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Implement declarative (`macro_rules!`) derive macros (RFC 3698) This is a draft for review, and should not be merged yet. This is layered atop https://github.com/rust-lang/rust/pull/145153 , and has only two additional commits atop that. The first handles parsing and provides a test for various parse errors. The second implements expansion and handles application. This implements RFC 3698, "Declarative (`macro_rules!`) derive macros". Tracking issue: https://github.com/rust-lang/rust/issues/143549 This has one remaining issue, which I could use some help debugging: in `tests/ui/macros/macro-rules-derive-error.rs`, the diagnostics for `derive(fn_only)` (for a `fn_only` with no `derive` rules) and `derive(ForwardReferencedDerive)` both get emitted twice, as a duplicate diagnostic. From what I can tell via adding some debugging code, `unresolved_macro_suggestions` is getting called twice from `finalize_macro_resolutions` for each of them, because `self.single_segment_macro_resolutions` has two entries for the macro, with two different `parent_scope` values. I'm not clear on why that happened; it doesn't happen with the equivalent code using attrs. I'd welcome any suggestions for fixing this.