mbe: Recover from an attr rule without parentheses

Emit the error, but parse the macro definition as if it had parentheses,
to go on and emit other errors.

Author: joshtriplett

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -564,7 +564,7 @@ pub fn compile_declarative_macro(
                 return dummy_syn_ext(guar);
             }
             let args = p.parse_token_tree();
-            check_emission(check_args_parens(sess, &args));
+            check_args_parens(sess, &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") {
@@ -651,17 +651,16 @@ fn check_no_eof(sess: &Session, p: &Parser<'_>, msg: &'static str) -> Option<Err
     None
 }
 
-fn check_args_parens(sess: &Session, args: &tokenstream::TokenTree) -> Result<(), ErrorGuaranteed> {
+fn check_args_parens(sess: &Session, 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
     {
-        return Err(sess.dcx().emit_err(errors::MacroArgsBadDelim {
+        sess.dcx().emit_err(errors::MacroArgsBadDelim {
             span: dspan.entire(),
             sugg: errors::MacroArgsBadDelimSugg { open: dspan.open, close: dspan.close },
-        }));
+        });
     }
-    Ok(())
 }
 
 fn check_lhs(sess: &Session, node_id: NodeId, lhs: &mbe::TokenTree) -> Result<(), ErrorGuaranteed> {

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

@@ -0,0 +1,19 @@
+#![crate_type = "lib"]
+#![feature(macro_attr)]
+
+macro_rules! attr {
+    attr[$($args:tt)*] { $($body:tt)* } => {
+        //~^ ERROR: macro attribute argument matchers require parentheses
+        //~v ERROR: attr:
+        compile_error!(concat!(
+            "attr: args=\"",
+            stringify!($($args)*),
+            "\" body=\"",
+            stringify!($($body)*),
+            "\"",
+        ));
+    };
+}
+
+#[attr]
+struct S;

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

@@ -0,0 +1,31 @@
+error: macro attribute argument matchers require parentheses
+  --> $DIR/macro-attr-recovery.rs:5:9
+   |
+LL |     attr[$($args:tt)*] { $($body:tt)* } => {
+   |         ^^^^^^^^^^^^^^
+   |
+help: the delimiters should be `(` and `)`
+   |
+LL -     attr[$($args:tt)*] { $($body:tt)* } => {
+LL +     attr($($args:tt)*) { $($body:tt)* } => {
+   |
+
+error: attr: args="" body="struct S;"
+  --> $DIR/macro-attr-recovery.rs:8:9
+   |
+LL | /         compile_error!(concat!(
+LL | |             "attr: args=\"",
+LL | |             stringify!($($args)*),
+LL | |             "\" body=\"",
+LL | |             stringify!($($body)*),
+LL | |             "\"",
+LL | |         ));
+   | |__________^
+...
+LL |   #[attr]
+   |   ------- in this attribute macro expansion
+   |
+   = note: this error originates in the attribute macro `attr` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+