely reasonable that Vec.len could be exposed. Whether it actually is exposed is a decision solely for T-libs-api. Likewise with the inner fields of the various nonzero types. I'd rather not tie down who this is intended for, as it truly is intended for everyone. I know I've written a binary that had fields relied upon in unsafe code — there was just no way to make it actually unsafe.

this seems closely related to mut(self) fields #3323 which should probably be mentioned.

Related, sure, but beyond the mention in unresolved questions, I'm not sure how it could be mentioned. Pretty much the only overlap is what's considered a "mutable access", which I didn't feel necessary to be copy-pasted.

[jsgf]

How does this look with functional struct update?

Foo {
// Stuff
.. unsafe { other }
}

?
Or does the whole initializer need to be unsafe?

Edit: or I guess it doesn't need unsafe if the source had been initialized with unsafe.

[jhpratt]

@jsgf Great question. I don't have an immediate answer, though I believe the mechanism currently in the compiler would require the entire initializer to be unsafe.

[mo8it]

I love it! This is much better than getters and setters, both for library authors and users.

typed-builder would have to adjust, but I would love to implement this feature there :)

@idanarye (the main author) Maybe you have some input regarding the builder pattern?

[djc]

The implicit notion that only mutation is unsafe (and reading is not) seems tricky. Do you have reasoning to prove that we'll never need fields that are unsafe to read? Maybe there should be an alternative syntax proposal (like unsafe(mut) or mut(unsafe)) that makes this more obvious/explicit?

[idanarye]

@mo8it I don't want to spam the comments here with a discussion about typed builder, so I've opened a discussion in my repository instead: idanarye/rust-typed-builder#103

[idanarye]

Regarding the RFC itself - I think you are trying to solve a visibility issue with the safety mechanism, which is the wrong tool for the job. You gave Vec as an example, and it looks like you want to grant public read access to its len field (so that you don't have to use len() as a method? Let's ignore the question if that's big enough an improvement to justify such a feature). To do so, you are willing to provide public write access to it as well but make that access unsafe.

But why?

I mean, it's obvious why you don't want to give regular write access to len. But why give any access at all? Even if you require an unsafe block, what good will come from letting external users modify the len field without going through a method that upholds the invariant? As long as we are devising a brand new feature, wouldn't it make more sense to add a feature that gives public read access without any write access at all (other than private access from inside the defining module, of course)?

I'm aware of the set_len method that grants such access, but this is an explicit decision to give such access, with a fully documented method. Not a side-effect of wanting to provide a non-method-call read access to the field.

---

Another thing - conceptually it never makes sense to define only one field as unsafe. The invariant is a property of the struct as a whole. If this is unsafe:

let mut vec = Vec::new();
vec.len = 4; // UNSAFE!!!

The why not this?

let mut vec = Vec::from([1, 2, 3, 4]);
vec.buf = RawVec::new(); // perfectly safe apparently

Yes, buf is not publicly exposed at all, but inside the module len will need unsafe block to modify and buf won't, even though the invariant is about both of them, together, and how they interact with each other.

Whatever the semantics of unsafe fields will be - conceptually it makes more sense to put the unsafe on the struct itself. If there are fields that are not part of the invariant, they should not be part of the struct - because unsafety should be contained as much as possible and not contaminated with unrelated data. The only reason putting unsafe only on len seems to make case in your example is that the goal - as I've said before - is about visibility, not about safety.

[Jules-Bertholet]

One could argue that ptr::addr_of_mut! on an unsafe field need not be unsafe, because writes through the pointer are unsafe.

[jhpratt]

Consider this in conjunction with the examples present in the RFC.

fn make_zero(p: *mut u8) {
    unsafe { *p = 0; }
}

let p = ptr::addr_of_mut!(foo.unsafe_field);
make_zero(p);

Ignoring thread-safety, which can be easily achieved with locks, no single step appears wrong. make_zero does not do anything wrong — assigning zero to an arbitrary *mut u8 is fine. Passing a pointer to the method is naturally okay. Yet it still results in unsoundness, as foo.unsafe_field must be odd.

[Jules-Bertholet]

"assigning zero to an arbitrary *mut u8 is fine" what‽ No it is not fine‽ An arbitrary *mut u8 could be null, dangling, aliased... fn make_zero is unsound.

