match: Use an aggregate equality comparison for constant array/slice patterns

compiler/rustc_middle/src/thir.rs

@@ -660,6 +660,14 @@ pub struct PatExtra<'tcx> {
     /// the pattern node back to the `DefId` of its original constant.
     pub expanded_const: Option<DefId>,
 
+    /// If present, the original constant value that this array or slice
+    /// pattern node was expanded from by `const_to_pat`.
+    ///
+    /// Match lowering uses this to compare the scrutinee against the original
+    /// constant as a whole via `PartialEq::eq`, rather than element by
+    /// element.
+    pub expanded_const_value: Option<ty::Value<'tcx>>,
+
     /// User-written types that must be preserved into MIR so that they can be
     /// checked.
     pub ascriptions: Vec<Ascription<'tcx>>,

compiler/rustc_mir_build/src/builder/matches/buckets.rs

@@ -323,6 +323,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     value: case_val,
                     kind: PatConstKind::Float | PatConstKind::Other,
                 },
+            )
+            | (
+                TestKind::AggregateEq { value: test_val, .. },
+                TestableCase::Constant { value: case_val, kind: PatConstKind::Aggregate },
             ) => {
                 if test_val == case_val {
                     fully_matched = true;
@@ -353,6 +357,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 | TestKind::Range { .. }
                 | TestKind::StringEq { .. }
                 | TestKind::ScalarEq { .. }
+                | TestKind::AggregateEq { .. }
                 | TestKind::Deref { .. },
                 _,
             ) => {

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -13,6 +13,40 @@ use crate::builder::matches::{
     FlatPat, MatchPairTree, PatConstKind, PatternExtraData, SliceLenOp, TestableCase,
 };
 
