Add pattern_complexity_limit to Limits.

It's similar to the other limits, e.g. obtained via `get_limit`. So it
makes sense to handle it consistently with the other limits. We now use
`Limit`/`usize` in most places instead of `Option<usize>`, so we use
`Limit::new(usize::MAX)`/`usize::MAX` to emulate how `None` used to work.

The commit also adds `Limit::unlimited`.

compiler/rustc_middle/src/middle/limits.rs

@@ -24,30 +24,36 @@ pub fn provide(providers: &mut Providers) {
             tcx.hir().krate_attrs(),
             tcx.sess,
             sym::move_size_limit,
-            tcx.sess.opts.unstable_opts.move_size_limit.unwrap_or(0),
+            Limit::new(tcx.sess.opts.unstable_opts.move_size_limit.unwrap_or(0)),
         ),
         type_length_limit: get_limit(
             tcx.hir().krate_attrs(),
             tcx.sess,
             sym::type_length_limit,
-            2usize.pow(24),
+            Limit::new(2usize.pow(24)),
+        ),
+        pattern_complexity_limit: get_limit(
+            tcx.hir().krate_attrs(),
+            tcx.sess,
+            sym::pattern_complexity,
+            Limit::unlimited(),
         ),
     }
 }
 
 pub fn get_recursion_limit(krate_attrs: &[impl AttributeExt], sess: &Session) -> Limit {
-    get_limit(krate_attrs, sess, sym::recursion_limit, 128)
+    get_limit(krate_attrs, sess, sym::recursion_limit, Limit::new(128))
 }
 
 fn get_limit(
     krate_attrs: &[impl AttributeExt],
     sess: &Session,
     name: Symbol,
-    default: usize,
+    default: Limit,
 ) -> Limit {
     match get_limit_size(krate_attrs, sess, name) {
         Some(size) => Limit::new(size),
-        None => Limit::new(default),
+        None => default,
     }
 }
 

compiler/rustc_middle/src/ty/context.rs

@@ -2168,6 +2168,10 @@ impl<'tcx> TyCtxt<'tcx> {
         self.limits(()).move_size_limit
     }
 
+    pub fn pattern_complexity_limit(self) -> Limit {
+        self.limits(()).pattern_complexity_limit
+    }
+
     /// All traits in the crate graph, including those not visible to the user.
     pub fn all_traits(self) -> impl Iterator<Item = DefId> + 'tcx {
         iter::once(LOCAL_CRATE)

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

@@ -10,7 +10,6 @@ use rustc_hir::{self as hir, BindingMode, ByRef, HirId};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::Level;
 use rustc_middle::bug;
-use rustc_middle::middle::limits::get_limit_size;
 use rustc_middle::thir::visit::Visitor;
 use rustc_middle::thir::*;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -25,7 +24,7 @@ use rustc_session::lint::builtin::{
 };
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::DesugaringKind;
-use rustc_span::{Ident, Span, sym};
+use rustc_span::{Ident, Span};
 use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::instrument;
 
@@ -404,18 +403,11 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         arms: &[MatchArm<'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);
-        let report = rustc_pattern_analysis::rustc::analyze_match(
-            &cx,
-            &arms,
-            scrut_ty,
-            pattern_complexity_limit,
-        )
-        .map_err(|err| {
-            self.error = Err(err);
-            err
-        })?;
+        let report =
+            rustc_pattern_analysis::rustc::analyze_match(&cx, &arms, scrut_ty).map_err(|err| {
+                self.error = Err(err);
+                err
+            })?;
 
         // Warn unreachable subpatterns.
         for (arm, is_useful) in report.arm_usefulness.iter() {

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -1084,12 +1084,16 @@ pub fn analyze_match<'p, 'tcx>(
     tycx: &RustcPatCtxt<'p, 'tcx>,
     arms: &[MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
-    pattern_complexity_limit: Option<usize>,
 ) -> Result<UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
     let scrut_validity = PlaceValidity::from_bool(tycx.known_valid_scrutinee);
-    let report =
-        compute_match_usefulness(tycx, arms, scrut_ty, scrut_validity, pattern_complexity_limit)?;
+    let report = compute_match_usefulness(
+        tycx,
+        arms,
+        scrut_ty,
+        scrut_validity,
+        tycx.tcx.pattern_complexity_limit().0,
+    )?;
 
     // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid hitting
     // `if let`s. Only run if the match is exhaustive otherwise the error is redundant.

