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Summary

Permit users to #[cfg(..)] on whether:

  • they have a certain minimum Rust version (#[cfg(version(1.27.0))]).
  • a certain external path is accessible (#[cfg(accessible(::std::mem::ManuallyDrop))]).

Motivation

A core tenet of Rust's story is "stability without stagnation". We have made great strides sticking to this story while continuously improving the language and the community. This is especially the case with the coming Rust 2018 edition.

However, while the situation for evolving the language is doing well, the situation for library authors is not as good as it could be. Today, crate authors often face a dilemma: - "Shall I provide more features and implementations for later versions of Rust, or should I stay compatible with more versions of the compiler".

While much thought has been given to how we can reduce "dependency hell" by enhancing cargo for:

  • the control users have over their dependencies.
  • the compatibility of crates with each other.
  • reducing the maintainability burden of having to make sure that versions work with each other.

...not much focus has been given to how conditional compilation can be improved to extend how many versions back a crate supports. This becomes critically important if and when we gain LTS channels as proposed by RFC 2483.

The current support for such conditional compilation is lacking. While it is possible to check if you are above a certain compiler version, such facilities are not particularly ergonomic at the moment. In particular, they require the setting up of a build.rs file and declaring up-front which versions you are interested in knowing about. These tools are also unable to check, without performing canary builds of simple programs with use ::std::some::path;, if a certain path exists and instead force you to know which version they were introduced in.

We can do better. In this RFC we aim to rectify this by giving library authors the tools they need in the language itself. With the features proposed in the summary we aim to make retaining compatibility and supporting more compiler versions pain-free and to give authors a lot of control over what is supported and what is not.

Another use case this RFC supports is to work around compiler bugs by checking if we are on a particular version. An example where this occurred is documented in rust-lang-nursery/error-chain#101.

Guide-level explanation

#[cfg(accessible($path))]

Consider for a moment that we would like to use the Iterator::flatten method of the standard library if it exists (because it has become soon in a certain Rust version), but otherwise fall back to Itertools::flatten. We can do that with the following snippet:

#[cfg(accessible(::std::iter::Flatten))]
fn make_iter(limit: u8) -> impl Iterator<Item = u8> {
    (0..limit).map(move |x| (x..limit)).flatten()
}

#[cfg(not(accessible(::std::iter::Flatten)))]
fn make_iter(limit: u8) -> impl Iterator<Item = u8> {
    use itertools::Itertools;
    (0..limit).map(move |x| (x..limit)).flatten()
}

// Even better
fn make_iter(limit: u8) -> impl Iterator<Item = u8> {
    #[cfg(not(accessible(::std::iter::Flatten)))]
    use itertools::Itertools;
    (0..limit).map(move |x| (x..limit)).flatten()
}

fn main() {
    println!("{:?}", make_iter(10).collect::<Vec<_>>());
}

What this snippet does is the following:

  1. If the path ::std::iter::Flatten exists, the compiler will compile the first version of make_iter. If the path does not exist, the compiler will instead compile the second version of make_iter.

The result of 1. is that your crate will use Iterator::flatten on newer versions of Rust and Itertools::flatten on older compilers. The result of this is that as a crate author, you don't have to publish any new versions of your crate for the compiler to switch to the libstd version when people use a newer version of Rust.

Once the standard library has stabilized iter::Flatten, future stable compilers will start using the first version of the function.

In this case we used the accessible flag to handle a problem that the addition of Iterator::flatten caused for us if we had used Itertools::flatten. We can also use these mechanisms for strictly additive cases as well. Consider for example the proptest crate adding support for RangeInclusive:

// #[cfg_attr(feature = "unstable", feature(inclusive_range))]
// ^-- If you include this line; then `cargo build --features unstable`
//     would cause nightly compilers to activate the feature gate.
//     Note that this has some inherent risks similar to those for
//     `#[cfg(nightly)]` (as discussed later in this RFC).

macro_rules! numeric_api {
    ($typ:ident) => {
        ...

