Add a hidden sequence point around the code that assigns pattern variables in switch (#14789)

Fixes #14740

src/Compilers/CSharp/Portable/Lowering/Instrumentation/CompoundInstrumenter.cs

@@ -208,6 +208,11 @@ public override BoundExpression InstrumentSwitchStatementExpression(BoundStateme
             return Previous.InstrumentSwitchStatementExpression(original, rewrittenExpression, factory);
         }
 
+        public override BoundStatement InstrumentPatternSwitchBindCasePatternVariables(BoundStatement bindings)
+        {
+            return Previous.InstrumentPatternSwitchBindCasePatternVariables(bindings);
+        }
+
         public override BoundStatement InstrumentForEachStatementDeconstructionVariablesDeclaration(BoundForEachStatement original, BoundStatement iterationVarDecl)
         {
             return Previous.InstrumentForEachStatementDeconstructionVariablesDeclaration(original, iterationVarDecl);

src/Compilers/CSharp/Portable/Lowering/Instrumentation/DebugInfoInjector.cs

@@ -431,5 +431,11 @@ public override BoundExpression InstrumentSwitchStatementExpression(BoundStateme
             return AddConditionSequencePoint(base.InstrumentSwitchStatementExpression(original, rewrittenExpression, factory), original.Syntax, factory);
         }
 
+        public override BoundStatement InstrumentPatternSwitchBindCasePatternVariables(BoundStatement bindings)
+        {
+            // Mark the code that binds pattern variables to their values as hidden.
+            // We do it to tell that this is not a part of previous statement.
+            return new BoundSequencePoint(null, base.InstrumentPatternSwitchBindCasePatternVariables(bindings));
+        }
     }
 }
\ No newline at end of file

src/Compilers/CSharp/Portable/Lowering/Instrumentation/Instrumenter.cs

@@ -298,5 +298,10 @@ public virtual BoundExpression InstrumentSwitchStatementExpression(BoundStatemen
             Debug.Assert(factory != null);
             return rewrittenExpression;
         }
+
+        public virtual BoundStatement InstrumentPatternSwitchBindCasePatternVariables(BoundStatement bindings)
+        {
+            return bindings;
+        }
     }
 }
\ No newline at end of file

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_PatternSwitchStatement.cs

@@ -278,6 +278,43 @@ private void LowerConstantValueDecision(DecisionTree.ByValue byValue)
                 LowerDecisionTree(byValue.Expression, byValue.Default);
             }
 
+            /// <summary>
+            /// Add a branch in the lowered decision tree to a label for a matched
+            /// pattern, and then produce a statement for the target of that branch
+            /// that binds the pattern variables.
+            /// </summary>
+            /// <param name="bindings">The source/destination pairs for the assignments</param>
+            private BoundStatement BindingsForCase(
+                ImmutableArray<KeyValuePair<BoundExpression, BoundExpression>> bindings)
+            {
+                var patternMatched = _factory.GenerateLabel("patternMatched");
+                _loweredDecisionTree.Add(_factory.Goto(patternMatched));
+
+                var addBindings = ArrayBuilder<BoundStatement>.GetInstance();
+                addBindings.Add(_factory.Label(patternMatched));
+                if (!bindings.IsDefaultOrEmpty)
+                {
+                    foreach (var kv in bindings)
+                    {
+                        var source = kv.Key;
+                        var dest = kv.Value;
+                        var rewriter = this.LocalRewriter;
+                            addBindings.Add(_factory.ExpressionStatement(
+                            rewriter.MakeStaticAssignmentOperator(
+                                _factory.Syntax, rewriter.VisitExpression(dest), rewriter.VisitExpression(source), RefKind.None, dest.Type, false)));
+                    }
+                }
+
+                BoundStatement result = _factory.Block(addBindings.ToImmutableAndFree());
+                if (this.LocalRewriter.Instrument)
+                {
+                    // Hide the code that binds pattern variables in a hidden sequence point
+                    result = this.LocalRewriter._instrumenter.InstrumentPatternSwitchBindCasePatternVariables(result);
+                }
+
+                return result;
+            }
+
             private void LowerDecisionTree(DecisionTree.Guarded guarded)
             {
                 var sectionBuilder = this.SwitchSections[guarded.Section];
@@ -294,20 +331,13 @@ private void LowerDecisionTree(DecisionTree.Guarded guarded)
                     else
                     {
                         // with bindings
-                        var matched = _factory.GenerateLabel("matched");
-                        _loweredDecisionTree.Add(_factory.Goto(matched));
-                        sectionBuilder.Add(_factory.Label(matched));
-                        AddBindings(sectionBuilder, guarded.Bindings);
+                        sectionBuilder.Add(BindingsForCase(guarded.Bindings));
                         sectionBuilder.Add(_factory.Goto(targetLabel));
                     }
                 }
                 else
                 {
-                    var checkGuard = _factory.GenerateLabel("checkGuard");
-                    _loweredDecisionTree.Add(_factory.Goto(checkGuard));
-                    sectionBuilder.Add(_factory.Label(checkGuard));
-                    AddBindings(sectionBuilder, guarded.Bindings);
-
+                    sectionBuilder.Add(BindingsForCase(guarded.Bindings));
                     var guardTest = _factory.ConditionalGoto(LocalRewriter.VisitExpression(guarded.Guard), targetLabel, true);
 