[jhpratt]

True — I typed that far too quickly and without thinking. Regardless, it's not immediately obvious to me that ptr::addr_of_mut! should be allowed safely.

[Jules-Bertholet]

I am not fully convinced either way, but I fear it would just be confusing to require unsafe for an operation that can't lead to unsoundness, especially as addr_of!(struct.field) as *mut _ would do the same thing with no unsafe.

[jhpratt]

especially as addr_of!(struct.field) as *mut _ would do the same thing

While you can do that, it's undefined behavior to actually mutate the resulting mut pointer. I've just confirmed this with miri.

[Jules-Bertholet]

It's UB under Stacked Borrows, but MIRIFLAGS=-Zmiri-tree-borrows accepts it.

[jhpratt]

That's surprising. I'm not familiar with tree borrows, admittedly.

[CodesInChaos]

ptr::from_mut(&mut struct).wrapping_offset(offset_of!(Struct, field)) should achieve the same thing as addr_of_mut!, and I doubt you'd want to make offset_of unsafe for unsafe fields. And even without offset_of, you could use ptr::from_mut(&mut struct).wrapping_offset(addr_of!(stuct.Field) - ptr::from_ref(&struct)). Both of these should be safe in every borrowing model.

[burdges]

Vec::len should not do this even if this feature exists, because Vec::set_len is better pedagogically.

static muts require unsafe blocks for both writing and reading. An unsafe field would likely be some similar construction, so unsafe for both writing and reading. An UnsafeCell already hits those requirements, but any variants should set their auto-traits.

Ain't clear this proposal handles auto-traits correctly, even if some use case exists. If you need this, then define your own type which provides this. We've the inner-builder or whatever deref polymorphism pattern, which comes up far more in practice, and can simulate this of desired.

pub struct ThingBuilder { ... }

impl ThingBuilder {
    fn build(self) -> Thing {
        ...
        Thing { ..., inner, self }
    }
}

pub struct Thing {
    ... 
    inner: ThingBuilder,
}

impl Deref for Thing {
    type Target = ThingBuilder;
    fn deref(&self) -> ThingBuilder {
        &self.inner
    }
}

// We stop mutating ThingBuilder once we create a Thing, so Thing: !DerefMut,
// but Thing: Deref<Target=ThingBuilder> to make reading & replicating the
// builder config easy.

[jhpratt]

Do you have reasoning to prove that we'll never need fields that are unsafe to read?

How could a field of a struct be unsafe to read?

---

You gave Vec as an example, and it looks like you want to grant public read access to its len field (so that you don't have to use len() as a method?

Within the module it's defined in (as the field is private), it is currently safe to assign any value, despite the fact that it can lead to undefined behavior. Said another way, the current behavior is inherently unsound.

Nothing in the RFC so much as hints at Vec.len being made public, nor is an RFC an appropriate place to make a change like that. It is an example of a field that should be unsafe to avoid unsoundness and nothing more.

Another thing - conceptually it never makes sense to define only one field as unsafe.

I never claimed that was the case.

The why not this?

let mut vec = Vec::from([1, 2, 3, 4]);
vec.buf = RawVec::new(); // perfectly safe apparently

Inclusion of one example does not mean that everything not included is forbidden. There is simply no reason to repeat the same thing for every field. Of course buf would also be unsafe. I used Vec.len as the example because it's a clear, obvious example where its safety invariants are publicly documented.

The invariant is a property of the struct as a whole.

If there are fields that are not part of the invariant, they should not be part of the struct

For Vec, yes, but only because all fields of the strict interact with all other fields.

I have real world code where this is not the case. The flags field is for whether other fields are initialized. Note that some fields have niche value optimization, and as such don't interact with other fields in any way. Are you asserting that month: Option<Month> and other similar fields should be in a separate struct solely because it has niche value optimization? That appears to a logical conclusion as a result of what you've said.

---

static muts require unsafe blocks for both writing and reading. An unsafe field would likely be some similar construction, so unsafe for both writing and reading.

static mut requires unsafe due to inherent data races between threads. Unsafe fields have no such issue.

Ain't clear this proposal handles auto-traits correctly, even if some use case exists.

I don't follow. What problems do you see with handling auto traits? The type of the field is unchanged, so there would be no impact on auto traits.