        #[cfg(accessible(::core::ops::RangeInclusive))]
        impl Strategy for ::core::ops::RangeInclusive<$typ> {
            type Tree = BinarySearch;
            type Value = $typ;

            fn new_tree(&self, runner: &mut TestRunner) -> NewTree<Self> {
                Ok(BinarySearch::new_clamped(
                    *self.start(),
                    $crate::num::sample_uniform_incl(runner, *self.start(), *self.end()),
                    *self.end()))
            }
        }

        ...
    }
}

macro_rules! unsigned_integer_bin_search {
    ($typ:ident) => {
        pub mod $typ {
            use rand::Rng;

            use strategy::*;
            use test_runner::TestRunner;

            int_any!($typ);
        }
    }
}

unsigned_integer_bin_search!(u8);
unsigned_integer_bin_search!(u16);
...

This means that proptest can continue to evolve and add support for RangeInclusive from the standard library and the x..=y syntax in the language without having to release a new breaking change version. Dependents of proptest simply need to be on a compiler version where ::core::ops::RangeInclusive is defined to take advantage of this.

So far we have only used accessible(..) to refer to paths in the standard library. However, while it will be a less likely use case, you can use the flag to test if a path exists in some library in the ecosystem. This can for example be useful if you need to support lower minor versions of a library but also add support for features in a higher minor version.

#[cfg(version(1.27.0))]

Until now, we have only improved our support for library features but never any language features. By checking if we are on a certain minimum version of Rust or any version above it, we can conditionally support such new features. For example:

#[cfg_attr(version(1.27), must_use)]
fn double(x: i32) -> i32 {
    2 * x
}

fn main() {
    double(4);
    // warning: unused return value of `double` which must be used
    // ^--- This warning only happens if we are on Rust >= 1.27.
}

Another example is opting into the system allocator on Rust 1.28 and beyond:

#[cfg(version(1.28))]
// or: #[cfg(accessible(::std::alloc::System))]
use std::alloc::System;

#[cfg_attr(version(1.28), global_allocator)]
static GLOBAL: System = System;

fn main() {
    let mut v = Vec::new();
    // This will allocate memory using the system allocator.
    // ^--- But only on Rust 1.28 and beyond!
    v.push(1);
}

Note that you won't be able to make use of #[cfg(version(..))] for these particular features since they were introduced before this RFC's features get stabilized. This means that you can't for example add version(1.28) to your code and expect Rust 1.28 compilers to enable the code. However, there will be features in the future to use this mechanism on.

Reference-level explanation

#[cfg(version(<semver>))]

To the cfg attribute, a version flag is added. This flag has the following grammar (where \d is any digit in 0 to 9):

flag : "version" "(" semver ")" ;
semver : digits ("." digits ("." digits)?)? ;
digits : \d+ ;

If and only if a Rust compiler considers itself to have a version which is greater or equal to the version in the semver string will the #[cfg(version(<semver>)] flag be considered active. Greater or equal is defined in terms of caret requirements.

#[cfg(accessible($path))]

To the cfg attribute, an accessible flag is added.

Syntactic form

This flag requires that a path fragment be specified in it inside parenthesis but not inside a string literal. The $path must start with leading :: and may not refer to any parts of the own crate (e.g. with ::crate::foo, ::self::foo, or ::super::foo if such paths are legal). This restriction exists to ensure that the user does not try to conditionally compile against parts of their own crate because that crate has not been compiled when the accessible flag is checked on an item.

Basic semantics

If and only if the path referred to by $path does exist and is public will the #[cfg(accessible($path))] flag be considered active.

#![feature(..)] gating

In checking whether the path exists or not, the compiler will consider feature gated items to exist if the gate has been enabled.

NOTE: In the section on #[cfg(nightly)] and in the guide level explanation we note that there are some risks when combining cfg(feature = "unstable") and accessible(..) to add conditional support for an unstable feature that is expected to stabilize. With respect to such usage:

  1. User-facing documentation, regarding accessible(..) should highlight risky scenarios, including with examples, with respect to possible breakage.

  2. Our stability policy is updated to state that breakage caused due to misuse of accessible(..) is allowed breakage. Consequently, rust teams will not delay releases or un-stabilize features because they broke a crate using accessible(..) to gate on those features.

