Unstable Features

Unstable features used by Crubit

For each feature we use, we document the following:

• Crubit feature: Under which Crubit feature flags do we use this? If it is under supported, then the Rust feature breaking can break Crubit users in general. If it is under experimental, then it only breaks tests and close partner teams.
• Use case: What do we use the feature for?
• Exit strategy: What would happen if the feature went away?

custom_inner_attributes

• Crubit feature: supported
• Use case: Used to suppress automatic formatting, to make our golden tests stable even in the face of e.g. non-idempotency bugs in rustfmt.
• Exit strategy: Disable rustfmt on the golden tests some other way.

negative_impls

• Crubit feature: supported
• Use case: Used to implement ctor / nontrivial intialization, so that we can dispatch on the existence of the Unpin trait (which is not possible with PhantomPinned). Also used so that we can pin/unpin a type without adding a field (which is not possible with PhantomPinned).
• Exit strategy: For dispatch, we can define a new auto trait (also an unstable feature) or use specialization (also an unstable feature). For the fields, this is a compatibility break, and we'd need to add a PhantomData field to all C++ types to mark them as !Send, !Unpin, etc.

vec_into_raw_parts

• Crubit feature: experimental
• Use case: Used for conversions of vectors of forward-declared objects, which is not yet released.
• Exit strategy: We could delete this if we had to. Hopefully low-risk, people love raw parts.

extern_types

• Crubit feature: experimental
• Use case: Used for forward declarations, which have an unknown size.
• Exit strategy: Hard to avoid: without this, we get UB for forward declarations if you use references, due to provenance rules. Don't need the forward_declare crate except for migration from existing C++ code to Rust, where it relies on forward declarations for build performance or cycle-breaking reasons.

sized_hierarchy

• Crubit feature: experimental

• Use case: Used for forward declarations, which have an unknown size.

• Exit strategy: Same as extern_types.

arbitrary_self_types

• Crubit feature: experimental
• Use case: We need this or an equivalent feature in order to make methods callable on rvalue references but not regular const references, so as to support C++ move semantics. Also likely need it for aliasing-safe references.
• Exit strategy: Likely can't live without this feature. Lowish risk, in that Rust needs user-defined types to be usable as self types (e.g. it already has this for Rc), although the exact mechanism is as-yet undecided.

never_type

• Crubit feature: experimental
• Use case: Used in a test to demonstrate we don't support it.
• Exit strategy: Delete test.

c_variadic

• Crubit feature: experimental
• Use case: Used in a test to demonstrate we don't support it.
• Exit strategy: Delete test.

abi_vectorcall

• Crubit feature: experimental
• Use case: Used in a test to test non-C calling conventions.
• Exit strategy: Delete test at worst, replace with different non-C calling convention at best.

impl_trait_in_assoc_type

• Crubit feature: experimental
• Use case: Used for returning impl Ctor for nontrivial construction.
• Exit strategy: Replace with RPITIT, which is stable now.

allocator_api

• Crubit feature: unreleased
• Use case: Used for an ABI-compatible implementation of std::vector that can reuse the C++ allocator.
• Exit strategy: Do not release, or release but require that the Rust global allocator is the C++ allocator.

cfg_sanitize

• Crubit feature: unreleased
• Use case: Used for an ABI-compatible implementation of std::vector that poisons memory in the same way as libc++.
• Exit strategy: Do not release, or release but with degraded AddressSanitizer results that hide bugs.

cfg_accessible, stmt_expr_attributes, proc_macro_hygiene

• Crubit feature: N/A, used internally by Crubit
• Use case: Crubit uses rustc as a library, which is subject to backwards-incompatible changes. cfg_accessible makes it much easier to write code which is compatible with two versions of rustc at the same time. stmt_expr_attributes, proc_macro_hygiene lets us handle backwards-incompatible changes affecting individual expressions.
• Exit strategy: If cfg_accessible changes during the very same update we are trying to be compatible with, we can't use it in the first place. If it changes in a future version, then the update has already completed successfully, so we can delete the usage (keeping the cfg_accessible usage that is currently used, and deleting the other one). Same for the rest.

unsized_const_params, adt_const_params

• Crubit feature: supported
• Use case: Better error messages when you get a type error on forward declarations. Symbol<(C<'F'>, C<'o'>, C<'o'>)> is so substantially worse than Symbol<"Foo"> that Rust will even hide the type entirely, forcing you to read the name from a separate file (which may not be available to you if you were doing a remote build).
• Exit strategy: The Symbol<(...)> approach works and is tested, and we can switch back with a single flag flip. The only impact is a regression on error messages.

super_let

• Crubit feature: wrapper
• Use case: Less painfully awkward syntax for in-place initialization.
• Exit strategy: Migrate all calls like let x = emplace!(y) to emplace!{let x = y;}.