[scottmcm]

(@jhpratt I'd love for you to steal something like the text in this post for the RFC)

I assume it's library authors a making unsafe fields a "reminder to self" about upholding some invariant as opposed to say expecting a unsafe field in an API surface.

A huge incentive for it, to me at least, is helping avoid misunderstandings about the model.

For example, a 2020 PACMPL paper contains the following statement:

To check how prevalent unsafe is used as a documentation feature, our queries gathered data for unsafe traits and unsafe functions with safe implementations.

set_len being an unsafe fn is of course not merely a "documentation feature". It's an absolutely critical part of the soundness of Vec. (And the paper does talk about "invariants that are potentially critical for upholding Rust’s safety guarantees" (emphasis added), so I don't think their analysis is incorrect, but I still find the phrasing curious.)

Thus a huge win of this would be to avoid the (from the same paper)

After all, the compiler does not force developers to declare such functions as unsafe -- in contrast to other unsafe features.

Having the body of set_len do something the compiler recognizes as unsafe is a big help to people understanding the soundness of Vec, and in general to people adding new features inside existing unsafety privacy boundaries.

Especially combined with other accepted work like https://rust-lang.github.io/rfcs/2316-safe-unsafe-trait-methods.html we could start even doing things like clippy lints for "why is this unsafe when it doesn't do anything unsafe? Should one of the types involved be marked unsafe?"

If we don't have to link to tootsie pops as often because changing things that are relied on by unsafe code is itself unsafe, I'd consider that a big win.

---

As another way to look at this, it's weird that when I'm writing a method on by type with a safety invariant that I can do Self { a, b } and it's totally "safe", whereas if I call Self::new_unchecked(a, b) I need an unsafe block and tidy nags me to write a safety comment.

Tidy should nag about a safety comment for the constructor too, so that I'm not disincentivized to use the other, correctly-marked-unsafe function when writing things.

[burdges]

We do not need unsafe per se when maintaining a safety invariant within its defining visibility boundary aka module:

"Because it relies on invariants of a struct field, this unsafe code does more than pollute a whole function: it pollutes a whole module. Generally, the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy."

In other words, rust does not have unsafe types because you must enforce invariants at module boundaries anyways. Also various discussions in https://github.com/rust-lang/unsafe-code-guidelines clarify this point.

A method like Vec::set_len must be unsafe due to being public. An extern fn is unsafe because it points outside the module. etc.

Anyways..

If I understand, you want this type:

pub UnsafeInvariant<T>(T);
impl<T> UnsafeInvariant<T> {
    fn new(t: T) -> UnsafeInvariant<T> { UnsafeInvariant(t) }
    unsafe fn get(&self) -> &T { &self.0 }
    unsafe fn get_mut(&mut self) -> &mut T { &mut self.0 }
}

It's similar to UnsafeCell but propagates all auto-traits normally, based upon your comment above.

You still enforce invariants by visibility but UnsafeInvariant could provide the documentation for which you propose unsafe fields. In practice, I suspect you'd be better off like this:

    fn invariant_get(&self) -> &T { &self.0 }
    fn invariant_get_mut(&mut self) -> &mut T { &mut self.0 }

Why? All those unsafe blocks you'll write risk other mistakes, so ideally they should not exist if they merely maintain some invarant. Instead, you want a safe but distinctively named accessor method, which flags that you maintain the invariant.

Anecdotally, this type winds up being much more common:

struct HideMut<T>(T);
impl<T> Deref for HideMut<T> {
    type Target = T;
    fn deref(&self) -> &T { &self.0 }
}
impl<T> HideMut<T> {
    fn new(t: T) -> HideMut<T> { HideMut(t) }
    unsafe fn get_mut(&mut self) -> &mut T { &mut self.0 }
}

And UnsafeCell remains more common than both of course.

In fact, if you wrap the HideMut declaration inside some macro_rules! use_hide_mut then HideMut becomes module local, so the local module can access pub foo: HideMut<Foo> fields freely, but the outside world has only immutable access, even if given a &mut for the containing struct. This is really the common pattern.

We made this a local type for visibility modifiers, so a language level construct helps here. Also conversely, if you do not require visibility modifiers then simple types like UnsafeInvariant suffice, no language change necessary.

I suppose one might imagine pub(positive_visibility) unsafe(negative_visibility) mut(positive_visibility) field: type, except this still cannot capture when mutation becomes unsafe but reading remains safe. Yet, visibility control types like UnsafeInvariant and HideMut work fine.

