Declarative macro_rules! derive macros

text/3698-declarative-derive-macros.md

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+- Feature Name: `declarative_derive_macros`
+- Start Date: 2024-09-20
+- RFC PR: [rust-lang/rfcs#3698](https://github.com/rust-lang/rfcs/pull/3698)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Support implementing `derive(Trait)` via a `macro_rules!` macro.
+
+# Motivation
+[motivation]: #motivation
+
+Many crates support deriving their traits with `derive(Trait)`. 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 derives 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.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+You can define a macro to implement `derive(MyTrait)` by defining a
+`macro_rules!` macro with one or more `derive()` rules. Such a macro can create
+new items based on a struct, enum, or union. Note that the macro can only
+append new items; it cannot modify the item it was applied to.
+
+For example:
+
+```rust
+trait Answer { fn answer(&self) -> u32; }
+
+macro_rules! Answer {
+    // Simplified for this example
+    derive() (struct $n:ident $_:tt) => {
+        impl Answer for $n {
+            fn answer(&self) -> u32 { 42 }
+        }
+    };
+}
+
+#[derive(Answer)]
+struct Struct;
+
+fn main() {
+    let s = Struct;
+    assert_eq!(42, s.answer());
+}
+```
+
+Derive 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.
+
+A derive macro may share the same path as a trait of the same name. For
+instance, the name `mycrate::MyTrait` can refer to both the `MyTrait` trait and
+the macro for `derive(MyTrait)`. This is consistent with existing derive
+macros.
+
+If a derive macro emits a trait impl for the type, it may want to add the
+[`#[automatically_derived]`](https://doc.rust-lang.org/reference/attributes/derive.html#the-automatically_derived-attribute)
+attribute, for the benefit of diagnostics.
+
+If a derive macro mistakenly emits the token stream it was applied to
+(resulting in a duplicate item definition), the error the compiler emits for
+the duplicate item should hint to the user that the macro was defined
+incorrectly, and remind the user that derive macros only append new items.
+
+A `derive()` rule can be marked as `unsafe`: `unsafe derive() (...)
+=> { ... }`. Invoking such a derive using a rule marked as unsafe
+requires unsafe derive syntax: either
+`#[unsafe(derive(DangerousTrait))]` or
+`#[derive(unsafe(DangerousTrait))]`. (The latter syntax allows
+isolating the `unsafe` to a single derive within a list of
+derives.) Invoking an unsafe derive rule without the unsafe derive
+syntax will produce a compiler error. Using the unsafe derive
+syntax without an unsafe derive will trigger an "unused unsafe"
+lint. (RFC 3715 defines the equivalent mechanism for proc macro
+derives.)
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The grammar for macros is extended as follows:
+
+> _MacroRule_ :\
+> &nbsp;&nbsp; ( `unsafe`<sup>?</sup> `derive` `(` `)` )<sup>?</sup>  _MacroMatcher_ `=>` _MacroTranscriber_
+
+The _MacroMatcher_ matches the entire construct the attribute was
+applied to, receiving precisely what a proc-macro-based attribute
+would in the same place.
+
+(The empty parentheses after `derive` reserve future syntax space
+for derives accepting arguments, at which time they'll be replaced
+by a second _MacroMatcher_ that matches the arguments.)
+
+A derive invocation that uses an `unsafe derive` rule will produce
+an error if invoked without using the `unsafe` derive syntax. A
+derive invocation that uses an `derive` rule (without `unsafe`)
+will trigger the "unused unsafe" lint if invoked using the `unsafe`
+derive syntax. A single derive macro may have both `derive` and
+`unsafe derive` 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 `derive` or `unsafe`.
+
+Adding `derive` rules to an existing macro is a semver-compatible change,
+though in practice, it will likely be uncommon.
+
+If a user invokes a macro as a derive and that macro does not have any `derive`
+rules, the compiler should give a clear error stating that the macro is not
+usable as a derive because it does not have any `derive` 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. We can and should attempt
+to avert and such pressure, such as by providing a post with guidance that
+crate maintainers can link to when responding to such requests.
+
+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 derive 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! { ... }`, rather than using
+derives. 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 derive. In addition, this would
+not preserve the property derive macros normally have that they cannot change
+the item they are applied to.
+
+A mechanism to define attribute macros would let people write attributes like
+`#[derive_mytrait]`, but that would not provide compatibility with existing
+derive syntax.
