Avoid hanging on complex matches

Author: Nadrieril

crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs

@@ -67,7 +67,9 @@ impl<'p> MatchCheckCtx<'p> {
     ) -> Result<UsefulnessReport<'p, Self>, ()> {
         // FIXME: Determine place validity correctly. For now, err on the safe side.
         let place_validity = PlaceValidity::MaybeInvalid;
-        compute_match_usefulness(self, arms, scrut_ty, place_validity, None)
+        // Measured to take ~100ms on modern hardware.
+        let complexity_limit = Some(500000);
+        compute_match_usefulness(self, arms, scrut_ty, place_validity, complexity_limit)
     }
 
     fn is_uninhabited(&self, ty: &Ty) -> bool {
@@ -476,7 +478,6 @@ impl<'p> PatCx for MatchCheckCtx<'p> {
     }
 
     fn complexity_exceeded(&self) -> Result<(), Self::Error> {
-        // FIXME(Nadrieril): make use of the complexity counter.
         Err(())
     }
 }

crates/ide-diagnostics/src/handlers/missing_match_arms.rs

@@ -1004,6 +1004,29 @@ fn f() {
         );
     }
 
+    #[test]
+    fn exponential_match() {
+        // Constructs a match where match checking takes exponential time. Ensures we bail early.
+        use std::fmt::Write;
+        let struct_arity = 30;
+        let mut code = String::new();
+        write!(code, "struct BigStruct {{").unwrap();
+        for i in 0..struct_arity {
+            write!(code, "  field{i}: bool,").unwrap();
+        }
+        write!(code, "}}").unwrap();
+        write!(code, "fn big_match(s: BigStruct) {{").unwrap();
+        write!(code, "  match s {{").unwrap();
+        for i in 0..struct_arity {
+            write!(code, "    BigStruct {{ field{i}: true, ..}} => {{}},").unwrap();
+            write!(code, "    BigStruct {{ field{i}: false, ..}} => {{}},").unwrap();
+        }
+        write!(code, "    _ => {{}},").unwrap();
+        write!(code, "  }}").unwrap();
+        write!(code, "}}").unwrap();
+        check_diagnostics_no_bails(&code);
+    }
+
     mod rust_unstable {
         use super::*;