+/// Below this length, an array or slice pattern is compared element by element
+/// rather than as a single aggregate, since the per-element comparisons are
+/// unlikely to be more expensive than a `PartialEq::eq` call.
+const AGGREGATE_EQ_MIN_LEN: usize = 4;
+
+impl<'a, 'tcx> Builder<'a, 'tcx> {
+    /// Check if we can use aggregate `PartialEq::eq` comparisons for constant array/slice patterns.
+    /// This is not possible in const contexts, because `PartialEq` is not const-stable yet.
+    fn can_use_aggregate_eq(&self) -> bool {
+        let in_const_context = self.tcx.is_const_fn(self.def_id.to_def_id())
+            || !self.tcx.hir_body_owner_kind(self.def_id).is_fn_or_closure();
+        !in_const_context
+    }
+
+    /// If the given array or slice pattern node was expanded from a constant
+    /// by `const_to_pat` and an aggregate comparison is both possible and
+    /// worthwhile, returns the original constant value, so that the scrutinee
+    /// can be compared against it as a whole via `PartialEq::eq`.
+    ///
+    /// Note that this deliberately does not apply to hand-written array or
+    /// slice patterns, which only ever match element by element.
+    fn aggregate_const_value(
+        &self,
+        pattern: &Pat<'tcx>,
+        element_count: usize,
+    ) -> Option<ty::Value<'tcx>> {
+        let value = pattern.extra.as_deref()?.expanded_const_value?;
+        if element_count < AGGREGATE_EQ_MIN_LEN || !self.can_use_aggregate_eq() {
+            return None;
+        }
+        Some(value)
+    }
+}
+
 /// For an array or slice pattern's subpatterns (prefix/slice/suffix), returns a list
 /// of those subpatterns, each paired with a suitably-projected [`PlaceBuilder`].
 fn prefix_slice_suffix<'a, 'tcx>(
@@ -344,10 +378,26 @@ impl<'tcx> InterPat<'tcx> {
                     _ => None,
                 };
                 if let Some(array_len) = array_len {
-                    for (subplace, subpat) in
-                        prefix_slice_suffix(&place_builder, Some(array_len), prefix, slice, suffix)
-                    {
-                        subpats.push(InterPat::lower_thir_pat(cx, subplace, subpat));
+                    // If this pattern was expanded from a constant, compare
+                    // the whole array against that constant at once via
+                    // `PartialEq::eq` rather than element by element.
+                    if let Some(aggregate_value) = cx.aggregate_const_value(pattern, prefix.len()) {
+                        debug_assert!(slice.is_none() && suffix.is_empty());
+                        Some(TestableCase::Constant {
+                            value: aggregate_value,
+                            kind: PatConstKind::Aggregate,
+                        })
+                    } else {
+                        for (subplace, subpat) in prefix_slice_suffix(
+                            &place_builder,
+                            Some(array_len),
+                            prefix,
+                            slice,
+                            suffix,
+                        ) {
+                            subpats.push(InterPat::lower_thir_pat(cx, subplace, subpat));
+                        }
+                        None
                     }
                 } else {
                     // If the array length couldn't be determined, ignore the
@@ -359,33 +409,57 @@ impl<'tcx> InterPat<'tcx> {
                             pattern.ty
                         ),
                     );
+                    None
                 }
-
-                None
             }
             PatKind::Slice { ref prefix, ref slice, ref suffix } => {
-                for (subplace, subpat) in
-                    prefix_slice_suffix(&place_builder, None, prefix, slice, suffix)
-                {
-                    subpats.push(InterPat::lower_thir_pat(cx, subplace, subpat));
-                }
-
-                if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
-                    // A slice pattern shaped like `[..]` is irrefutable.
-                    // It can match a slice of any length, so no length test is needed.
-                    None
-                } else {
-                    // Any other shape of slice pattern requires a length test.
-                    // Slice patterns with a `..` subpattern require a minimum
-                    // length; those without `..` require an exact length.
+                // If this pattern was expanded from a constant, compare the
+                // whole slice against that constant at once via
+                // `PartialEq::eq` after the length check, rather than
+                // element by element.
+                if let Some(aggregate_value) = cx.aggregate_const_value(pattern, prefix.len()) {
+                    debug_assert!(slice.is_none() && suffix.is_empty());
+                    subpats.push(InterPat {
+                        place,
+                        testable_case: Some(TestableCase::Constant {
+                            value: aggregate_value,
+                            kind: PatConstKind::Aggregate,
+                        }),
+                        subpats: Vec::new(),
+                        or_subpats: None,
+                        ascriptions: Vec::new(),
+                        binding: None,
+                        pattern_span: pattern.span,
+                        is_never: false,
+                    });
                     Some(TestableCase::Slice {
-                        len: u64::try_from(prefix.len() + suffix.len()).unwrap(),
-                        op: if slice.is_some() {
-                            SliceLenOp::GreaterOrEqual
-                        } else {
-                            SliceLenOp::Equal
-                        },
+                        len: u64::try_from(prefix.len()).unwrap(),
+                        op: SliceLenOp::Equal,
                     })
+                } else {
+                    for (subplace, subpat) in
+                        prefix_slice_suffix(&place_builder, None, prefix, slice, suffix)
+                    {
+                        subpats.push(InterPat::lower_thir_pat(cx, subplace, subpat));
+                    }
+
+                    if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
+                        // A slice pattern shaped like `[..]` is irrefutable.
+                        // It can match a slice of any length, so no length test is needed.
+                        None
+                    } else {
+                        // Any other shape of slice pattern requires a length test.
+                        // Slice patterns with a `..` subpattern require a minimum
+                        // length; those without `..` require an exact length.
+                        Some(TestableCase::Slice {
+                            len: u64::try_from(prefix.len() + suffix.len()).unwrap(),
+                            op: if slice.is_some() {
+                                SliceLenOp::GreaterOrEqual
+                            } else {
+                                SliceLenOp::Equal
+                            },
+                        })
+                    }
                 }
             }
 

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -1248,6 +1248,10 @@ enum PatConstKind {
     Float,
     /// Constant string values, tested via string equality.
     String,
+    /// Constant array or slice values that array/slice patterns were expanded
+    /// from. Tested by calling `PartialEq::eq` on the whole aggregate at once,
+    /// rather than comparing element by element.
+    Aggregate,
     /// Any other constant-pattern is usually tested via some kind of equality
     /// check. Types that might be encountered here include:
     /// - raw pointers derived from integer values
@@ -1333,6 +1337,10 @@ enum TestKind<'tcx> {
     /// Tests the place against a constant using scalar equality.
     ScalarEq { value: ty::Value<'tcx> },
 
