Only do parser recovery on retried macro matching

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -22,7 +22,7 @@ use rustc_lint_defs::builtin::{
     RUST_2021_INCOMPATIBLE_OR_PATTERNS, SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
 };
 use rustc_lint_defs::BuiltinLintDiagnostics;
-use rustc_parse::parser::Parser;
+use rustc_parse::parser::{Parser, Recovery};
 use rustc_session::parse::ParseSess;
 use rustc_session::Session;
 use rustc_span::edition::Edition;
@@ -219,6 +219,8 @@ pub(super) trait Tracker<'matcher> {
 
     /// For tracing.
     fn description() -> &'static str;
+
+    fn recovery() -> Recovery;
 }
 
 /// A noop tracker that is used in the hot path of the expansion, has zero overhead thanks to monomorphization.
@@ -230,6 +232,9 @@ impl<'matcher> Tracker<'matcher> for NoopTracker {
     fn description() -> &'static str {
         "none"
     }
+    fn recovery() -> Recovery {
+        Recovery::Forbidden
+    }
 }
 
 /// Expands the rules based macro defined by `lhses` and `rhses` for a given
@@ -330,15 +335,20 @@ fn expand_macro<'cx>(
     let mut tracker = CollectTrackerAndEmitter::new(cx, sp);
 
     let try_success_result = try_match_macro(sess, name, &arg, lhses, &mut tracker);
-    assert!(try_success_result.is_err(), "Macro matching returned a success on the second try");
+
+    if try_success_result.is_ok() {
+        // Nonterminal parser recovery might turn failed matches into successful ones,
+        // but for that it must have emitted an error already
+        tracker.cx.sess.delay_span_bug(sp, "Macro matching returned a success on the second try");
+    }
 
     if let Some(result) = tracker.result {
         // An irrecoverable error occurred and has been emitted.
         return result;
     }
 
     let Some((token, label, remaining_matcher)) = tracker.best_failure else {
-        return tracker.result.expect("must have encountered Error or ErrorReported");
+        return DummyResult::any(sp);
     };
 
     let span = token.span.substitute_dummy(sp);
@@ -360,7 +370,7 @@ fn expand_macro<'cx>(
     // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
     if let Some((arg, comma_span)) = arg.add_comma() {
         for lhs in lhses {
-            let parser = parser_from_cx(sess, arg.clone());
+            let parser = parser_from_cx(sess, arg.clone(), Recovery::Allowed);
             let mut tt_parser = TtParser::new(name);
 
             if let Success(_) =
@@ -406,7 +416,12 @@ impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx,
     fn after_arm(&mut self, result: &NamedParseResult) {
         match result {
             Success(_) => {
-                unreachable!("should not collect detailed info for successful macro match");
+                // Nonterminal parser recovery might turn failed matches into successful ones,
+                // but for that it must have emitted an error already
+                self.cx.sess.delay_span_bug(
+                    self.root_span,
+                    "should not collect detailed info for successful macro match",
+                );
             }
             Failure(token, msg) => match self.best_failure {
                 Some((ref best_token, _, _)) if best_token.span.lo() >= token.span.lo() => {}
@@ -432,6 +447,10 @@ impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx,
     fn description() -> &'static str {
         "detailed"
     }
+
+    fn recovery() -> Recovery {
+        Recovery::Allowed
+    }
 }
 
 impl<'a, 'cx> CollectTrackerAndEmitter<'a, 'cx, '_> {
@@ -477,7 +496,7 @@ fn try_match_macro<'matcher, T: Tracker<'matcher>>(
     // 68836 suggests a more comprehensive but more complex change to deal with
     // this situation.)
     // FIXME(Nilstrieb): Stop recovery from happening on this parser and retry later with recovery if the macro failed to match.
-    let parser = parser_from_cx(sess, arg.clone());
+    let parser = parser_from_cx(sess, arg.clone(), T::recovery());
     // Try each arm's matchers.
     let mut tt_parser = TtParser::new(name);
     for (i, lhs) in lhses.iter().enumerate() {
@@ -1559,8 +1578,8 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
     }
 }
 
-fn parser_from_cx(sess: &ParseSess, tts: TokenStream) -> Parser<'_> {
-    Parser::new(sess, tts, true, rustc_parse::MACRO_ARGUMENTS)
+fn parser_from_cx(sess: &ParseSess, tts: TokenStream, recovery: Recovery) -> Parser<'_> {
+    Parser::new(sess, tts, true, rustc_parse::MACRO_ARGUMENTS).recovery(recovery)
 }
 
 /// Generates an appropriate parsing failure message. For EOF, this is "unexpected end...". For

compiler/rustc_parse/src/parser/mod.rs

@@ -503,8 +503,8 @@ impl<'a> Parser<'a> {
         parser
     }
 
-    pub fn forbid_recovery(mut self) -> Self {
-        self.recovery = Recovery::Forbidden;
+    pub fn recovery(mut self, recovery: Recovery) -> Self {
+        self.recovery = recovery;
         self
     }
 

src/test/ui/macros/recovery-allowed.rs

@@ -0,0 +1,8 @@
+macro_rules! please_recover {
+    ($a:expr) => {};
+}
+
+please_recover! { not 1 }
+//~^ ERROR unexpected `1` after identifier
+
+fn main() {}

src/test/ui/macros/recovery-allowed.stderr

@@ -0,0 +1,10 @@
+error: unexpected `1` after identifier
+  --> $DIR/recovery-allowed.rs:5:23
+   |
+LL | please_recover! { not 1 }
+   |                   ----^
+   |                   |
+   |                   help: use `!` to perform bitwise not
+
+error: aborting due to previous error
+

src/test/ui/macros/recovery-forbidden.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+macro_rules! dont_recover_here {
+    ($e:expr) => {
+        compile_error!("Must not recover to single !1 expr");
+    };
+
+    (not $a:literal) => {};
+}
+
+dont_recover_here! { not 1 }
+
+fn main() {}