[scottmcm]

EDIT: see below; it looks like the thing I was worried about here is probably impossible for other reasons.

I do thing that "safe to read; unsafe to modify" is the 99%+ case, and should certainly be the default, but

How could a field of a struct be unsafe to read?

Well, the field in AtomicPtr is unsafe to read, because it could be a race, for example. https://github.com/rust-lang/rust/blob/7a5814f922f85370e773f2001886b8f57002811c/library/core/src/sync/atomic.rs#L176

Or the value field of a ShardedLock in crossbeam https://docs.rs/crossbeam-utils/0.8.11/src/crossbeam_utils/sync/sharded_lock.rs.html#81

So perhaps some nuance for !Freeze could make sense? I'm not sure what the semver implications of that would be, though.

[Jules-Bertholet]

Well, the field in AtomicPtr is unsafe to read, because it could be a race

I don't think that is correct? The unsafe operation is dereferencing the pointer returned by UnsafeCell::get(), accessing the field can't lead to UB on its own.

[scottmcm]

accessing the field can't lead to UB on its own

Ah, I guess an access can't actually read an UnsafeCell (without ownership) because it's never Copy.

So I think the PlaceMention is always ok for everything, and a read would be unsafe for an UnsafeCell, but you can't actually do a read of an UnsafeCell directly in Rust.

[jhpratt]

We do not need unsafe per se when maintaining a safety invariant within its defining visibility boundary aka module

rust does not have unsafe types because you must enforce invariants at module boundaries anyways

The nomicon describes current behavior; using it as an argument against this RFC is counter to the purpose of the RFC. It's circular reasoning at best.

if they merely maintain some invarant.

The invariants are "merely" there for soundness. If the invariant is violated, the result is undefined behavior. That's far more serious than you make it sound.

Yet, visibility control types like UnsafeInvariant and HideMut work fine.

I have never seen anyone write code like this in practice. The standard library and my own code (in time) is included in this. That is a significant argument in favor of something better.

---

@scottmcm I'll definitely include parts of that into the RFC. Also reading that blog post now — I'd never seen it before.

[burdges]

If the invariant is violated, the result is undefined behavior.

That is a significant argument in favor of something better.

This RFC is not better because memory safety also helps when writing unsafe code.

The unsafe keyword is not simply a marker to tell you where to read more carefully. Its a marker of where safety rules must be violated.

In other words, we always convert regular code invariants into memory safety assurance, but these regular code invariants have exactly the same risks as other regular code, including their own memory safety concerns. This RFC confuses the memory safety consumed in maintaining the regular code invariant with the actually unsafe options the code requires.

In the past, unsafe fn bodies were unsafe blocks, but rust changed this to reduce the unsafe code surface area. This RFC is a mistake because it increases the unsafe code surface area with no benefits, given the same cautions can be maintained in other ways, like by variable naming, etc.

UnsafeInvariant not being used is evidence this feature is not required. UnsafeInvariant would make sense if you wanted to split the regular code invariant across distant visibility boundaries. In practice, unsafe fns always sufficed, or indeed proved more nuanced than UnsafeInvariant.

Anyways..

I think this discussion belongs in https://github.com/rust-lang/unsafe-code-guidelines where at least some people think formally about the unsafe code boundary.

[jhpratt]

In other words, we always convert regular code invariants into memory safety assurance, but these regular code invariants have exactly the same risks as other regular code, including their own memory safety concerns. This RFC confuses the memory safety consumed in maintaining the regular code invariant with the actually unsafe options the code requires.

This is your fundamental misunderstanding.

Other code in Vec relies on the invariants of Vec.len in ways that leads to undefined behavior if the invariants are broken. Fields like Vec.len are not "regular code invariants" — they are tightly coupled to whether the code is sound or not. It is a soundness invariant. You cannot possibly claim otherwise.

given the same cautions can be maintained in other ways, like by variable naming, etc.

Frankly, it's thinking like this that led to the creation of Rust. Thread safety can be maintained if everyone is super careful, but we all know how that works out. Likewise with a million other things. Programmers can not be relied upon to do the right thing. We have to force them to do it by leveraging the compiler wherever possible.

I think this discussion belongs in https://github.com/rust-lang/unsafe-code-guidelines where at least some people think formally about the unsafe code boundary.