Inherent implementations

If a path refers to an item inside an inherent implementation, the path will be considered to exist if any configuration of generic parameters can lead to the item. To check whether an item exists for an implementation with a specific sequence of concrete types applied to a type constructor, it is possible to use the ::foo::bar::<T>::item syntax.

Fields

It is also possible to refer to fields of structs, enums, and unions. Assuming that we have the following definitions in the foobar crate:

pub struct Person { pub ssn: SSN, age: u16 }

pub enum Shape<Unit> {
    Triangle { pub sides: [Unit; 3] },
    ...
}

pub union MaybeUninit<T> { uninit: (), pub value: T }

We can then refer to them like so:

#[cfg(all(
    accessible(::foobar::Person::ssn),
    accessible(::foobar::Shape::Triangle::sides),
    accessible(::foobar::Shape::MaybeUninit::value)
))]
fn do_stuff() {
    ...
}

Macros

Finally, bang macros, derive macros, attributes of all sorts including built-in, user provided, as well as latent derive helper attributes, will be considered when determining if a path is accessible.

cfg_attr and cfg!

Note that the above sections also apply to the attribute #[cfg_attr(..)] as well as the special macro cfg!(..) in that version(..) and accessible(..) are added to those as well.

Drawbacks

One argument is that hypothetically, if the standard library removed some unstable item, then we might "not notice" if everyone uses it through #[cfg(accessible(..))].

Incremental garbage code and its collection

It sometimes happens that feature gates never make it to stable and that they instead get scrapped. This occurs infrequently. However, when this does happen, code that is conditionally compiled under #[cfg(accessible(::std::the::obsoleted::path))] will become garbage that just sits around. Over time, this garbage can grow to a non-trivial amount.

However, if we provide LTS channels in the style of RFC 2483, then there are opportunities to perform some "garbage collection" of definitions that won't be used when the LTS version changes.

Rationale and alternatives

accessible(..)

The primary rationale for the accessible mechanism is that when you want to support some library feature, it is some path you are thinking of rather than what version it was added. For example, if you want to use ManuallyDrop, you can just ask if it exists. The version is instead a proxy for the feature. Instead of detecting if the path we want is available or not via an indirection, we can just check if the path exists directly. This way, a user does not have to look up the minimum version number for the feature.

You may think that version(..) subsumes accessible(..). However, we argue that it does not. This is the case because at the time of enabling the feature = "unstable" feature that enables the path in libstd, we do not yet know what minimum version it will be supported under. If we try to support it with version(..), it is possible that we may need to update the minimum version some small number of times. However, doing so even once means that you will need to release new versions of your crate. If you instead use accessible(..) you won't need to use it even once unless the name of the path changes in-between.

Another use case accessible(..) supports that version(..) doesn't is checking support for atomic types, e.g. accessible(::std::sync::atomic::AtomicU8). This subsumes the proposed #[cfg(target_has_atomic = "..")] construct.

Preventing relative paths

The reason why we have enforced that all paths must start with :: inside accessible(..) is that if we allow relative paths, and users write accessible(self::foo), then they can construct situations such as:

#[cfg(accessible(self::bar)]
fn foo() {}

#[cfg(accessible(self::foo)]
fn bar() {}

One way around this is to collect all items before cfg-stripping, but this can cause problems with respect to stage separation. Therefore, we prevent this from occurring with a simple syntactic check.

One mechanism we could use to make relative paths work is to use a different resolution algorithm for accessible(..) than for use. We would first syntactically reject self::$path, super::$path, and crate::$path. The resolution algorithm would then need to deal with situations such as:

#[cfg(accessible(bar)]
fn foo() {}

#[cfg(accessible(foo)]
fn bar() {}

by simply not considering local items and assuming that bar and foo are crates. While that would make accessible($path) a bit more ergonomic by shaving off two characters, chances are, assuming the uniform_paths system, that it would lead to surprises for some users who think that bar and foo refer to the local crate. This is problematic because it is not immediately evident for the user which is which since a different crate is needed to observe the difference.