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -795,20 +795,21 @@ struct UsefulnessCtxt<'a, 'p, Cx: PatCx> {
     /// Track information about the usefulness of branch patterns (see definition of "branch
     /// pattern" at [`BranchPatUsefulness`]).
     branch_usefulness: FxHashMap<PatId, BranchPatUsefulness<'p, Cx>>,
-    complexity_limit: Option<usize>,
+    // Ideally this field would have type `Limit`, but this crate is used by
+    // rust-analyzer which cannot have a dependency on `Limit`, because `Limit`
+    // is from crate `rustc_session` which uses unstable Rust features.
+    complexity_limit: usize,
     complexity_level: usize,
 }
 
 impl<'a, 'p, Cx: PatCx> UsefulnessCtxt<'a, 'p, Cx> {
     fn increase_complexity_level(&mut self, complexity_add: usize) -> Result<(), Cx::Error> {
         self.complexity_level += complexity_add;
-        if self
-            .complexity_limit
-            .is_some_and(|complexity_limit| complexity_limit < self.complexity_level)
-        {
-            return self.tycx.complexity_exceeded();
+        if self.complexity_level <= self.complexity_limit {
+            Ok(())
+        } else {
+            self.tycx.complexity_exceeded()
         }
-        Ok(())
     }
 }
 
@@ -1834,7 +1835,7 @@ pub fn compute_match_usefulness<'p, Cx: PatCx>(
     arms: &[MatchArm<'p, Cx>],
     scrut_ty: Cx::Ty,
     scrut_validity: PlaceValidity,
-    complexity_limit: Option<usize>,
+    complexity_limit: usize,
 ) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> {
     let mut cx = UsefulnessCtxt {
         tycx,

compiler/rustc_pattern_analysis/tests/common/mod.rs

@@ -124,7 +124,7 @@ pub fn compute_match_usefulness<'p>(
     arms: &[MatchArm<'p, Cx>],
     ty: Ty,
     scrut_validity: PlaceValidity,
-    complexity_limit: Option<usize>,
+    complexity_limit: usize,
 ) -> Result<UsefulnessReport<'p, Cx>, ()> {
     init_tracing();
     rustc_pattern_analysis::usefulness::compute_match_usefulness(

compiler/rustc_pattern_analysis/tests/complexity.rs

@@ -14,7 +14,7 @@ fn check(patterns: &[DeconstructedPat<Cx>], complexity_limit: usize) -> Result<(
     let ty = *patterns[0].ty();
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
-    compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, Some(complexity_limit))
+    compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, complexity_limit)
         .map(|_report| ())
 }
 

compiler/rustc_pattern_analysis/tests/exhaustiveness.rs

@@ -14,7 +14,8 @@ fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<WitnessPat<Cx>> {
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
     let report =
-        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, None).unwrap();
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX)
+            .unwrap();
     report.non_exhaustiveness_witnesses
 }
 

compiler/rustc_pattern_analysis/tests/intersection.rs

@@ -14,7 +14,8 @@ fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<Vec<usize>> {
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
     let report =
-        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, None).unwrap();
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX)
+            .unwrap();
     report.arm_intersections.into_iter().map(|bitset| bitset.iter().collect()).collect()
 }
 

compiler/rustc_session/src/session.rs

@@ -67,6 +67,11 @@ impl Limit {
         Limit(value)
     }
 
+    /// Create a new unlimited limit.
+    pub fn unlimited() -> Self {
+        Limit(usize::MAX)
+    }
+
     /// Check that `value` is within the limit. Ensures that the same comparisons are used
     /// throughout the compiler, as mismatches can cause ICEs, see #72540.
     #[inline]
@@ -119,6 +124,8 @@ pub struct Limits {
     pub move_size_limit: Limit,
     /// The maximum length of types during monomorphization.
     pub type_length_limit: Limit,
+    /// The maximum pattern complexity allowed (internal only).
+    pub pattern_complexity_limit: Limit,
 }
 
 pub struct CompilerIO {

src/tools/rust-analyzer/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs

@@ -95,7 +95,7 @@ impl<'db> MatchCheckCtx<'db> {
 
         let place_validity = PlaceValidity::from_bool(known_valid_scrutinee.unwrap_or(true));
         // Measured to take ~100ms on modern hardware.
-        let complexity_limit = Some(500000);
+        let complexity_limit = 500000;
         compute_match_usefulness(self, arms, scrut_ty, place_validity, complexity_limit)
     }