I have no idea why you think the discussion belongs there, particularly as you're the one that initiated it here.

[burdges]

Other code in Vec relies on the invariants of Vec.len in ways that leads to undefined behavior if the invariants are broken.

Yes, but this does not make altering Vec.len within the Vec module an unsafe operation. That's not how safety works.

What happens if I've unsafe code which relies upon an invariant between the values in a slice, so your unsafe field is a &[*mut Foo] or &[usize]? I want memory safety around the regular code invariants in how I maintain this slice. Yes, those regular code invariants control memory safety in how the module get used, hence privacy. It's clearly worse if I've many more unsafe blocks merely to access this slice, which now might intermix with some real unsafe code blocks for the *mut Foos or even violate slice invariants.

We often have this "catch your tail" phenomenon in formalalisms.

Frankly, it's thinking like this that led to the creation of Rust

No. There is a formal model from the rustbelt project about how unsafe works, which gets discussed in the unsafe code guidlines repo. We should only expand what falls under unsafe code if the formal model says so. This RFC confuses those really important formal models with mere "read this" markers.

In brief, you don't have a formal conception of when unsafe types should be used. It's unlikely one exists. That makes this change a regression towards the C days.

[jhpratt]

Yes, but this does not make altering Vec.len within the Vec module an unsafe operation. That's not how safety works.

It actually is. If something can result in undefined behavior, it is unsafe. Where that happens is wholly irrelevant. You're using circular logic regarding the UCG, which is by definition written about Rust as it currently is. You seem to be claiming that library undefined behavior doesn't exist, which is bizarre.

I'll leave it at that as there's clearly no convincing you that the mere concept of an unsafe field has merit. I won't be responding further to anything along those lines.

[tgross35]

Could you add another motivating example to the RFC? It seems like the vec.len example isn't super convincing because changing anything in Vec/RawVec breaks the model down and is "unsafe", by definition: that is protected via the private/public module interface. This makes me think that the resolution would be something like unsafe modules or #[unsafe] types rather than fine grained per-field control, so I'm curious to see what motivates this specific solution.

[jhpratt]

@tgross35 I linked this earlier in the thread. Is that sufficient for you? It demonstrates that field-level safety is appropriate, as some of the fields are perfectly safe to set as they have no interaction with any other field. Yet at the same time, all fields are one logical unit that should not be split into separate structs.

[tgross35]

Thank you for the link, I meant specifically to add an example to the RFC document (which perhaps you planned to do anyway).

Even with that example, it does seem to me that it would be more correct to nest flags into a separate type with the other items it has the invariant with. To me, it illustrates the concept better; a field alone is never unsafe but rather it is unsafe in the context of other fields, and it seems like this is a suitable level for abstraction to a separate type. Also:

• If a struct of 12 fields has 8 marked unsafe, it isn't immediately clear how they may be related. Are they all tied together via an invariant? Are 6 tied together and the remaining 2 associated somehow? This becomes immediately clear by separating things that are related into separate types, and I think that behavior should be encouraged
• For the example Parsed struct, if sufficiently many fields are related to flags then it seems like it would make sense to mark the entire struct #[upholds_unsafe_invariant] or something like that. Maybe a few fields aren't related to the invariant, but commenting // SAFETY: not related to the invariant is easy enough to make this clear (and forces you to make the change if they do become related to the invariant for some reason)

Niche optimization was mentioned as well as a reason for not wanting to split off types, but I think this is more applicable to all separations of logic, and not just those with safety concerns. I don't wish to derail this conversation, but the thought has crossed my mind before about how Rust could potentially use a #[flatten] attribute for nested struct fields that tells the compiler to merge the child struct's fields into the parent's and rearrange for best possible size. (yes - possible method duplication, but this would be meant for one-off structs).

I think that investigating something like that may be more widely useful than using niche optimization as a justification for why a single flat struct is the correct solution. (edit: I brought this up for some discussion on Zulip https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/.60.23.5Bflatten.5D.60.20struct.20field.20attribute)

[RalfJung]

That seems like a rather odd example -- why would I even have this field then? It's also an example of a "subtractive" invariant.

There are more convincing examples involving less artificial, and "additive", invariants. For instance,

    /// Must contain an even number.
  unsafe r: &'static Cell<i32>,

[jswrenn]

Your example is appropriate for the next heading ("Field Copies are Safe"), but not this one, since &T is unconditionally Copy. The example here should be one where moving the field (and invalidating the original memory) should not be safe.