Also do note that requiring absolute paths with leading :: is fully forward-compatible with not requiring leading ::. If we experience that this restriction is a problem in the future, we may remove the restriction.

#[cfg(accessible(..)) or #[cfg(accessible = ..)

We need to decide between the syntax accessible(..) or accessible = ... The reason we've opted for the former rather than the latter is that the former syntax looks more like a question/query whilst the latter looks more like a statement of fact.

In addition, if we would like to enable accessible = $path we would need to extend the meta grammar. We could justify that change in and of itself by observing that crates such as serde_derive permit users to write things like #[serde(default = "some::function")]. By changing the grammar we can allow users to instead write: #[serde(default = some::function)]. However, in this case, accessible($path) seems the optimal notation.

If we would like to extend the meta grammar, we could do so by changing:

named_value : "=" lit ;

meta_or_lit : meta | lit ;
meta_or_lit_list : meta_or_lit "," meta_or_lit_list ","? ;
meta_list : "(" meta_or_lit_list ")" ;
meta : path ( named_value | meta_list )? ;

into:

lit_or_path : path | lit ;
named_value : "=" lit_or_path ;

meta_or_lit : meta | lit ;
meta_or_lit_list : meta_or_lit "," meta_or_lit_list ","? ;
meta_list : "(" meta_or_lit_list ")" ;
meta : path ( named_value | meta_list )? ;

The bikeshed

One might consider other names for the flag instead of accessible. Some contenders are:

  • reachable
  • path_accessible
  • usable
  • can_use
  • path_exists
  • have_path (or has_path)
  • have
  • have_item
  • path_reachable
  • item_reachable
  • item_exists

accessible

Currently accessible is the choice because it clearly signals the intent while also being short enough to remain ergonomic to use. In particular, while path_accessible might be somewhat more unambiguous, we argue that from the context of seeing accessible(::std::foo::bar) it is clear that it is paths we are talking about because the argument is a path and not something else.

reachable

The word reachable is also a synonym of accessible and is one character shorter. However, it tends to have a different meaning in code. Examples include:

  • std::hint::unreachable_unchecked
  • std::unreachable

All in all, we have chosen to go with accessible instead as the more intuitive option.

usable

While can_use and usable are also strong contenders, we reject these options because they may imply to the user that only things that you may use $path; can go in there. Meanwhile, you may #[cfg(accessible(::foo::MyTrait::my_method)) which is not possible as use ::foo::MyTrait::my_method;. This also applies to other associated items and inherent methods as well as struct fields.

has_path

Another strong contender is has_path or have_path.

However, this variant is vague with respect to what "having" something means. In other words, it does not say whether it refers to being accessible and public, or whether it is usable, and so on.

As we previously noted, having path in the name is also somewhat redundant because it is clear that ::std::bar is a path.

Another small wrinkle is that it is unclear whether it should be have or has. That choice depends on what one things the subject is. For example, if one considers a module to be an "it", then it should probably be has.

One upside to has_path is that it has precedent from the clang compiler. For example, a user may write: #if __has_feature(cxx_rvalue_references) or __has_feature(c_generic_selections).

Another benefit is that has_ gives us the opportunity to introduce a family of has_path, has_feature, and has_$thing if we so wish.

#[cfg(version(..))

When it comes to version(..), it is needed to support conditional compilation of language features as opposed to library features as previously shown. Also, as we've seen, version(..) does not subsume accessible(..) but is rather a complementary mechanism.

One problem specific to version(..) is that it might get too rustc specific. It might be difficult for other Rust implementations than rustc to work with this version numbering as libraries will compile against rustcs release numbering. However, it is possible for other implementations to follow rustc in the numbering and what features it provides. This is probably not too unreasonable as we can expect rustc to be the reference implementation and that other ones will probably lag behind. Indeed, this is the experience with GHC and alternative Haskell compilers.