                     // Only add instrumentation (such as a sequence point) if the node is not compiler-generated.
@@ -326,22 +356,6 @@ private void LowerDecisionTree(DecisionTree.Guarded guarded)
                 }
             }
 
-            private void AddBindings(ArrayBuilder<BoundStatement> sectionBuilder, ImmutableArray<KeyValuePair<BoundExpression, BoundExpression>> bindings)
-            {
-                if (!bindings.IsDefaultOrEmpty)
-                {
-                    foreach (var kv in bindings)
-                    {
-                        var source = kv.Key;
-                        var dest = kv.Value;
-                        var rewriter = this.LocalRewriter;
-                        sectionBuilder.Add(_factory.ExpressionStatement(
-                            rewriter.MakeStaticAssignmentOperator(
-                                _factory.Syntax, rewriter.VisitExpression(dest), rewriter.VisitExpression(source), RefKind.None, dest.Type, false)));
-                    }
-                }
-            }
-
             // For switch statements, we have an option of completely rewriting the switch header
             // and switch sections into simpler constructs, i.e. we can rewrite the switch header
             // using bound conditional goto statements and the rewrite the switch sections into

src/Compilers/CSharp/Test/Emit/PDB/PDBTests.cs

@@ -2305,9 +2305,12 @@ class Student : Person { public double GPA; }
         <entry offset=""0x0"" startLine=""19"" startColumn=""5"" endLine=""19"" endColumn=""6"" />
         <entry offset=""0x1"" startLine=""20"" startColumn=""9"" endLine=""20"" endColumn=""19"" />
         <entry offset=""0x4"" hidden=""true"" />
+        <entry offset=""0x32"" hidden=""true"" />
         <entry offset=""0x35"" startLine=""22"" startColumn=""28"" endLine=""22"" endColumn=""44"" />
         <entry offset=""0x49"" startLine=""23"" startColumn=""17"" endLine=""23"" endColumn=""57"" />
+        <entry offset=""0x6a"" hidden=""true"" />
         <entry offset=""0x6f"" startLine=""25"" startColumn=""17"" endLine=""25"" endColumn=""57"" />
+        <entry offset=""0x90"" hidden=""true"" />
         <entry offset=""0x95"" startLine=""27"" startColumn=""17"" endLine=""27"" endColumn=""59"" />
         <entry offset=""0xb1"" startLine=""29"" startColumn=""17"" endLine=""29"" endColumn=""43"" />
         <entry offset=""0xc5"" startLine=""31"" startColumn=""5"" endLine=""31"" endColumn=""6"" />