mbe: Parse macro derive rules

This handles various kinds of errors, but does not allow applying the
derive yet.

This adds the feature gate `macro_derive`.

Author: joshtriplett

compiler/rustc_expand/messages.ftl

@@ -70,7 +70,7 @@ expand_invalid_fragment_specifier =
     invalid fragment specifier `{$fragment}`
     .help = {$help}
 
-expand_macro_args_bad_delim = macro attribute argument matchers require parentheses
+expand_macro_args_bad_delim = macro `{$rule_kw}` argument matchers require parentheses
 expand_macro_args_bad_delim_sugg = the delimiters should be `(` and `)`
 
 expand_macro_body_stability =

compiler/rustc_expand/src/errors.rs

@@ -490,6 +490,7 @@ pub(crate) struct MacroArgsBadDelim {
     pub span: Span,
     #[subdiagnostic]
     pub sugg: MacroArgsBadDelimSugg,
+    pub rule_kw: Symbol,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -26,7 +26,7 @@ use rustc_session::Session;
 use rustc_session::parse::{ParseSess, feature_err};
 use rustc_span::edition::Edition;
 use rustc_span::hygiene::{MacroKinds, Transparency};
-use rustc_span::{Ident, Span, kw, sym};
+use rustc_span::{Ident, Span, Symbol, kw, sym};
 use tracing::{debug, instrument, trace, trace_span};
 
 use super::diagnostics::failed_to_match_macro;
@@ -137,6 +137,9 @@ pub(super) enum MacroRule {
         body_span: Span,
         rhs: mbe::TokenTree,
     },
+    /// A derive rule, for use with `#[m]`
+    #[expect(unused)]
+    Derive { body: Vec<MatcherLoc>, body_span: Span, rhs: mbe::TokenTree },
 }
 
 pub struct MacroRulesMacroExpander {
@@ -156,6 +159,7 @@ impl MacroRulesMacroExpander {
             MacroRule::Attr { args_span, body_span, ref rhs, .. } => {
                 (MultiSpan::from_spans(vec![args_span, body_span]), rhs)
             }
+            MacroRule::Derive { body_span, ref rhs, .. } => (MultiSpan::from_span(body_span), rhs),
         };
         if has_compile_error_macro(rhs) { None } else { Some((&self.name, span)) }
     }
@@ -568,7 +572,7 @@ pub fn compile_declarative_macro(
     let mut rules = Vec::new();
 
     while p.token != token::Eof {
-        let args = if p.eat_keyword_noexpect(sym::attr) {
+        let (args, is_derive) = if p.eat_keyword_noexpect(sym::attr) {
             kinds |= MacroKinds::ATTR;
             if !features.macro_attr() {
                 feature_err(sess, sym::macro_attr, span, "`macro_rules!` attributes are unstable")
@@ -578,16 +582,46 @@ pub fn compile_declarative_macro(
                 return dummy_syn_ext(guar);
             }
             let args = p.parse_token_tree();
-            check_args_parens(sess, &args);
+            check_args_parens(sess, sym::attr, &args);
             let args = parse_one_tt(args, RulePart::Pattern, sess, node_id, features, edition);
             check_emission(check_lhs(sess, node_id, &args));
             if let Some(guar) = check_no_eof(sess, &p, "expected macro attr body") {
                 return dummy_syn_ext(guar);
             }
-            Some(args)
+            (Some(args), false)
+        } else if p.eat_keyword_noexpect(sym::derive) {
+            kinds |= MacroKinds::DERIVE;
+            let derive_keyword_span = p.prev_token.span;
+            if !features.macro_derive() {
+                feature_err(sess, sym::macro_attr, span, "`macro_rules!` derives are unstable")
+                    .emit();
+            }
+            if let Some(guar) = check_no_eof(sess, &p, "expected `()` after `derive`") {
+                return dummy_syn_ext(guar);
+            }
+            let args = p.parse_token_tree();
+            check_args_parens(sess, sym::derive, &args);
+            let args_empty_result = check_args_empty(sess, &args);
+            let args_not_empty = args_empty_result.is_err();
+            check_emission(args_empty_result);
+            if let Some(guar) = check_no_eof(sess, &p, "expected macro derive body") {
+                return dummy_syn_ext(guar);
+            }
+            // If the user has `=>` right after the `()`, they might have forgotten the empty
+            // parentheses.
+            if p.token == token::FatArrow {
+                let mut err = sess
+                    .dcx()
+                    .struct_span_err(p.token.span, "expected macro derive body, got `=>`");
+                if args_not_empty {
+                    err.span_label(derive_keyword_span, "need `()` after this `derive`");
+                }
+                return dummy_syn_ext(err.emit());
+            }
+            (None, true)
         } else {
             kinds |= MacroKinds::BANG;
-            None
+            (None, false)
         };
         let lhs_tt = p.parse_token_tree();
         let lhs_tt = parse_one_tt(lhs_tt, RulePart::Pattern, sess, node_id, features, edition);
@@ -618,6 +652,8 @@ pub fn compile_declarative_macro(
             let args = mbe::macro_parser::compute_locs(&delimited.tts);
             let body_span = lhs_span;
             rules.push(MacroRule::Attr { args, args_span, body: lhs, body_span, rhs: rhs_tt });
+        } else if is_derive {
+            rules.push(MacroRule::Derive { body: lhs, body_span: lhs_span, rhs: rhs_tt });
         } else {
             rules.push(MacroRule::Func { lhs, lhs_span, rhs: rhs_tt });
         }
@@ -664,18 +700,29 @@ fn check_no_eof(sess: &Session, p: &Parser<'_>, msg: &'static str) -> Option<Err
     None
 }
 
