Declarative macro_rules! attribute macros

text/3697-declarative-attribute-macros.md

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+- Feature Name: `declarative_attribute_macros`
+- Start Date: 2024-09-20
+- RFC PR: [rust-lang/rfcs#3697](https://github.com/rust-lang/rfcs/pull/3697)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Support defining `macro_rules!` macros that work as attribute macros.
+
+# Motivation
+[motivation]: #motivation
+
+Many crates provide attribute macros. Today, this requires defining proc
+macros, in a separate crate, typically with several additional dependencies
+adding substantial compilation time, and typically guarded by a feature that
+users need to remember to enable.
+
+However, many common cases of attribute macros don't require any more power
+than an ordinary `macro_rules!` macro. Supporting these common cases would
+allow many crates to avoid defining proc macros, reduce dependencies and
+compilation time, and provide these macros unconditionally without requiring
+the user to enable a feature.
+
+The [`macro_rules_attribute`](https://crates.io/crates/macro_rules_attribute)
+crate defines proc macros that allow invoking declarative macros as attributes,
+demonstrating a demand for this. This feature would allow defining such
+attributes without requiring proc macros at all, and would support the same
+invocation syntax as a proc macro.
+
+Some macros in the ecosystem already implement the equivalent of attribute
+using declarative macros; for instance, see
+[smol-macros](https://crates.io/crates/smol-macros), which provides a `main!`
+macro and recommends using it with `macro_rules_attribute::apply`.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+When defining a `macro_rules!` macro, you can prefix some of the macro's rules
+with `attr(...)` to allow using the macro as an attribute. The
+arguments to the attribute, if any, are parsed by the *MacroMatcher* in the
+first set of parentheses; the second *MacroMatcher* parses the entire construct
+the attribute was applied to. The resulting macro will work anywhere an
+attribute currently works.
+
+```rust
+macro_rules! main {
+    attr() ($func:item) => { make_async_main!($func) };
+    attr(threads = $threads:literal) ($func:item) => { make_async_main!($threads, $func) };
+}
+
+#[main]
+async fn main() { ... }
+
+#[main(threads = 42)]
+async fn main() { ... }
+```
+
+Attribute macros defined using `macro_rules!` follow the same scoping rules as
+any other macro, and may be invoked by any path that resolves to them.
+
+An attribute macro must not require itself for resolution, either directly or
+indirectly (e.g. applied to a containing module or item).
+
+Note that a single macro can have both `attr` and non-`attr` rules. Attribute
+invocations can only match the `attr` rules, and non-attribute invocations can
+only match the non-`attr` rules. This allows adding `attr` rules to an existing
+macro without breaking backwards compatibility.
+
+An attribute macro may emit code containing another attribute, including one
+provided by an attribute macro. An attribute macro may use this to recursively
+invoke itself.
+
+An `attr` rule may be prefixed with `unsafe`. Invoking an attribute macro in a
+way that makes use of a rule declared with `unsafe attr` requires the unsafe
+attribute syntax `#[unsafe(attribute_name)]`.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The grammar for macros is extended as follows:
+
+> _MacroRule_ :\
+> &nbsp;&nbsp; ( `unsafe`<sup>?</sup> `attr` _MacroMatcher_ )<sup>?</sup>  _MacroMatcher_ `=>` _MacroTranscriber_
+
+The first _MacroMatcher_ matches the attribute's arguments, which will be an
+empty token tree if either not present (`#[myattr]`) or empty (`#[myattr()]`).
+The second _MacroMatcher_ matches the entire construct the attribute was
+applied to, receiving precisely what a proc-macro-based attribute would in the
+same place.
+
+Only a rule matching both the arguments to the attribute and the construct the
+attribute was applied to will apply. Note that the captures in both
+`MacroMatcher`s share the same namespace; attempting to use the same name for
+two captures will give a "duplicate matcher binding" error.
+
+An attribute macro invocation that uses an `unsafe attr` rule will produce an
+error if invoked without using the `unsafe` attribute syntax. An attribute
+macro invocation that uses an `attr` rule will trigger the "unused unsafe" lint
+if invoked using the `unsafe` attribute syntax. A single attribute macro may
+have both `attr` and `unsafe attr` rules, such as if only some invocations are
+unsafe.
+
+This grammar addition is backwards compatible: previously, a _MacroRule_ could
+only start with `(`, `[`, or `{`, so the parser can easily distinguish rules
+that start with `attr` or `unsafe`.
+
+Attribute macros declared using `macro_rules!` are
+[active](https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes),
+just like those declared using proc macros.
+
+Adding `attr` rules to an existing macro is a semver-compatible change.
+
+If a user invokes a macro as an attribute and that macro does not have any
+`attr` rules, the compiler should give a clear error stating that the macro is
+not usable as an attribute because it does not have any `attr` rules.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This feature will not be sufficient for *all* uses of proc macros in the
+ecosystem, and its existence may create social pressure for crate maintainers
+to switch even if the result is harder to maintain.