Without this feature, Crubit's ctor.rs crate must use a syntax like emplace!{let x = ...} to in-place initialize a pinned value using a C++ constructor. It cannot use the same trick that pin!() used to use, because it needs to actually call a function to initialize the memory, and so lifetime extension would not apply: the memory, being passed to a function, would have its lifetime end at the end of the statement.

This doesn't work great with rustfmt, which gives up inside the macro call. It is also confusing, and even alarming, and makes the ctor.rs crate look less like a set of ecosystem and vocabulary types/traits, and more like a language dialect.

super_let allows us to instead write let x = emplace!(...), with the macro expanding to a use of super_let to lifetime-extend the storage subexpression.

allow_internal_unstable

• Crubit feature: wrapper
• Use case: Allow callers of the super_let version of emplace not to require the feature.
• Exit strategy: Add feature(super_let) to all callers.

Unstable features not used by Crubit

The following features are ones we'd hypothetically like to use, but do not.

try_trait_v2

Crubit does aim to support the Abseil error type, absl::Status, which ideally would be usable everywhere a Result is for ergonomics. For example, we'd expect you to be able to write code like the following:

pub fn read() -> Status {
  foo()?;
}

Where foo returns a Status, or even an io::Result.

For this to work with Status being exactly the same type as it is in C++, with the same layout, we need to use try_trait_v2.

Until it is stabilized, or the cost/benefit becomes worthwhile, we can work around it by using Result, and converting when a Result is passed or returned by value.

fn_traits

Much like try_traits_v2, we'd like to support C++ “function objects” which implement the call operator. In Rust, without fn_traits, the API is awkward: given a foo of type std::function, absl::AnyInvocable, or absl::FunctionRef, you cannot just write foo() to call it, because it cannot implement the Fn* family of traits. This makes these types less convenient to use than they are in C++, requiring something like foo.as_closure()() to create a closure out of a FunctionRef or similar.

(Another use case we've examined in the past: in addition to implementing function objects, fn_traits could also be used to implement overloading, including overloaded function objects. For example, an overloaded function could, instead, be a constant with many different implementations of Fn, for different parameter types. Though, this ends up looking a bit odd: (mystruct.overloaded)())

min_specialization

We have many use cases for min_specialization, including bindings for C++ templates that have explicit template specialization or partial template specialization. However, they are not yet pressing, and specialization related Rust features are especially risky.

inherent_associated_types

Inherent associated types give a natural Rust spelling of the following C++ type definition:

struct Foo {
    using MyAlias = int;
};

disjoint_associated_types

disjoint_associated_types was closed, but not necessarily permanently. See also https://github.com/rust-lang/rust/issues/20400.

Not every C++ type supports the Rust move operation. For more on this, see Classes and Structs.

Crubit (in experimental mode) supports passing and returning these non-Rust-movable C++ objects by value. But since they are not Rust-movable, they cannot literally be returned in Rust by value: pub fn foo() -> X performs a Rust move of its return type by definition.

Instead, these objects support lazy construction, in the same style as moveit. See ctor.rs. This results in, for example, the following API differences:

X is rust-movable	X is not rust-movable
pub fn foo() -> X	pub fn foo() -> impl Ctor<Output=X>
impl Add<X> for &C	impl<T: Ctor<Output=X>> Add<T> for &C

The problem comes in with operator overloading: the following is valid:

impl Add<X> for &C {...}
impl Add<Y> for &C {...}

But the equivalent using this lazy-construction trait is not:

impl<T: Ctor<Output=X>> Add<T> for &C {...}
impl<T: Ctor<Output=Y>> Add<T> for &C {...}

Rust doesn't know that these two are disjoint, meaning that we cannot use the Ctor trait approach for traits.

An alternative fix would be language support for in-place pinned construction. That would render the Ctor trait obsolete, and reduce Crubit's needs around trait coherence (as well as negative_impls, above).

register_tool

We previously used this attribute to allow Crubit to read annotations on types/functions. However, the need for end-users to enable a feature gate and re-declare the tool caused us to migrate away to using #[doc] attributes.

We use custom attributes so that Crubit can round-trip a type correctly, or to implement automated bridging so that a C++ Status becomes a Rust Result<(), StatusError>, or what have you.