The bikeshed - Argument syntax

We have roughly two options with respect to how the version flag may be specified:

  1. version = "<semver>"
  2. version(<semver>)

The syntax in 2. is currently an error in #[cfg(..)] as you may witness with:

// error[E0565]: unsupported literal
#[cfg(abracadabra(1.27))] fn bar() {}
                  ^^^^

However, the attribute grammar is technically:

attribute  : "#" "!"? "[" path attr_inner? "]" ;
attr_inner : "[" token_stream "]"
           | "(" token_stream ")"
           | "{" token_stream "}"
           | "=" token_tree
           ;

Note in particular that #[my_attribute(<token_stream>)] is a legal production in the grammar wherefore we can support #[cfg(version(1.27.0))] if we so wish.

Given that syntax 2. is possible, we have decided to use it because as @eddyb has noted, the cfg flags that use the flag = ".." syntax are all static as opposed to dynamic. In other words, the semantics of cfg(x = "y") is that of checking for a membership test within a fixed set determined ahead of time. This set is also available through rustc --print=cfg.

What a user may infer from how other cfg(flag = "..") flags work is that version = ".." checks for an exact version. But as we've seen before, this interpretation is not the one in this RFC.

However, one reason to pick syntax 1. is that version(..) looks like a list.

The bikeshed - Attribute name

Naturally, there are other possible names for the flag. For example:

  • rustc_version
  • compiler_version
  • min_version

We pick the current naming because we believe it is sufficiently clear while also short and sweet. However, min_version is a good alternative to consider because it telegraphs the >= nature of the flag.

As for the <semver> syntax, it could also be adjusted such that you could write version(>= 1.27). We could also support exact version checking (==) as well as checking if the compiler is below a certain version (<=). There are also the "tilde requirements" and "wildcard requirements" that Cargo features that we could add. However, as a first iteration, version(1.27.0) is simple and covers most use cases.

version_check as an alternative

Using the crate version_check we may conditionally compile using a build.rs file. For example, the dbg crate does this:

// src/lib.rs:
// -----------------------------------------------------------------------------

#![cfg_attr(use_nightly, feature(core_intrinsics, specialization))]

// Deal with specialization:
// On nightly: typeof(expr) doesn't need to be Debug.
#[allow(dead_code)]
#[doc(hidden)]
pub struct WrapDebug<T>(pub T);
use std::fmt::{Debug, Formatter, Result};

#[cfg(use_nightly)]
impl<T> Debug for WrapDebug<T> {
    default fn fmt(&self, f: &mut Formatter) -> Result {
        use ::std::intrinsics::type_name;
        write!(f, "[<unknown> of type {} is !Debug]",
            unsafe { type_name::<T>() })
    }
}

...

// build.rs:
// -----------------------------------------------------------------------------

//!
//! This build script detects if we are nightly or not
//!

extern crate version_check;

fn main() {
    println!("cargo:rerun-if-changed=build.rs");
    if let Some(true) = version_check::is_nightly() {
        println!("cargo:rustc-cfg=use_nightly");
    }
}

The version_check crate also supports testing for a minimum version(..) with:

extern crate version_check;

if let Some((true, _)) = version_check::is_min_version("1.13.0") {
    println!("cargo:rustc-cfg=MIN_COMPILER_1_13");
}

However, this is quite verbose in comparison and requires you to invent ad-hoc and crate-specific names for your #[cfg(..)] flags such as MIN_COMPILER_1_13 that will not be the same for every crate. You will also need to repeat this per version you want to support. This causes the mechanism to scale poorly as compared to version(1.27) which we argue is simple and intuitive.

Conditional compilation on feature gates

An alternative to version(..) and accessible(..) is to allow users to query where a certain feature gate is stable or not. However, it has been argued that allowing this would essentially stabilize the names of the gates which we've historically not done.

We also argue that accessible(..) is more intuitive because it is more natural to think of a feature in terms of how you would make use of it (via its path) rather than the sometimes somewhat arbitrarily named feature gate.

Prior art

Crates

As previously mentioned, the version_check crate provides precedent for doing the desired conditional compilation in this RFC. There is also the rustc_version crate. Together, these crates have 18 + 67 direct reverse dependencies. This suggests that the feature is both desired and used.

