Unsafe derives and attributes

text/0000-unsafe-derives-and-attrs.md

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+- Feature Name: `unsafe_derives_and_attrs`
+- Start Date: 2024-10-22
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Allow declaring proc macro attributes and derive macros as unsafe, and
+requiring `unsafe` to invoke them.
+
+# Motivation
+[motivation]: #motivation
+
+Some traits place requirements on implementations that the Rust compiler cannot
+verify. Those traits can mark themselves as unsafe, requiring `unsafe impl`
+syntax to implement. However, trait `derive` macros cannot currently require
+`unsafe`. This RFC defines a syntax for declaring and using unsafe `derive`
+macros.
+
+This RFC also defines a syntax for declaring proc macro attributes as unsafe.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+## Derives
+
+When declaring a proc macro `derive`, you can add the `unsafe` parameter to the
+`proc_macro_derive` attribute to indicate that the derive requires `unsafe`:
+
+```rust
+#[proc_macro_derive(DangerousTrait, unsafe)]
+pub fn derive_helper_attr(_item: TokenStream) -> TokenStream {
+    TokenStream::new()
+}
+```
+
+Invoking this derive requires writing 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 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.
+
+A `proc_macro_derive` attribute can include both `attributes` for helper
+attributes and `unsafe` to declare the derive unsafe, in any order.
+
+## Attributes
+
+When declaring a proc macro attribute, you can add the `unsafe` parameter to
+the `proc_macro_attribute` attribute to indicate that the attribute requires
+`unsafe`:
+
+```rust
+#[proc_macro_attribute]
+pub fn dangerous(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    item
+}
+```
+
+Invoking an unsafe attribute requires the unsafe attribute syntax:
+`#[unsafe(dangerous)]`.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Should we support the `#[unsafe(derive(DangerousTrait))]` syntax, or only
+`#[derive(unsafe(DangerousTrait))]`? The former elevates the `unsafe` to be
+more visible, and allows deriving several traits using one `unsafe`. The latter
+isolates the `unsafe` to a specific trait. This RFC proposes supporting both,
+but we could choose to only support the latter instead.
+
+# Prior art
+[prior-art]: #prior-art
+
+RFC 3325 defined unsafe attributes. This RFC provides a natural extension of
+that mechanism to derives.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+When we add support for `macro_rules!`-based attributes and derives, we should
+provide a means for such attributes and derives to declare themselves unsafe as
+well.
+
+We could provide a syntax to declare specific helper attributes of a derive as
+unsafe, without declaring the entire derive unsafe.