Summary

This RFC proposes a temporary solution to the problem of letting tools use attributes. We outline a (partial) long-term solution and propose a step towards that solution for tools which are part of the Rust distribution.

The long-term solution is that a crate can use attributes for a specific tool by using some explicit (but unspecified) opt-in mechanism. The tool name then becomes the root of a module hierarchy for attribute naming. E.g., by opting-in to a tool named my_tool, a crate can use #[my_tool::foo] and #[my_tool::bar(42)], etc.

This RFC is a special case of the long-term solution: any tool distributed with Rust creates a scope for attributes (without any opt-in). So any crate can use #[rustdoc::hidden] or #[rustfmt::skip].

E.g.,

#[rustfmt::skip]
fn foo() {}

This would be allowed by the compiler but ignored. When Rustfmt is run on the crate, it will read the attibute and skip formatting foo (note that we make no provision for reading the attribute or doing anything with it, that is all up to the tool).

This RFC proposes a second mechanism for scoping lints for tools. Similar to attributes, we propose a subset of a hypothetical long-term solution.

This RFC supersedes #1755.

Motivation

Attributes are a useful, general-purpose mechanism for annotating code with metadata. They are used in the language (e.g., repr), for macros (e.g., derive, and for user-supplied attribute- like macros), and by tools (e.g., rustfmt_skip which instructs Rustfmt not to format an item). Attributes could also be used by compiler plugins such as lints.

Currently, custom attributes (i.e., those not known to the compiler, e.g., rustfmt_skip) are unstable. There is a future compatibility hazard with custom attributes: if we add #[foo] to the language, then any users using a foo custom attribute will suffer breakage.

There is a potential problem with the interaction between custom attributes and attribute-like macros. Given an attribute, the compiler cannot tell if the attribute is intended to be a macro invocation or an attribute that might only be used by a tool (either outside or inside the compiler). Currently, the compiler tries to find a macro and if it cannot, ignores the attribute (giving a stability error if not on nightly or the custom_attribute feature is not enabled). However, if the user intended the attribute to be a macro, silently ignoring the missing macro error is not the right thing to do. The compiler needs to know whether an attribute is intended to be a macro or not.

Given the above constraints, an opt-in solution is attractive. However, any such solution ends up being closely related to mechanisms for importing crates (extern crate) and macro naming. These features are being re-examined or are unstable and so now is a bad time to fully specify a long-term solution.

We do wish to make progress on allowing tools to use attributes. For example, Rustfmt is mostly ready to move towards stabilisation, but requires some kind of skip attribute. So we are proposing a solution that should work well with any reasonable long-term solution and addresses the needs of some important tools today.

Similarly, tools (e.g., Clippy) may want to use their own lints without the compiler warning about unused lints. E.g., we want a user to be able to write #![allow(clippy::some_lint)] in their crate without warning.

Guide-level explanation

Attributes

This section assumes that attributes (e.g., #[test]) have already been taught.

You can use attibutes in your crate to pass information to tools. For now, this facility is limited to the tools we include with the Rust distribution.

The names of these attributes are a path starting with the name of a tool, and then one or more identifiers, e.g., #[tool_name::foo] or #[tool_name::bar::baz::qux(argument)]. Such paths hide any attribute-like macros with the same name and location.

For example, using #[rustfmt::skip] indicates that an item (such as a function) should not be formatted by Rustfmt:

#[rustfmt::skip]
fn foo() { this_will_be_kept_as_is_by_rustfmt(); }

fn bar() { this_will_be_reformatted }

mod baz {
    #![rustfmt::skip]
    // Rustfmt will skip this whole module.
}

Lints

This section assumes lints have already been taught.

Lints can be defined hierarchically as a path, as well as just a single name. For example, nonstandard_style::non_snake_case_functions and nonstandard_style::uppercase_variables. Note this RFC is not proposing changing any existing lints, just extending the current lint naming system. Lint names cannot be imported using use.

Lints can be enforced by tools other than the compiler. For example, Clippy provides a large suite of lints to catch common mistakes and improve your Rust code. Lints for tools are prefixed with the tool name, e.g., clippy::box_vec.

Reference-level explanation

Long-term solution

There will be some opt-in mechanism for crates to declare that they want to allow use of a tool's attributes. This might be in the source text (an attribute as in #1755 or new syntax, e.g., extern attribute foo;) or passed to rustc as a command line flag (e.g., --extern-attr foo). The exact mechanism is deliberately unspecifed.

