Summary

The #[cfg(...)] attribute provides a mechanism for conditional compilation of items in a Rust crate. This RFC proposes to change the syntax of #[cfg] to make more sense as well as enable expansion of the conditional compilation system to attributes while maintaining a single syntax.

Motivation

In the current implementation, #[cfg(...)] takes a comma separated list of key, key = "value", not(key), or not(key = "value"). An individual #[cfg(...)] attribute "matches" if all of the contained cfg patterns match the compilation environment, and an item preserved if it either has no #[cfg(...)] attributes or any of the #[cfg(...)] attributes present match.

This is problematic for several reasons:

It is excessively verbose in certain situations. For example, implementing the equivalent of (a AND (b OR c OR d)) requires three separate attributes and a to be duplicated in each.
It differs from all other attributes in that all #[cfg(...)] attributes on an item must be processed together instead of in isolation. This change will move #[cfg(...)] closer to implementation as a normal syntax extension.

Detailed design

The  inside of #[cfg()] will be called a cfg pattern and have a simple recursive syntax:

key is a cfg pattern and will match if key is present in the compilation environment.
key = "value" is a cfg pattern and will match if a mapping from key to value is present in the compilation environment. At present, key-value pairs only exist for compiler defined keys such as target_os and endian.
not() is a cfg pattern if  is and matches if  does not match.
all(, ...) is a cfg pattern if all of the comma-separated s are cfg patterns and all of them match.
any(, ...) is a cfg pattern if all of the comma-separated s are cfg patterns and any of them match.

If an item is tagged with #[cfg()], that item will be stripped from the AST if the cfg pattern  does not match.

One implementation hazard is that the semantics of

#[cfg(a)]
#[cfg(b)]
fn foo() {}

will change from "include foo if either of a and b are present in the compilation environment" to "include foo if both of a and b are present in the compilation environment". To ease the transition, the old semantics of multiple #[cfg(...)] attributes will be maintained as a special case, with a warning. After some reasonable period of time, the special case will be removed.

In addition, #[cfg(a, b, c)] will be accepted with a warning and be equivalent to #[cfg(all(a, b, c))]. Again, after some reasonable period of time, this behavior will be removed as well.

The cfg!() syntax extension will be modified to accept cfg patterns as well. A #[cfg_attr(, <attr>)] syntax extension will be added (PR 16230) which will expand to #[<attr>] if the cfg pattern  matches. The test harness's #[ignore] attribute will have its built-in cfg filtering functionality stripped in favor of #[cfg_attr(, ignore)].

Drawbacks

While the implementation of this change in the compiler will be straightforward, the effects on downstream code will be significant, especially in the standard library.

Alternatives

all and any could be renamed to and and or, though I feel that the proposed names read better with the function-like syntax and are consistent with Iterator::all and Iterator::any.

Issue #2119 proposed the addition of || and && operators and parentheses to the attribute syntax to result in something like #[cfg(a || (b && c)]. I don't favor this proposal since it would result in a major change to the attribute syntax for relatively little readability gain.

Unresolved questions

How long should multiple #[cfg(...)] attributes on a single item be forbidden? It should probably be at least until after 0.12 releases.

Should we permanently keep the behavior of treating #[cfg(a, b)] as #[cfg(all(a, b))]? It is the common case, and adding this interpretation can reduce the noise level a bit. On the other hand, it may be a bit confusing to read as it's not immediately clear if it will be processed as and(..) or all(..).

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