[DO NOT MERGE] crater: emit a warning if a match is too complex

compiler/rustc_pattern_analysis/src/lib.rs

@@ -145,6 +145,8 @@ pub trait TypeCx: Sized + fmt::Debug {
 
     /// The maximum pattern complexity limit was reached.
     fn complexity_exceeded(&self) -> Result<(), Self::Error>;
+
+    fn too_complex_match(&self);
 }
 
 /// The arm of a match expression.

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -900,6 +900,10 @@ impl<'p, 'tcx: 'p> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
         let span = self.whole_match_span.unwrap_or(self.scrut_span);
         Err(self.tcx.dcx().span_err(span, "reached pattern complexity limit"))
     }
+
+    fn too_complex_match(&self) {
+        panic!("match too complex!");
+    }
 }
 
 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -747,6 +747,10 @@ impl<'a, Cx: TypeCx> UsefulnessCtxt<'a, Cx> {
         {
             return self.tycx.complexity_exceeded();
         }
+        // FIXME: Will be turned into a warning once we have a somewhat useable threeshold.
+        if self.complexity_level > 10_000_000 {
+            self.tycx.too_complex_match();
+        }
         Ok(())
     }
 }