After opting-in to foo, a crate can use foo as the base of a path in any attribute in the crate. E.g., allowing #[foo::bar] to be used (but not #[foo]). This mechanism is follows the normal macro hygiene rules. Depending on the opt-in mechanism a tool might be able to specify to the compiler which paths are valid, e.g., allow #[foo::bar] but disallow #[foo::baz]. I would hope that we'd be able to reuse most of the macro naming feature (see #1561) here (i.e., this won't be a whole new specification, we'll just allow a new way to base paths).

Unscoped attributes will be reserved for the language and can't be used by tools.

During macro expansion, when faced with an attribute, the compiler first tries to find a macro using the macro name resolution rules. The compiler then checks if the attribute matches any of the declared or built- in attributes. If this fails, then it reports a macro not found error. The compiler may suggest mis-typed attributes (declared or built-in).

A similar opt-in mechanism will exist for lints.

Proposed for immediate implementation

There is an attribute path white list of the names of tools shipped with the Rust distribution. Any crate can use an attibute path starting with those names and the attribute will not trigger the custom attribute lint or require a macro feature gate.

E.g., #[rustdoc::foo] will be permitted in stable Rust code; #[rustdoc] will still be treated as a custom attribute.

The initial list of allowed prefixes is rustc, rustdoc, and rls (but see note below on activation). As tools are added to the distribution, they will be allowed as path prefixes in attributes. We expect to add rustfmt and clippy in the near future. Note that whether one of these names can be used does not depend on whether the relevant component is installed on the user's system; this is a simple, universal white list.

Given the earlier rules on name resolution, these attributes would shadow any attribute macro with the same name. This is not problematic because a macro would have to be in a module starting with a tool name (e.g., rustdoc::foo), naming macros in such a way is currently unstable, and this can be worked around by using an import (use).

Tool-scoped attributes should be preserved by the compiler for as long as possible through compilation. This allows tools which plug into the compiler (like Clippy) to observe these attributes on items during type checking, etc.

Likewise, white-listed tools may be used as a prefix for lints. So for example, rustfmt::foo and clippy::bar are both valid lint names, from the compiler's perspective.

Activation and unused attibutes/lints

For each name on the whitelist, it is indicated if the name is active for attributes or lints. A name is only activated if required. So for example, rustdoc will not be activated at all until it takes advantage of this feature. I expect clippy will be activated only for lints and attributes, and rustfmt only for attributes.

A tool that has an active name must check for unused lints/attibutes. For example, if rustfmt becomes active for attributes, and only recognises rustfmt::skip, it must produce a warning if a user uses rustfmt::foo in their code.

These two requirements together mean that we do not lose checking of unused attributes/lints in any circumstance and we can move to having the compiler check for unused attributes/lints as part of a possible long-term solution without introducing new warnings or errors.

Forward and backward compatability

Since custom attributes are feature gated and scoped attributes are part of the unstable macros 2.0 work, there is no backwards compatability issue.

For tools who want to move to these newly stable attributes (e.g., from rustfmt_skip to rustfmt::skip) they will have to manage the change themselves.

Although the mechanism for opt-in for the long-term solution is unspecified, the actual usage of tool attributes seems pretty clear. Therefore we can be reasonably confident that this proposal is forward-compatible in its syntax, etc.

For the white-listed tools, will their names be implicitly imported in the long- term solution? One could imagine either leaving them implicit (similar to the libraries prelude) or using warning cycles or an epoch to move them to explicit opt-in.

Drawbacks

The proposed scheme does not allow tools or macros to use custom top-level attributes (I consider this a feature, not a bug, but others may differ).

Some tools are clearly given special treatment.

We permit some useless attributes without warning from the compiler (e.g., #[rustfmt::foo], assuming Rustfmt does nothing with foo). However, tools should warn or error on such attributes.

We are not planning any infrastructure to help tools use these attributes. That seems fine for now, I imagine a long-term solution should include some library or API for this.

No interaction with imports or other parts of the module system.

Alternatives

We could continue to force tools to rely on cfg_attr - this is very unergonomic, e.g., #[cfg_attr(rustfmt, rustfmt_skip)].

We could allow all scoped attributes without checks. This feels like it introduces too much scope for error.

Unresolved questions

Are there other tools that should be included on the whitelist (#[test] perhaps)?

Should we try and move some top-level attributes that are compiler-specific (rather than language-specific) to use #[rustc::]? (E.g., crate_type).

How should the compiler expose path lints to lint plugins/lint tools?

RFC 2126 may change how paths are written, the paths used in attributes in this RFC should be adjusted accordingly.