+    /// Tests the place against a constant array or slice using `PartialEq::eq`,
+    /// comparing the whole aggregate at once rather than element by element.
+    AggregateEq { value: ty::Value<'tcx> },
+
     /// Test whether the value falls within an inclusive or exclusive range.
     Range(Arc<PatRange<'tcx>>),
 

compiler/rustc_mir_build/src/builder/matches/test.rs

@@ -40,6 +40,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             TestableCase::Constant { value, kind: PatConstKind::String } => {
                 TestKind::StringEq { value }
             }
+            TestableCase::Constant { value, kind: PatConstKind::Aggregate } => {
+                TestKind::AggregateEq { value }
+            }
             TestableCase::Constant { value, kind: PatConstKind::Float | PatConstKind::Other } => {
                 TestKind::ScalarEq { value }
             }
@@ -137,44 +140,59 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.cfg.terminate(block, self.source_info(match_start_span), terminator);
             }
 
-            TestKind::StringEq { value } => {
+            TestKind::StringEq { value } | TestKind::AggregateEq { value } => {
                 let tcx = self.tcx;
                 let success_block = target_block(TestBranch::Success);
                 let fail_block = target_block(TestBranch::Failure);
 
-                let ref_str_ty = Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, tcx.types.str_);
-                assert!(ref_str_ty.is_imm_ref_str(), "{ref_str_ty:?}");
-
-                // The string constant we're testing against has type `str`, but
-                // calling `<str as PartialEq>::eq` requires `&str` operands.
-                //
-                // Because `str` and `&str` have the same valtree representation,
-                // we can "cast" to the desired type by just replacing the type.
-                assert!(value.ty.is_str(), "unexpected value type for StringEq test: {value:?}");
-                let expected_value = ty::Value { ty: ref_str_ty, valtree: value.valtree };
+                let inner_ty = value.ty;
+                if matches!(test.kind, TestKind::StringEq { .. }) {
+                    assert!(
+                        inner_ty.is_str(),
+                        "unexpected value type for StringEq test: {value:?}"
+                    );
+                }
+                let ref_ty = Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, inner_ty);
+
+                // The constant we're testing against has type `str`, `[T; N]`, or `[T]`,
+                // but calling `<T as PartialEq>::eq` requires a reference operand
+                // (`&str`, `&[T; N]`, or `&[T]`). Valtree representations are the same
+                // with or without the reference wrapper, so we can "cast" to the
+                // desired type by just replacing the type.
+                let expected_value = ty::Value { ty: ref_ty, valtree: value.valtree };
                 let expected_value_operand =
                     self.literal_operand(test.span, Const::from_ty_value(tcx, expected_value));
 
-                // Similarly, the scrutinized place has type `str`, but we need `&str`.
-                // Get a reference by doing `let actual_value_ref_place: &str = &place`.
-                let actual_value_ref_place = self.temp(ref_str_ty, test.span);
+                // Similarly, the scrutinised place has the inner type, but we need a
+                // reference. Get one by doing `let actual_value_ref_place = &place`.
+                let actual_value_ref_place = self.temp(ref_ty, test.span);
                 self.cfg.push_assign(
                     block,
                     self.source_info(test.span),
                     actual_value_ref_place,
                     Rvalue::Ref(tcx.lifetimes.re_erased, BorrowKind::Shared, place),
                 );
 