+
+Before stabilizing this feature, we should receive feedback from crate
+maintainers, and potentially make further improvements to `macro_rules` to make
+it easier to use for their use cases. This feature will provide motivation to
+evaluate many new use cases that previously weren't written using
+`macro_rules`, and we should consider quality-of-life improvements to better
+support those use cases.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Adding this feature will allow many crates in the ecosystem to drop their proc
+macro crates and corresponding dependencies, and decrease their build times.
+
+This will also give attribute macros access to the `$crate` mechanism to refer
+to the defining crate, which is simpler than mechanisms currently used in proc
+macros to achieve the same goal.
+
+Macros defined this way can more easily support caching, as they cannot depend
+on arbitrary unspecified inputs.
+
+Crates could instead define `macro_rules!` macros and encourage users to invoke
+them using existing syntax like `macroname! { ... }`. This would provide the
+same functionality, but would not support the same syntax people are accustomed
+to, and could not maintain semver compatibility with an existing
+proc-macro-based attribute.
+
+We could require the `!` in attribute macros (`#[myattr!]` or similar).
+However, proc-macro-based attribute macros do not require this, and this would
+not allow declarative attribute macros to be fully compatible with
+proc-macro-based attribute macros.
+
+Many macros will want to parse their arguments and separately parse the
+construct they're applied to, rather than a combinatorial explosion of both.
+This problem is not unique to attribute macros. In both cases, the standard
+solution is to parse one while carrying along the other.
+
+We could leave out support for writing a function-like macro and an attribute
+macro with the same name. However, this would prevent crates from preserving
+backwards compatibility when adding attribute support to an existing
+function-like macro.
+
+Instead of or in addition to marking the individual rules, we could mark the
+whole macro with `#[attribute_macro]` or similar, and allow having an attribute
+macro and a non-attribute macro with the same name.
+
+We could include another `=>` or other syntax between the first and second
+macro matchers.
+
+We could use `attribute` rather than `attr`. Rust usually avoids abbreviating
+except for the most common constructs; however, `cfg_attr` provides precedent
+for this abbreviation, and `attr` appears repeatedly in multiple rules which
+motivates abbreviating it.
+
+# Prior art
+[prior-art]: #prior-art
+
+We have had proc-macro-based attribute macros for a long time, and the
+ecosystem makes extensive use of them.
+
+The [`macro_rules_attribute`](https://crates.io/crates/macro_rules_attribute)
+crate defines proc macros that allow invoking declarative macros as attributes,
+demonstrating a demand for this. This feature would allow defining such
+attributes without requiring proc macros at all, and would support the same
+invocation syntax as a proc macro.
+
+Some macros in the ecosystem already implement the equivalent of attribute
+using declarative macros; for instance, see
+[smol-macros](https://crates.io/crates/smol-macros), which provides a `main!`
+macro and recommends using it with `macro_rules_attribute::apply`.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Is an attribute macro allowed to recursively invoke itself by emitting the
+attribute in its output? If there is no technical issue with allowing this, then
+we should do so, to allow simple recursion (e.g. handling defaults by invoking
+the same rule as if they were explicitly specified).
+
+Are there any places where we currently allow an attribute, but where
+implementation considerations make it difficult to allow a `macro_rules`
+attribute? (For instance, places where we currently allow attributes but don't
+allow proc-macro attributes.)
+
+Before stabilizing this feature, we should make sure it doesn't produce wildly
+worse error messages in common cases.
+
+Before stabilizing this feature, we should receive feedback from crate
+maintainers, and potentially make further improvements to `macro_rules` to make
+it easier to use for their use cases. This feature will provide motivation to
+evaluate many new use cases that previously weren't written using
+`macro_rules`, and we should consider quality-of-life improvements to better
+support those use cases.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+We should provide a way to define `derive` macros declaratively, as well.
+
+We should provide a way for `macro_rules!` macros to provide better error
+reporting, with spans, rather than just pointing to the macro.
+
+We may want to provide more fine-grained control over the requirement for
+`unsafe`, to make it easier for attribute macros to be safe in some
+circumstances and unsafe in others (e.g. unsafe only if a given parameter is
+provided).
+
+As people test this feature and run into limitations of `macro_rules!` parsing,
+we should consider additional features to make this easier to use for various
+use cases.
+
+Some use cases involve multiple attribute macros that users expect to be able
+to apply in any order. For instance, `#[test]` and `#[should_panic]` can appear
+on the same function in any order. Implementing that via this mechanism for
+attribute macros would require making both of those attributes into macros that
+both do all the parsing regardless of which got invoked first, likely by
+invoking a common helper. We should consider if we consider that mechanism
+sufficient, or if we should provide another mechanism for a set of related
+attribute macros to appear in any order.
+
+If it turns out many users of attribute macros want to emit new tokens but
+leave the tokens they were applied to unmodified, we may want to have a
+convenient mechanism for that.