[RalfJung]

I was thinking of "move" and "copy" as the same thing for a Copy type, but I take your point.

Still, I think the example here is not convincing at all. If that's the best we got, we should just allow moves of unsafe fields instead.

[jswrenn]

Disagree. We should keep unsafe moves for three reasons:

• Over and over again, we've thought "can't we just make X safe" and the answer has been "no". I am not confident that this time is different; that this is "the best we got".
• There are cases, such as the example here, where making moves unsafe really does prevent programmer mistakes earlier than if we made moves safe. Isn't that the point of this RFC?
• There is pedagogic value in having a simple rule — "uses of unsafe fields are unsafe" — than having one with niche carve-outs, particularly as odd as "moves are safe, but only if they invalidate the original memory".

[RalfJung]

A truly destructive move leaves the field inaccessible. The field has to be re-assigned a new value before the entire outer type is even considered fully initialized again. That re-assignment is unsafe, and that unsafety is where you have to check the invariant. So I am quite confident that there's no way a move can cause issues.

Having a field that nobody is allowed to read or write is not the purpose of this RFC and is not covered by the motivation stated for this RFC. What bug would be prevented by this? When would one even have such a field?

[jswrenn]

Thank you for the examples. It might also be helpful to see what some use-sites of these fields looks like. Would you agree that these are the drawbacks of the approach:

• the onus is on the user to correctly deduce what combination of unsafe/unsafe(mut)/Unsafe they need, and the cost of getting this wrong can be that invariants are violatable in unsafe code
• the safety proofs required to call Unsafe's constructor and accessors depends on program dataflow

[Jules-Bertholet]

the onus is on the user to correctly deduce what combination of unsafe/unsafe(mut)/Unsafe they need, and the cost of getting this wrong can be that invariants are violatable in unsafe code

On the other hand, by forcing you to think deeply about exactly what your safety invariants are, this choice could also help you not violate them. And, as I have said before, false-positive unsafe also has a cost. Taken to the extreme: if everything is unsafe, then nothing is.

the safety proofs required to call Unsafe's constructor and accessors depends on program dataflow

Yes for the accessors, but this is also true of the unsafe-to-move-out-of fields in the current iteration of this RFC. No for the constructor, Unsafe’s constructor is safe.

[Jules-Bertholet]

It might also be helpful to see what some use-sites of these fields looks like.

Example 1: UnalignedRef

pub struct UnalignedRef<'a, T> {
    /// # Safety
    ///
    /// `ptr` is a shared reference to a valid-but-unaligned instance of `T`.
    unsafe(mut) ptr: *const T,
    _lifetime: PhantomData<&'a T>,
}

// Usage:

fn main() {
    let array: [u8; 4] = [0; 4];
    let unaligned_u32: *const u32 = (&raw const array).cast();

    // SAFETY: `unaligned_u32` points to 4 initialized bytes
    let mut unaligned_ref = unsafe {
        UnalignedRef {
            ptr: unaligned_u32,
            _lifetime: PhantomData,
        }
    };

    let _ref = &unaligned_ref.ptr;
    // SAFETY: `_mut_ref` is entirely unused
    let _mut_ref = unsafe { &mut unaligned_ref.ptr };

    let _ptr = &raw const unaligned_ref.ptr;
    let _mut_ptr = &raw mut unaligned_ref.ptr;

    let copy_out = unaligned_ref.ptr;

    // SAFETY: doesn't modify the value at all
    unsafe {
        unaligned_ref.ptr = copy_out;
    }
}

Example 2: NonZeroCell

pub struct NonZeroCell {
    /// # Safety
    ///
    /// Contents never equal zero.
    unsafe cell: Cell<u32>,
}

// Usage:

fn main() {
    // SAFETY: `cell` is not 0
    let mut nonzero_cell = unsafe {
        NonZeroCell {
            cell: Cell::new(42),
        }
    };

    // SAFETY: `_ref` is entirely unused
    let _ref =  unsafe {  &nonzero_cell.cell };
    // SAFETY: `_mut_ref` is entirely unused
    let _mut_ref = unsafe {  &mut nonzero_cell.cell };

    let _ptr = &raw const nonzero_cell.cell;
    let _mut_ptr = &raw mut nonzero_cell.cell;

    let move_out = nonzero_cell.cell;

