Rollup merge of #145208 - joshtriplett:mbe-derive, r=petrochenkov

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.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 42ca2cb4175..3ed483db3c4 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1329,6 +1329,7 @@ symbols! {
         macro_attr,
         macro_attributes_in_derive_output,
         macro_concat,
+        macro_derive,
         macro_escape,
         macro_export,
         macro_lifetime_matcher,