-fn check_args_parens(sess: &Session, args: &tokenstream::TokenTree) {
+fn check_args_parens(sess: &Session, rule_kw: Symbol, args: &tokenstream::TokenTree) {
     // This does not handle the non-delimited case; that gets handled separately by `check_lhs`.
     if let tokenstream::TokenTree::Delimited(dspan, _, delim, _) = args
         && *delim != Delimiter::Parenthesis
     {
         sess.dcx().emit_err(errors::MacroArgsBadDelim {
             span: dspan.entire(),
             sugg: errors::MacroArgsBadDelimSugg { open: dspan.open, close: dspan.close },
+            rule_kw,
         });
     }
 }
 
+fn check_args_empty(sess: &Session, args: &tokenstream::TokenTree) -> Result<(), ErrorGuaranteed> {
+    match args {
+        tokenstream::TokenTree::Delimited(.., delimited) if delimited.is_empty() => Ok(()),
+        _ => {
+            let msg = "`derive` rules do not accept arguments; `derive` must be followed by `()`";
+            Err(sess.dcx().span_err(args.span(), msg))
+        }
+    }
+}
+
 fn check_lhs(sess: &Session, node_id: NodeId, lhs: &mbe::TokenTree) -> Result<(), ErrorGuaranteed> {
     let e1 = check_lhs_nt_follows(sess, node_id, lhs);
     let e2 = check_lhs_no_empty_seq(sess, slice::from_ref(lhs));

compiler/rustc_feature/src/unstable.rs

@@ -556,6 +556,8 @@ declare_features! (
     (incomplete, loop_match, "1.90.0", Some(132306)),
     /// Allow `macro_rules!` attribute rules
     (unstable, macro_attr, "CURRENT_RUSTC_VERSION", Some(83527)),
+    /// Allow `macro_rules!` derive rules
+    (unstable, macro_derive, "CURRENT_RUSTC_VERSION", Some(143549)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.

compiler/rustc_span/src/symbol.rs

@@ -1314,6 +1314,7 @@ symbols! {
         macro_attr,
         macro_attributes_in_derive_output,
         macro_concat,
+        macro_derive,
         macro_escape,
         macro_export,
         macro_lifetime_matcher,

tests/ui/feature-gates/feature-gate-macro-derive.rs

@@ -0,0 +1,4 @@
+#![crate_type = "lib"]
+
+macro_rules! MyDerive { derive() {} => {} }
+//~^ ERROR `macro_rules!` derives are unstable

tests/ui/feature-gates/feature-gate-macro-derive.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `macro_rules!` derives are unstable
+  --> $DIR/feature-gate-macro-derive.rs:3:1
+   |
+LL | macro_rules! MyDerive { derive() {} => {} }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #83527 <https://github.com/rust-lang/rust/issues/83527> for more information
+   = help: add `#![feature(macro_attr)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/parser/macro/macro-attr-bad.rs

@@ -14,10 +14,10 @@ macro_rules! attr_incomplete_4 { attr() {} => }
 //~^ ERROR macro definition ended unexpectedly
 
 macro_rules! attr_noparens_1 { attr{} {} => {} }
-//~^ ERROR macro attribute argument matchers require parentheses
+//~^ ERROR macro `attr` argument matchers require parentheses
 
 macro_rules! attr_noparens_2 { attr[] {} => {} }
-//~^ ERROR macro attribute argument matchers require parentheses
+//~^ ERROR macro `attr` argument matchers require parentheses
 
 macro_rules! attr_noparens_3 { attr _ {} => {} }
 //~^ ERROR invalid macro matcher

tests/ui/parser/macro/macro-attr-bad.stderr

@@ -22,7 +22,7 @@ error: macro definition ended unexpectedly
 LL | macro_rules! attr_incomplete_4 { attr() {} => }
    |                                              ^ expected right-hand side of macro rule
 
-error: macro attribute argument matchers require parentheses
+error: macro `attr` argument matchers require parentheses
   --> $DIR/macro-attr-bad.rs:16:36
    |
 LL | macro_rules! attr_noparens_1 { attr{} {} => {} }
@@ -34,7 +34,7 @@ LL - macro_rules! attr_noparens_1 { attr{} {} => {} }
 LL + macro_rules! attr_noparens_1 { attr() {} => {} }
    |
 
-error: macro attribute argument matchers require parentheses
+error: macro `attr` argument matchers require parentheses
   --> $DIR/macro-attr-bad.rs:19:36
    |
 LL | macro_rules! attr_noparens_2 { attr[] {} => {} }

tests/ui/parser/macro/macro-attr-recovery.rs

@@ -3,7 +3,7 @@
 
 macro_rules! attr {
     attr[$($args:tt)*] { $($body:tt)* } => {
-        //~^ ERROR: macro attribute argument matchers require parentheses
+        //~^ ERROR: macro `attr` argument matchers require parentheses
         //~v ERROR: attr:
         compile_error!(concat!(
             "attr: args=\"",