    // SAFETY: doesn't modify the value at all
    unsafe {
        nonzero_cell.cell = move_out;
    }
}

Definition of Unsafe

/// A wrapper for a `T` that may have broken safety invariants.
///
/// Validity invariants must still be upheld.
/// Additionally, the destructor is not supressed,
/// so the minimal invariants required by the destructor
/// must still be upheld. (Use `Unsafe<ManuallyDrop<...>>`
/// if you can't provide this.)
#[repr(transparent)]
#[derive(Default)]
pub struct Unsafe<T: ?Sized>(T);

impl<T> Unsafe<T> {
    pub fn new(val: T) -> Self {
        Self(val)
    }

    pub fn set(&mut self, val: T) {
        self.0 = val;
    }

    /// # Safety
    ///
    /// `T` may have broken safety invariants
    pub unsafe fn into_inner(self) -> T {
        self.0
    }
}

impl<T: ?Sized> Unsafe<T> {
    /// # Safety
    ///
    /// `T` may have broken safety invariants
    pub unsafe fn get(&self) -> &T {
        &self.0
    }

    /// # Safety
    ///
    /// `T` may have broken safety invariants
    pub unsafe fn get_mut(&mut self) -> &mut T {
        &mut self.0
    }
}

impl<T> Clone for Unsafe<T>
where
    T: Copy,
{
    fn clone(&self) -> Self {
        *self
    }
}

impl<T> Copy for Unsafe<T> where T: Copy {}

Example 3: MaybeInvalidString

struct MaybeInvalidString {
    /// # Safety
    ///
    /// May contain invalid UTF-8.
    /// But must still point to a valid allocation
    /// with `length` bytes fully initialized.
    unsafe(mut) maybe_invalid: Unsafe<String>,
}

/// Returns a `String` that is invalid UTF-8.
fn make_an_invalid_string() -> Unsafe<String> {
    let mut invalid = Unsafe::new("hello".to_owned());
    // SAFETY: we turn the String into invalid UTF-8,
    // but it's wrapped in an `Unsafe` so this is fine
    unsafe {
        let bytes_mut = invalid.get_mut().as_mut_str().as_bytes_mut();
        bytes_mut[0] = 0xFF;
        bytes_mut[1] = 0xFF;
    }
    invalid
}

fn main() {
    // SAFETY: `maybe_invalid` points to a valid allocation
    // and is fully initialized up to its length
    // (though it is not valid UTF-8
    let mut string = unsafe {
        MaybeInvalidString {
            maybe_invalid: make_an_invalid_string(),
        }
    };

    let ref_ = &string.maybe_invalid;
    // SAFETY: `_inner_ref` is entirely unused
    let _inner_ref = unsafe {  ref_.get() };
    // SAFETY: `mut_ref` is not used to swap out the value
    let mut_ref = unsafe {  &mut string.maybe_invalid };
    // SAFETY: `_inner_mut_ref` is entirely unused
    let _inner_mut_ref = unsafe {  mut_ref.get_mut() };

    let _ptr = &raw const string.maybe_invalid;
    let _mut_ptr = &raw mut string.maybe_invalid;

    let move_out = string.maybe_invalid;

    // SAFETY: doesn't modify the value at all
    unsafe {
        string.maybe_invalid = move_out;
    }

    let move_out_again = string.maybe_invalid;
    // SAFETY: `_inner_ref` is entirely unused
    let _inner_ref = unsafe {  move_out_again.get() }; 
}

Example 4: UniqueArc

/// An `Arc` that provides exclusive access to its referent.
///
/// A `UniqueArc` may have any number of `KeepAlive` handles which ensure that
/// the inner value is not dropped. These handles only control dropping, and do
/// not provide read or write access to the value.
pub struct UniqueArc<T: 'static> {
    /// # Safety
    ///
    /// While this `UniqueArc` exists, the value pointed by this `arc` may not
    /// be accessed (read or written) other than via this `UniqueArc`.
    unsafe arc: Arc<UnsafeCell<T>>,
}