+
+We could allow `macro_rules!` derive macros to emit a replacement token stream.
+That would be inconsistent with the restriction preventing proc macros from
+doing the same, but it would give macros more capabilities, and simplify some
+use cases. Notably, that would make it easy for derive macros to re-emit a
+structure with another `derive` attached to it.
+
+We could allow directly invoking a `macro_rules!` derive macro as a
+function-like macro. This has the potential for confusion, given the
+append-only nature of derive macros versus the behavior of normal function-like
+macros. It might potentially be useful for code reuse, however.
+
+## Syntax alternatives
+
+Rather than using `derive()` rules, we could have `macro_rules!` macros use a
+`#[macro_derive]` attribute, similar to the `#[proc_macro_derive]` attribute
+used for proc macros.
+
+However, this would be inconsistent with `attr()` rules as defined in RFC 3697.
+This would also make it harder to add parameterized derives in the future (e.g.
+`derive(MyTrait(params))`).
+
+# Prior art
+[prior-art]: #prior-art
+
+We have had proc-macro-based derive 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 derives,
+demonstrating a demand for this. This feature would allow defining such derives
+without requiring proc macros at all, and would support the same invocation
+syntax as a proc macro.
+
+Some crates in the ecosystem already implement the equivalent of derives using
+declarative macros; for instance, see
+[merde](https://github.com/bearcove/merde).
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Before stabilizing this feature, we should ensure there's a mechanism macros
+can use to ensure that an error when producing an impl does not result in a
+cascade of additional errors caused by a missing impl. This may take the form
+of a fallback impl, for instance.
+
+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.
+
+Before stabilizing this feature, we should have clear public guidance
+recommending against pressuring crate maintainers to adopt this feature
+rapidly, and encourage crate maintainers to link to that guidance if such
+requests arise.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+We should provide a way for derive macros to declare themselves `unsafe` to
+invoke, requiring an unsafe attribute syntax to invoke.
+
+We should provide a way for derive macros to invoke other derive macros.
+
+We should provide a means to perform a `derive` on a struct without being
+directly attached to that struct. (This would also potentially require
+something like a compile-time reflection mechanism.)
+
+We could support passing parameters to derive macros (e.g.
+`#[derive(Trait(params), OtherTrait(other, params))]`). This may benefit from
+having `derive(...)` rules inside the `macro_rules!` macro declaration, similar
+to the `attr(...)` rules proposed in RFC 3697.
+
+In the future, if we support something like `const Trait` or similar trait
+modifiers, we'll want to support `derive(const Trait)`, and define how a
+`macro_rules!` derive handles that.
+
+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 support error recovery, so that a derive can produce an error
+but still provide enough for the remainder of the compilation to proceed far
+enough to usefully report further errors.
+
+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.
+
+We could provide a macro matcher to match an entire struct field, along with
+syntax (based on macro metavariable expressions) to extract the field name or
+type (e.g. `${f.name}`). This would simplify many common cases by leveraging
+the compiler's own parser.
+
+We could do the same for various other high-level constructs.
+
+We may want to provide simple helpers for generating/propagating `where`
+bounds, which would otherwise be complex to do in a `macro_rules!` macro.
+
+We may want to add a lint for macro names, encouraging macros with derive rules
+to use `CamelCase` names, and encouraging macros without derive rules to use
+`snake_case` names.
+
+## Helper attribute namespacing and hygiene
+
+We should provide a way for derive macros to define helper attributes ([inert
+attributes](https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes)
+that exist for the derive macro to parse and act upon). Such attributes are
+supported by proc macro derives; however, such attributes have no namespacing,
+and thus currently represent compatibility hazards because they can conflict.
+We should provide a namespaced, hygienic mechanism for defining and using
+helper attributes.
+
+For instance, could we have `pub macro_helper_attr! skip` in the standard
+library, namespaced under `core::derives` or similar? Could we let macros parse
+that in a way that matches it in a namespaced fashion, so that:
+- If you write `#[core::derives::skip]`, the macro matches it
+- If you `use core::derives::skip;` and `write #[skip]`, the macro matches it
+- If you `use elsewhere::skip` (or no import at all) and write `#[skip]`, the
+  macro *doesn't* match it.
+
+We already have *some* interaction between macros and name resolution, in order
+to have namespaced `macro_rules!` macros. Would something like this be feasible?
+
+(We would still need to specify the exact mechanism by which macros match these
+helper attributes.)