support locals before return in block-bodied switch cases

src/ExpressiveSharp.Generator/Emitter/ExpressionTreeEmitter.cs

@@ -2107,8 +2107,12 @@ private string EmitSwitchStatement(ISwitchOperation switchStmt)
         return currentVar;
     }
 
-    /// <summary>Returns the single return operation in a case body, or null if the shape isn't supported.</summary>
-    private static IReturnOperation? FindCaseReturn(ISwitchCaseOperation switchCase)
+    /// <summary>
+    /// Returns the case's return operation, inlining any leading local declarations (their
+    /// initializer expressions are mapped so the returned value embeds them and evaluation stays
+    /// inside the arm); null if the shape isn't supported.
+    /// </summary>
+    private IReturnOperation? FindCaseReturn(ISwitchCaseOperation switchCase)
     {
         foreach (var op in switchCase.Body)
         {
@@ -2123,6 +2127,10 @@ private string EmitSwitchStatement(ISwitchOperation switchStmt)
                         {
                             return innerRet;
                         }
+                        if (inner is IVariableDeclarationGroupOperation declGroup && TryInlineCaseLocals(declGroup))
+                        {
+                            continue;
+                        }
                         return null;
                     }
                     return null;
@@ -2133,6 +2141,24 @@ private string EmitSwitchStatement(ISwitchOperation switchStmt)
         return null;
     }
 
+    private bool TryInlineCaseLocals(IVariableDeclarationGroupOperation declGroup)
+    {
+        foreach (var declaration in declGroup.Declarations)
+        {
+            foreach (var declarator in declaration.Declarators)
+            {
+                if (declarator.Initializer is null)
+                {
+                    return false;
+                }
+
+                _localToVar[declarator.Symbol] = EmitOperation(declarator.Initializer.Value);
+            }
+        }
+
+        return true;
+    }
+
     private string EmitSwitchExpression(ISwitchExpressionOperation switchExpr)
     {
         var governingVar = EmitOperation(switchExpr.Value);

tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/BlockBodyTests.cs

@@ -389,4 +389,36 @@ public int Foo()
 
         return Verifier.Verify(result.GeneratedTrees[0].ToString());
     }
+
+    [TestMethod]
+    public void SwitchCase_LocalBeforeReturn_IsSupported()
+    {
+        var compilation = CreateCompilation(
+            """
+            namespace Foo {
+                class C {
+                    [Expressive(AllowBlockBody = true)]
+                    public static string Label(int x)
+                    {
+                        switch (x)
+                        {
+                            case 1:
+                            {
+                                var y = "one";
+                                return y;
+                            }
+                            default:
+                                return "other";
+                        }
+                    }
+                }
+            }
+            """);
+
+        var result = RunExpressiveGenerator(compilation);
+
+        Assert.IsFalse(result.Diagnostics.Any(d => d.Id == "EXP0008"),
+            "A case block with a local before the return should be emitted, not degraded to a " +
+            "default value via EXP0008. Diagnostics: " + string.Join("; ", result.Diagnostics));
+    }
 }