/// Keeps the parent [`UniqueArc`] alive without providing read or write access
/// to its value.
pub struct KeepAlive<T> {
    /// # Safety
    ///
    /// `T` may not be accessed (read or written) via this `Arc`.
    #[expect(unused)]
    arc: Unsafe<Arc<UnsafeCell<T>>>,
}


fn main() {
    // SAFETY: The `Arc` is newly created,
    // so there are no other clones of it around
    let unique_arc: UniqueArc<i32> = unsafe {
        UniqueArc {
            arc: Arc::new(UnsafeCell::new(42)),
        }
    };

    let keep_alive = KeepAlive {
        // SAFETY: we immediately put the cloned `Arc` in a `KeepAlive`,
        // where it will be inaccessible
        arc: Unsafe::new(unsafe { unique_arc.arc.clone() }),
    };

    let keep_alive_arc = keep_alive.arc;

    // By moving out of `arc`, we destroy the `UniqueArc`¸
    // and are no longer bound by its uniqueness requirement
    let no_longer_unique_arc = unique_arc.arc;
    let cloned_arc = no_longer_unique_arc.clone();

    drop(keep_alive_arc);
}

[Jules-Bertholet]

Another way to think about it:

• An unsafe(mut) field strengthens the requirements on the field contents. (Not every value of the type that is normally safe to get an &mut to, is allowed.)
• An unsafe field strengthens them further. (Not every value of the type that is normally safe to get an & to, is allowed.)
• An Unsafe<...> wrapper type weakens the requirements compared to the baseline for the type. (Some values are accepted even if it is not normally safe to get an & to them.)
• unsafe(mut) Unsafe<...> (combining unsafe fields and wrapper type) can weaken, strengthen, or both at once.

[jswrenn]

In d1d7388, I've attempted to document this syntax+wrapper proposal. While I'm sure there's much more to say, it's not feasible to embed a whole second RFC in the Alternatives section of this RFC. Overall, I favor the comparative pedagogic simplicity and resistance to misuse of the RFC's current proposal, and I think these benefits outweigh its drawbacks.

[Jules-Bertholet]

I am not convinced that changing the type of the field over an edition is viable. (However, I agree that safe-to-& unsafe fields is something that can be left for later.)

[jswrenn]

It would be sound, but not seamless. Since the Unsafe wrapper is presumably #[repr(transparent)], its addition doesn't have opsem implications. Crate authors with public unsafe fields affected by this could not migrate editions without releasing a SemVer breaking change, since changing a public field's type is a SemVer breaking change.

I raise this all as a future possibility only for completeness — I don't think we'd walk through this door likely or lightly. I remain confident that the design proposed by this RFC offers the best tradeoffs.

[Jules-Bertholet]

The way I conceptualized it in my proposal at #3458 (comment): the argument to unsafe denotes the weakest reference type that is unsafe to obtain. So unsafe() makes & and &mut references unsafe, while unsafe(mut) only makes &mut unsafe.

[Jules-Bertholet]

If Rust ever gets partial borrows, the syntax for that could be adopted to extend this feature as well.

[Jules-Bertholet]

Again, the same is true of this RFC’s design. With this RFC, the unsafe is required at the site of the copy/move, and to determine whether the safety condition is met, you must trace dataflow forward to where the value is later used. With Unsafe wrappers, the unsafe is required at the use-site, and dataflow must be traced back to the copy/move site. Both solutions require tracing dataflow, just in different directions.

[jswrenn]

But isn't that doubly true for Unsafe? Unsafe's accessors also require unsafe, forcing you to further trace the dataflow forward to check that uses don't violate the field invariant (which you discovered by tracing the dataflow backwards).

[Jules-Bertholet]

True, but you will probably not use the accessor until the last possible moment. So you are unlikely to have to trace forward very far.

[tmandry]

According to @jswrenn this RFC has reached a fixed point.

There are two leading alternatives. This RFC picks the simpler one of making exactly one kind of unsafe field, where all accesses are unsafe. Cases where e.g. reads are always safe can be expressed via a helper method.

Nominating for lang team discussion to see if we can get to a quick consensus on this RFC. I'd like to hear from @scottmcm as the lang team champion before moving forward.

@rustbot label I-lang-nominated

[Jules-Bertholet]

Even with partial borrows, it may not be possible to write a safe helper method for moving out of a field (while leaving the rest of the struct intact).

[joshtriplett]

I would definitely love to also have the future work of defining "unsafe to write but safe to read" fields. But I think this initial proposal as written is great and provides a good solution for many cases.

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.