Haskell

Using the Glasgow Haskell Compiler (GHC), it is possible to conditionally compile using it's provided preprocessor:

{-# LANGUAGE CPP #-}

module Main where

version :: String
#if __GLASGOW_HASKELL__ >= 706
version = "Version 7.0.6"
#else
version = "Below."
#endif

main :: IO ()
main = putStrLn version

Clang

The clang compiler gives you a suite of feature checking macros with which you can for example check whether a certain feature, extension, or attribute is supported. An example of this is:

#if __has_feature(cxx_rvalue_references)

// This code will only be compiled with the -std=c++11 and -std=gnu++11
// options, because rvalue references are only standardized in C++11.

#endif

This would be analogous to checking for the existence of a feature gate in Rust.

Clang also supports checking whether an include will succeed. For example, you may write:

#if __has_include("myinclude.h") && __has_include(<stdint.h>)
#include "myinclude.h"
#endif

This is similar in spirit to accessible($path).

Unresolved questions

The ability to have optional cargo dependencies is out of scope for this RFC.

  1. Is it technically feasible to implement accessible(..)? For example it could be hard if cfg-stripping runs before resolving things.

    @eddyb has indicated that:

    The good news is that we should be able to resolve that during macro expansion nowadays. The bad news is I don't know how hard early stability checking would be although, no, we should be able to easily add a DefId -> Option<Stability> method somewhere, with enough information to check against feature-gates (assuming the set of #![feature(...)]s in the local crate is known at cfg-stripping time).

  2. Should we allow referring to fields of type definitions in accessible(..)?

  3. In the reference-level-explanation, we note that:

    If and only if a Rust compiler considers itself to have a version which is greater or equal to the version in the semver string will the #[cfg(version(<semver>)] flag be considered active.

    However, it is currently not well specified what "considers itself" exactly means. To be more precise, if querying a mid-cycle nightly compiler with rustc --version results in rustc 1.29.0-nightly (31f1bc7b4 2018-07-15), but 1.29.0 has not been released on the stable channel, will then version(1.29.0) be active for this nightly or will it not?

    The reason this question matters is because on one 1.29.0-nightly compiler, a feature may not have been stabilized. Some days later, but before 1.29.0 hits a beta or stable compiler, a feature does get stabilized.

    To resolve this question, there are broadly 3 approaches:

    1. Answer the question in the affirmative. This entails that some breakage might sometimes occur when using a nightly compiler.

    2. Answer it in the negative by changing the date when the version constant is bumped in the compiler. That is, a version would only be bumped when releasing new stable or beta compilers and nightly compilers would always be versioned as the latest stable/beta. This also means that given #[stable(feature = "foobar", since = "1.42.0")] for some feature foobar, the feature would not be available first when the feature actually reaches stable/beta.

    3. As 2. but separate versions reported by rustc --version and to version(..). This would for example mean that if the last stable compiler is 1.42.0, then that would be used by version(..) while rustc --version would report 1.43.0-nightly. This approach could be technically achieved by for example maintaining one version constant that tracks the last stable/beta compiler as x.y.z and then --version would report x.(y + 1).0-nightly.

    Two arguments in favour of either 2. or 3. is that they would be more principled as we have not really stabilized something until it reaches stable or beta.

    We consider this unresolved question to be a matter of implementation detail which may be resolved during implementation.

Possible future work

#[cfg(rust_feature(..))]

One possible extension we might want to do in the future is to allow users to check whether a certain rustc feature gate is enabled or not. For example, we might write #[cfg(rustc_feature(generic_associated_types))] to check whether the GAT feature is supported in the compiler or not.

The main benefit of such an approach is that it is more direct than checking for a particular version. Also note that clang uses this approach as noted in the prior art.

However, there are some drawbacks as well:

  1. The names of feature gates are not always aptly named and usually do not follow a coherent naming system. As a frequent author of RFCs, the author of this one knows that they do not have a principled approach to naming RFCs. The feature name that is then used in the compiler is usually drawn directly from the RFC, so we would either need to accept the random naming of feature gates, or we would need to impose some system.

