mir_build: Separate match lowering for string-equality and scalar-equality

compiler/rustc_middle/src/ty/sty.rs

@@ -1218,6 +1218,12 @@ impl<'tcx> Ty<'tcx> {
         *self.kind() == Str
     }
 
+    /// Returns true if this type is `&str`. The reference's lifetime is ignored.
+    #[inline]
+    pub fn is_imm_ref_str(self) -> bool {
+        matches!(self.kind(), ty::Ref(_, inner, hir::Mutability::Not) if inner.is_str())
+    }
+
     #[inline]
     pub fn is_param(self, index: u32) -> bool {
         match self.kind() {

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

@@ -314,7 +314,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             (
-                TestKind::Eq { value: test_val, .. },
+                TestKind::StringEq { value: test_val, .. },
+                TestableCase::Constant { value: case_val, kind: PatConstKind::String },
+            )
+            | (
+                TestKind::ScalarEq { value: test_val, .. },
                 TestableCase::Constant {
                     value: case_val,
                     kind: PatConstKind::Float | PatConstKind::Other,
@@ -347,7 +351,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 | TestKind::If
                 | TestKind::SliceLen { .. }
                 | TestKind::Range { .. }
-                | TestKind::Eq { .. }
+                | TestKind::StringEq { .. }
+                | TestKind::ScalarEq { .. }
                 | TestKind::Deref { .. },
                 _,
             ) => {

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

@@ -2,6 +2,7 @@ use std::sync::Arc;
 
 use rustc_abi::FieldIdx;
 use rustc_middle::mir::*;
+use rustc_middle::span_bug;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
@@ -173,9 +174,21 @@ impl<'tcx> MatchPairTree<'tcx> {
                     PatConstKind::IntOrChar
                 } else if pat_ty.is_floating_point() {
                     PatConstKind::Float
+                } else if pat_ty.is_str() {
+                    // Deref-patterns can cause string-literal patterns to have
+                    // type `str` instead of the usual `&str`.
+                    if !cx.tcx.features().deref_patterns() {
+                        span_bug!(
+                            pattern.span,
+                            "const pattern has type `str` but deref_patterns is not enabled"
+                        );
+                    }
+                    PatConstKind::String
+                } else if pat_ty.is_imm_ref_str() {
+                    PatConstKind::String
                 } else {
                     // FIXME(Zalathar): This still covers several different
-                    // categories (e.g. raw pointer, string, pattern-type)
+                    // categories (e.g. raw pointer, pattern-type)
                     // which could be split out into their own kinds.
                     PatConstKind::Other
                 };

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

@@ -1290,9 +1290,10 @@ enum PatConstKind {
     /// These types don't support `SwitchInt` and require an equality test,
     /// but can also interact with range pattern tests.
     Float,
+    /// Constant string values, tested via string equality.
+    String,
     /// Any other constant-pattern is usually tested via some kind of equality
     /// check. Types that might be encountered here include:
-    /// - `&str`
     /// - raw pointers derived from integer values
     /// - pattern types, e.g. `pattern_type!(u32 is 1..)`
     Other,
@@ -1368,14 +1369,20 @@ enum TestKind<'tcx> {
     /// Test whether a `bool` is `true` or `false`.
     If,
 
-    /// Test for equality with value, possibly after an unsizing coercion to
-    /// `cast_ty`,
-    Eq {
+    /// Tests the place against a string constant using string equality.
+    StringEq {
+        /// Constant `&str` value to test against.
         value: ty::Value<'tcx>,
-        // Integer types are handled by `SwitchInt`, and constants with ADT
-        // types and `&[T]` types are converted back into patterns, so this can
-        // only be `&str` or floats.
-        cast_ty: Ty<'tcx>,
+        /// Type of the corresponding pattern node. Usually `&str`, but could
+        /// be `str` for patterns like `deref!("..."): String`.
+        pat_ty: Ty<'tcx>,
+    },
+
+    /// Tests the place against a constant using scalar equality.
+    ScalarEq {
+        value: ty::Value<'tcx>,
+        /// Type of the corresponding pattern node.
+        pat_ty: Ty<'tcx>,
     },
 
     /// Test whether the value falls within an inclusive or exclusive range.

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

@@ -38,11 +38,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             TestableCase::Constant { value: _, kind: PatConstKind::IntOrChar } => {
                 TestKind::SwitchInt
             }
-            TestableCase::Constant { value, kind: PatConstKind::Float } => {
-                TestKind::Eq { value, cast_ty: match_pair.pattern_ty }
+            TestableCase::Constant { value, kind: PatConstKind::String } => {
+                TestKind::StringEq { value, pat_ty: match_pair.pattern_ty }
             }
-            TestableCase::Constant { value, kind: PatConstKind::Other } => {
-                TestKind::Eq { value, cast_ty: match_pair.pattern_ty }
+            TestableCase::Constant { value, kind: PatConstKind::Float | PatConstKind::Other } => {
+                TestKind::ScalarEq { value, pat_ty: match_pair.pattern_ty }
             }
 
             TestableCase::Range(ref range) => {
@@ -141,17 +141,19 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.cfg.terminate(block, self.source_info(match_start_span), terminator);
             }
 
