Rollup merge of rust-lang#121917 - GuillaumeGomez:pattern-complexity_limit.rs, r=Nadrieril

Add new `pattern_complexity` attribute to add possibility to limit and check recursion in pattern matching

Needed for rust-lang/rust-analyzer#9528.

This PR adds a new attribute only available when running rust testsuite called `pattern_complexity` which allows to set the maximum recursion for the pattern matching. It is quite useful to ensure the complexity doesn't grow, like in `tests/ui/pattern/usefulness/issue-118437-exponential-time-on-diagonal-match.rs`.

r? `@Nadrieril`

Author: GuillaumeGomez

compiler/rustc_feature/src/builtin_attrs.rs

@@ -929,6 +929,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         omit_gdb_pretty_printer_section, Normal, template!(Word), WarnFollowing,
         "the `#[omit_gdb_pretty_printer_section]` attribute is just used for the Rust test suite",
     ),
+    rustc_attr!(
+        TEST, pattern_complexity, CrateLevel, template!(NameValueStr: "N"),
+        ErrorFollowing, @only_local: true,
+    ),
 ];
 
 pub fn deprecated_attributes() -> Vec<&'static BuiltinAttribute> {

compiler/rustc_feature/src/unstable.rs

@@ -213,6 +213,8 @@ declare_features! (
     (internal, negative_bounds, "1.71.0", None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (internal, omit_gdb_pretty_printer_section, "1.5.0", None),
+    /// Set the maximum pattern complexity allowed (not limited by default).
+    (internal, pattern_complexity, "CURRENT_RUSTC_VERSION", None),
     /// Allows using `#[prelude_import]` on glob `use` items.
     (internal, prelude_import, "1.2.0", None),
     /// Used to identify crates that contain the profiler runtime.

compiler/rustc_middle/src/middle/limits.rs

@@ -40,14 +40,21 @@ pub fn get_recursion_limit(krate_attrs: &[Attribute], sess: &Session) -> Limit {
 }
 
 fn get_limit(krate_attrs: &[Attribute], sess: &Session, name: Symbol, default: usize) -> Limit {
+    match get_limit_size(krate_attrs, sess, name) {
+        Some(size) => Limit::new(size),
+        None => Limit::new(default),
+    }
+}
+
+pub fn get_limit_size(krate_attrs: &[Attribute], sess: &Session, name: Symbol) -> Option<usize> {
     for attr in krate_attrs {
         if !attr.has_name(name) {
             continue;
         }
 
         if let Some(s) = attr.value_str() {
             match s.as_str().parse() {
-                Ok(n) => return Limit::new(n),
+                Ok(n) => return Some(n),
                 Err(e) => {
                     let value_span = attr
                         .meta()
@@ -69,5 +76,5 @@ fn get_limit(krate_attrs: &[Attribute], sess: &Session, name: Symbol, default: u
             }
         }
     }
-    return Limit::new(default);
+    None
 }

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

@@ -17,6 +17,7 @@ use rustc_hir as hir;
 use rustc_hir::def::*;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::HirId;
+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;
@@ -26,7 +27,7 @@ use rustc_session::lint::builtin::{
 };
 use rustc_session::Session;
 use rustc_span::hygiene::DesugaringKind;
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 
 pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGuaranteed> {
     let typeck_results = tcx.typeck(def_id);
@@ -403,8 +404,11 @@ 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::analyze_match(&cx, &arms, scrut_ty).map_err(|err| {
+            rustc_pattern_analysis::analyze_match(&cx, &arms, scrut_ty, pattern_complexity_limit)
+                .map_err(|err| {
                 self.error = Err(err);
                 err
             })?;

compiler/rustc_pattern_analysis/src/lib.rs

@@ -142,6 +142,9 @@ pub trait TypeCx: Sized + fmt::Debug {
         _overlaps_with: &[&DeconstructedPat<Self>],
     ) {
     }
+
+    /// The maximum pattern complexity limit was reached.
+    fn complexity_exceeded(&self) -> Result<(), Self::Error>;
 }
 
 /// The arm of a match expression.
@@ -167,10 +170,12 @@ pub fn analyze_match<'p, 'tcx>(
     tycx: &RustcMatchCheckCtxt<'p, 'tcx>,
     arms: &[rustc::MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
+    pattern_complexity_limit: Option<usize>,
 ) -> Result<rustc::UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
     let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
-    let report = compute_match_usefulness(tycx, arms, scrut_ty, scrut_validity)?;
+    let report =
+        compute_match_usefulness(tycx, arms, scrut_ty, scrut_validity, pattern_complexity_limit)?;
 
     // 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/rustc.rs

@@ -895,6 +895,11 @@ impl<'p, 'tcx: 'p> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
             errors::OverlappingRangeEndpoints { overlap: overlaps, range: pat_span },
         );
     }
+
+    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"))
+    }
 }
 
 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -734,6 +734,21 @@ struct UsefulnessCtxt<'a, Cx: TypeCx> {
     /// Collect the patterns found useful during usefulness checking. This is used to lint
     /// unreachable (sub)patterns.
     useful_subpatterns: FxHashSet<PatId>,
+    complexity_limit: Option<usize>,
+    complexity_level: usize,
+}
+
+impl<'a, Cx: TypeCx> UsefulnessCtxt<'a, 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();
+        }
+        Ok(())
+    }
 }
 
 /// Context that provides information local to a place under investigation.
@@ -1552,6 +1567,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
     }
 
     let Some(place) = matrix.head_place() else {
+        mcx.increase_complexity_level(matrix.rows().len())?;
         // The base case: there are no columns in the matrix. We are morally pattern-matching on ().
         // A row is useful iff it has no (unguarded) rows above it.
         let mut useful = true; // Whether the next row is useful.
@@ -1690,8 +1706,14 @@ pub fn compute_match_usefulness<'p, Cx: TypeCx>(
     arms: &[MatchArm<'p, Cx>],
     scrut_ty: Cx::Ty,
     scrut_validity: ValidityConstraint,
+    complexity_limit: Option<usize>,
 ) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> {
-    let mut cx = UsefulnessCtxt { tycx, useful_subpatterns: FxHashSet::default() };
+    let mut cx = UsefulnessCtxt {
+        tycx,
+        useful_subpatterns: FxHashSet::default(),
+        complexity_limit,
+        complexity_level: 0,
+    };
     let mut matrix = Matrix::new(arms, scrut_ty, scrut_validity);
     let non_exhaustiveness_witnesses = compute_exhaustiveness_and_usefulness(&mut cx, &mut matrix)?;
 

compiler/rustc_span/src/symbol.rs

@@ -1303,6 +1303,7 @@ symbols! {
         pat,
         pat_param,
         path,
+        pattern_complexity,
         pattern_parentheses,
         phantom_data,
         pic,

tests/ui/feature-gates/feature-gate-pattern-complexity.rs

@@ -0,0 +1,6 @@
+// check that `pattern_complexity` is feature-gated
+
+#![pattern_complexity = "42"]
+//~^ ERROR: the `#[pattern_complexity]` attribute is just used for rustc unit tests
+
+fn main() {}

tests/ui/feature-gates/feature-gate-pattern-complexity.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[pattern_complexity]` attribute is just used for rustc unit tests and will never be stable
+  --> $DIR/feature-gate-pattern-complexity.rs:3:1
+   |
+LL | #![pattern_complexity = "42"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.