  2. Permitting dependence on the names of feature gates on stable would require us to be more principled with feature gates. For example, rustc, or any other Rust compiler, would be unable to remove gates or drastically change their implementations without changing their names. Being more principled could potentially add an undue burden on the library and compiler teams.

#[cfg(has_attr($attribute))]

One possible extension would be to introduce a has_attr(..) feature. has_attr would check if the specified attribute would be usable on the item the cfg (or cfg_attr) directive is attached to. For instance:

#[cfg_attr(have_attr(must_use), must_use)]
fn double(x: i32) -> i32 {
    2 * x
}

This would allow code to detect the availability of an attribute before using it, while not failing if the attribute did not exist.

Using has_attr in a cfg block may be useful for conditionally compiling code that only makes sense if a given attribute exists (e.g. global_allocator), while using has_attr in a cfg_attr block may be useful for adding an attribute to an item if supported but still support compilers that don't support that attribute.

As previously discussed, currently, the names of feature gates do not tend to appear in code targeting stable versions of Rust. Allowing code to detect the availability of specified feature gates by name would require committing to stable names for these features, and would require that those names refer to a fixed set of functionality. This would require additional curation. However, as attribute names already have to be standardized, has_attr(..) would not suffer the same problems wherefore it may be the better solution.

#[cfg(nightly)]

In a previous iteration of this RFC, a #[cfg(nightly)] flag was included. However, this flag has since been removed from the RFC. We may still add such a feature in the future if we wish. Therefore, we have outlined what nightly would have meant and some upsides and drawbacks to adding it.

Technical specification

To the cfg attribute, a nightly flag is added.

If and only if a Rust compiler permits a user to specify #![feature(..)] will the nightly flag be considered active.

Drawbacks: Combining nightly and accessible(..)

Consider that a popular library writes:

#![cfg_attr(nightly, feature(some_feature))]
#[cfg(accessible(::std::foo:SomeFeature))]
use std::foo::SomeFeature;

#[cfg(not(accessible(::std::foo:SomeFeature)))]
struct SomeFeature { ... }

One potential hazard when writing this migrating construct is that once SomeFeature finally gets stabilized, it may have been shipped in a modified form. Such modification may include changing the names of SomeFeature's methods, their type signatures, or what trait implementations exist for SomeFeature.

This problem only occurs when you combine nightly and accessible(..) or indeed nightly and version(..). However, there is a risk of breaking code that worked on one stable release of Rust in one or more versions after.

A few mitigating factors to consider are:

  • It is possible to check if the methods of SomeFeature are accessible or not by using their paths. This reduces the risk somewhat.

  • If a crate author runs continuous integration (CI) builds that include testing the crate on a nightly toolchain, breakage can be detected well before any crates are broken and a patch release of the crate can be made which either removes the nightly feature or adjusts the usage of it. The remaining problem is that dependent crates may have Cargo.lock files that have pinned the patch versions of the crate.

  • Users should take care not to use this mechanism unless they are fairly confident that no consequential changes will be made to the library. A risk still exists, but it is opt-in.

However, at the end, compared to feature = "unstable", which reverse dependencies may opt out of, nightly can't be opted out of (unless we add a mechanism to Cargo to perform such an override, but this would be anti-modular). This is the fundamental reason that for the time being, we have not included nightly in the proposal.

Upsides

One reason for the inclusion of #[cfg(nightly)] is that it is useful on its own to conditionally compile based on nightly/not as opposed to providing an unstable feature in Cargo.toml. An example of this is provided by the dbg crate which currently uses version_check to provide this automation.

Alternative #![if_possible_feature(<feature>)]

As an alternative to #[cfg_attr(nightly, feature(<feature>))] we could permit the user to write #![if_possible_feature(<feature>)]. The advantage of this is that it is quite direct with respect to intent. However, adding this in terms of nightly already has precedent in version_check. In addition, nightly also composes with other flags using any, not, and all.

This alternative also suffers from the problems previously noted.

Naming of the attribute

If this flag were to be proposed again, it would probably be proposed under a different name than nightly. Instead, a more apt name with respect to intent would be unstable_features.