-                // Compare two strings using `<str as std::cmp::PartialEq>::eq`.
-                // (Interestingly this means that exhaustiveness analysis relies, for soundness,
-                // on the `PartialEq` impl for `str` to be correct!)
-                self.string_compare(
+                // Compare the two values using `<T as std::cmp::PartialEq>::eq`.
+                // (Interestingly this means that, for `str`, exhaustiveness analysis
+                // relies for soundness on the `PartialEq` impl for `str` to be correct!)
+                //
+                // The aggregate comparisons, unlike the long-standing string ones, are
+                // asserted not to unwind, since an unwind edge would make
+                // borrow-checking stricter than for the `SwitchInt`s they replace.
+                // That is sound because a constant is only allowed in a pattern if its
+                // type is structural match, so the array/slice impl and every element
+                // impl it delegates to are derived or primitive, and cannot panic.
+                let can_unwind = matches!(test.kind, TestKind::StringEq { .. });
+                self.non_scalar_compare(
                     block,
                     success_block,
                     fail_block,
                     source_info,
+                    inner_ty,
                     expected_value_operand,
                     Operand::Copy(actual_value_ref_place),
+                    can_unwind,
                 );
             }
 
@@ -409,19 +427,31 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         );
     }
 
-    /// Compare two values of type `&str` using `<str as std::cmp::PartialEq>::eq`.
-    fn string_compare(
+    /// Compare two reference values using `<T as PartialEq>::eq`.
+    ///
+    /// `compared_ty` is the *inner* type (e.g. `str`, `[u8; 64]`);
+    /// `expect` and `val` must already be references to that type.
+    ///
+    /// When `can_unwind` is false, the call is given `UnwindAction::Unreachable`
+    /// and no unwind edge, asserting that the `PartialEq::eq` implementation
+    /// cannot panic. This matters beyond codegen: an unwinding call would make
+    /// borrow-checking of the surrounding match stricter, because the unwind
+    /// path can create drop-order conflicts that the ordinary path does not
+    /// have.
+    fn non_scalar_compare(
         &mut self,
         block: BasicBlock,
         success_block: BasicBlock,
         fail_block: BasicBlock,
         source_info: SourceInfo,
+        compared_ty: Ty<'tcx>,
         expect: Operand<'tcx>,
         val: Operand<'tcx>,
+        can_unwind: bool,
     ) {
-        let str_ty = self.tcx.types.str_;
         let eq_def_id = self.tcx.require_lang_item(LangItem::PartialEq, source_info.span);
-        let method = trait_method(self.tcx, eq_def_id, sym::eq, &[str_ty.into(), str_ty.into()]);
+        let method =
+            trait_method(self.tcx, eq_def_id, sym::eq, &[compared_ty.into(), compared_ty.into()]);
 
         let bool_ty = self.tcx.types.bool;
         let eq_result = self.temp(bool_ty, source_info.span);
@@ -448,12 +478,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 .into(),
                 destination: eq_result,
                 target: Some(eq_block),
-                unwind: UnwindAction::Continue,
+                unwind: if can_unwind { UnwindAction::Continue } else { UnwindAction::Unreachable },
                 call_source: CallSource::MatchCmp,
                 fn_span: source_info.span,
             },
         );
-        self.diverge_from(block);
+        if can_unwind {
+            self.diverge_from(block);
+        }
 
         // check the result
         self.cfg.terminate(

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

@@ -473,7 +473,15 @@ impl<'tcx> ConstToPat<'tcx> {
             }
         };
 
-        Box::new(Pat { span, ty, kind, extra: None })
+        let mut pat = Box::new(Pat { span, ty, kind, extra: None });
+        if matches!(ty.kind(), ty::Array(..) | ty::Slice(_)) {
+            // Record the original constant value on array and slice nodes, so
+            // that match lowering can compare the scrutinee against the whole
+            // constant at once via `PartialEq::eq`, rather than element by
+            // element.
+            pat.extra.get_or_insert_default().expanded_const_value = Some(value);
+        }
+        pat
     }
 }
 

compiler/rustc_mir_build/src/thir/print.rs

@@ -703,10 +703,15 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
             return;
         };
 
-        let PatExtra { expanded_const, ascriptions } = extra;
+        let PatExtra { expanded_const, expanded_const_value, ascriptions } = extra;
 
         print_indented!(self, "extra: PatExtra {", depth_lvl);
         print_indented!(self, format_args!("expanded_const: {expanded_const:?}"), depth_lvl + 1);
+        print_indented!(
+            self,
+            format_args!("expanded_const_value: {expanded_const_value:?}"),
+            depth_lvl + 1
+        );
         self.print_list(
             "ascriptions",
             ascriptions,