Add fuel to match checking

crates/hir-ty/src/diagnostics/expr.rs

@@ -11,7 +11,6 @@ use hir_def::{ItemContainerId, Lookup};
 use hir_expand::name;
 use itertools::Itertools;
 use rustc_hash::FxHashSet;
-use rustc_pattern_analysis::usefulness::{compute_match_usefulness, PlaceValidity};
 use syntax::{ast, AstNode};
 use tracing::debug;
 use triomphe::Arc;
@@ -234,13 +233,7 @@ impl ExprValidator {
             return;
         }
 
-        let report = match compute_match_usefulness(
-            &cx,
-            m_arms.as_slice(),
-            scrut_ty.clone(),
-            PlaceValidity::ValidOnly,
-            None,
-        ) {
+        let report = match cx.compute_match_usefulness(m_arms.as_slice(), scrut_ty.clone()) {
             Ok(report) => report,
             Err(()) => return,
         };
@@ -282,13 +275,7 @@ impl ExprValidator {
                 continue;
             }
 
-            let report = match compute_match_usefulness(
-                &cx,
-                &[match_arm],
-                ty.clone(),
-                PlaceValidity::ValidOnly,
-                None,
-            ) {
+            let report = match cx.compute_match_usefulness(&[match_arm], ty.clone()) {
                 Ok(v) => v,
                 Err(e) => {
                     debug!(?e, "match usefulness error");

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

@@ -8,6 +8,7 @@ use rustc_hash::FxHashMap;
 use rustc_pattern_analysis::{
     constructor::{Constructor, ConstructorSet, VariantVisibility},
     index::IdxContainer,
+    usefulness::{compute_match_usefulness, PlaceValidity, UsefulnessReport},
     Captures, PatCx, PrivateUninhabitedField,
 };
 use smallvec::{smallvec, SmallVec};
@@ -59,6 +60,18 @@ impl<'p> MatchCheckCtx<'p> {
         Self { module, body, db, exhaustive_patterns, min_exhaustive_patterns }
     }
 
+    pub(crate) fn compute_match_usefulness(
+        &self,
+        arms: &[MatchArm<'p>],
+        scrut_ty: Ty,
+    ) -> Result<UsefulnessReport<'p, Self>, ()> {
+        // FIXME: Determine place validity correctly. For now, err on the safe side.
+        let place_validity = PlaceValidity::MaybeInvalid;
+        // 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 {
         is_ty_uninhabited_from(ty, self.module, self.db)
     }
@@ -465,7 +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

@@ -23,6 +23,7 @@ mod tests {
         },
         DiagnosticsConfig,
     };
+    use test_utils::skip_slow_tests;
 
     #[track_caller]
     fn check_diagnostics_no_bails(ra_fixture: &str) {
@@ -1004,6 +1005,32 @@ fn f() {
         );
     }
 
+    #[test]
+    fn exponential_match() {
+        if skip_slow_tests() {
+            return;
+        }
+        // Constructs a match where match checking takes exponential time. Ensures we bail early.
+        use std::fmt::Write;
+        let struct_arity = 50;
+        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::*;