-            TestKind::Eq { value, mut cast_ty } => {
+            TestKind::StringEq { value, pat_ty } => {
                 let tcx = self.tcx;
                 let success_block = target_block(TestBranch::Success);
                 let fail_block = target_block(TestBranch::Failure);
 
-                let mut expect_ty = value.ty;
-                let mut expect = self.literal_operand(test.span, Const::from_ty_value(tcx, value));
+                let expected_value_ty = value.ty;
+                let expected_value_operand =
+                    self.literal_operand(test.span, Const::from_ty_value(tcx, value));
 
-                let mut place = place;
+                let mut actual_value_ty = pat_ty;
+                let mut actual_value_place = place;
 
-                match cast_ty.kind() {
+                match pat_ty.kind() {
                     ty::Str => {
                         // String literal patterns may have type `str` if `deref_patterns` is
                         // enabled, in order to allow `deref!("..."): String`. In this case, `value`
@@ -172,74 +174,85 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             ref_place,
                             Rvalue::Ref(re_erased, BorrowKind::Shared, place),
                         );
-                        place = ref_place;
-                        cast_ty = ref_str_ty;
+                        actual_value_place = ref_place;
+                        actual_value_ty = ref_str_ty;
                     }
+                    _ => {}
+                }
+
+                assert_eq!(expected_value_ty, actual_value_ty);
+                assert!(actual_value_ty.is_imm_ref_str());
+
+                // 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(
+                    block,
+                    success_block,
+                    fail_block,
+                    source_info,
+                    expected_value_operand,
+                    Operand::Copy(actual_value_place),
+                );
+            }
+
+            TestKind::ScalarEq { value, pat_ty } => {
+                let tcx = self.tcx;
+                let success_block = target_block(TestBranch::Success);
+                let fail_block = target_block(TestBranch::Failure);
+
+                let mut expected_value_ty = value.ty;
+                let mut expected_value_operand =
+                    self.literal_operand(test.span, Const::from_ty_value(tcx, value));
+
+                let mut actual_value_ty = pat_ty;
+                let mut actual_value_place = place;
+
+                match pat_ty.kind() {
                     &ty::Pat(base, _) => {
-                        assert_eq!(cast_ty, value.ty);
+                        assert_eq!(pat_ty, value.ty);
                         assert!(base.is_trivially_pure_clone_copy());
 
                         let transmuted_place = self.temp(base, test.span);
                         self.cfg.push_assign(
                             block,
                             self.source_info(scrutinee_span),
                             transmuted_place,
-                            Rvalue::Cast(CastKind::Transmute, Operand::Copy(place), base),
+                            Rvalue::Cast(
+                                CastKind::Transmute,
+                                Operand::Copy(actual_value_place),
+                                base,
+                            ),
                         );
 
                         let transmuted_expect = self.temp(base, test.span);
                         self.cfg.push_assign(
                             block,
                             self.source_info(test.span),
                             transmuted_expect,
-                            Rvalue::Cast(CastKind::Transmute, expect, base),
+                            Rvalue::Cast(CastKind::Transmute, expected_value_operand, base),
                         );
 
-                        place = transmuted_place;
-                        expect = Operand::Copy(transmuted_expect);
-                        cast_ty = base;
-                        expect_ty = base;
+                        actual_value_place = transmuted_place;
+                        actual_value_ty = base;
+                        expected_value_operand = Operand::Copy(transmuted_expect);
+                        expected_value_ty = base;
                     }
                     _ => {}
                 }
 
-                assert_eq!(expect_ty, cast_ty);
-                if !cast_ty.is_scalar() {
-                    // Use `PartialEq::eq` instead of `BinOp::Eq`
-                    // (the binop can only handle primitives)
-                    // Make sure that we do *not* call any user-defined code here.
-                    // The only type that can end up here is string literals, which have their
-                    // comparison defined in `core`.
-                    // (Interestingly this means that exhaustiveness analysis relies, for soundness,
-                    // on the `PartialEq` impl for `str` to b correct!)
-                    match *cast_ty.kind() {
-                        ty::Ref(_, deref_ty, _) if deref_ty == self.tcx.types.str_ => {}
-                        _ => {
-                            span_bug!(
-                                source_info.span,
-                                "invalid type for non-scalar compare: {cast_ty}"
-                            )
-                        }
-                    };
-                    self.string_compare(
-                        block,
-                        success_block,
-                        fail_block,
-                        source_info,
-                        expect,
-                        Operand::Copy(place),
-                    );
-                } else {
-                    self.compare(
-                        block,
-                        success_block,
-                        fail_block,
-                        source_info,
-                        BinOp::Eq,
-                        expect,
-                        Operand::Copy(place),
-                    );
-                }
+                assert_eq!(expected_value_ty, actual_value_ty);
+                assert!(actual_value_ty.is_scalar());
+
+                self.compare(
+                    block,
+                    success_block,
+                    fail_block,
+                    source_info,
+                    BinOp::Eq,
+                    expected_value_operand,
+                    Operand::Copy(actual_value_place),
+                );
             }
 
             TestKind::Range(ref range) => {