Emit a warning if a match is too complex and set the default trigger when complexity reaches 10_000_000

Author: GuillaumeGomez

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -405,7 +405,8 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         scrut_ty: Ty<'tcx>,
     ) -> Result<UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
         let pattern_complexity_limit =
-            get_limit_size(cx.tcx.hir().krate_attrs(), cx.tcx.sess, sym::pattern_complexity);
+            get_limit_size(cx.tcx.hir().krate_attrs(), cx.tcx.sess, sym::pattern_complexity)
+            .or(Some(10_000_000)); // Default value to emit the warning for "too complex" match.
         let report =
             rustc_pattern_analysis::analyze_match(&cx, &arms, scrut_ty, pattern_complexity_limit)
                 .map_err(|err| {

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -913,7 +913,8 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
 
     fn complexity_exceeded(&self) -> Result<(), Self::Error> {
         let span = self.whole_match_span.unwrap_or(self.scrut_span);
-        Err(self.tcx.dcx().span_err(span, "reached pattern complexity limit"))
+        self.tcx.dcx().span_warn(span, "`match` is too complex");
+        Ok(())
     }
 
     fn lint_non_contiguous_range_endpoints(

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -745,6 +745,8 @@ impl<'a, Cx: PatCx> UsefulnessCtxt<'a, Cx> {
             .complexity_limit
             .is_some_and(|complexity_limit| complexity_limit < self.complexity_level)
         {
+            // We change it to `None` to prevent it from being called more than once.
+            self.complexity_limit = None;
             return self.tycx.complexity_exceeded();
         }
         Ok(())