Don't hoist ref locals

src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/IteratorAndAsyncCaptureWalker.cs

@@ -121,7 +121,7 @@ private static bool HoistInDebugBuild(Symbol symbol)
                 ParameterSymbol parameter =>
                     // in Debug build hoist all parameters that can be hoisted:
                     !parameter.Type.IsRestrictedType(),
-                LocalSymbol { IsConst: false, IsPinned: false } local =>
+                LocalSymbol { IsConst: false, IsPinned: false, IsRef: false } local =>
                     // hoist all user-defined locals and long-lived temps that can be hoisted:
                     local.SynthesizedKind.MustSurviveStateMachineSuspension() &&
                     !local.Type.IsRestrictedType(),

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncTests.cs

@@ -5606,5 +5606,60 @@ static async Task Main()
             var comp = CSharpTestBase.CreateCompilation(source, options: TestOptions.ReleaseExe);
             CompileAndVerify(comp, expectedOutput: "StructAwaitable");
         }
+
+        [Fact, WorkItem(40251, "https://github.com/dotnet/roslyn/issues/40251")]
+        public void AssignRefAfterAwait()
+        {
+            const string source = @"
+using System.Threading.Tasks;
+
+class IntCode
+{
+    public async Task Step(int i, Task t)
+    {
+        await t;
+        ReadMemory() = i switch
+        {
+            _ => throw null
+        };
+    }
+
+    private ref long ReadMemory() => throw null;
+}
+";
+
+            var comp = CreateCompilation(source, options: TestOptions.DebugDll);
+            comp.VerifyEmitDiagnostics();
+            comp = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact, WorkItem(40251, "https://github.com/dotnet/roslyn/issues/40251")]
+        public void AssignRefWithAwait()
+        {
+            const string source = @"
+using System.Threading.Tasks;
+
+class IntCode
+{
+    public async Task Step(Task<int> t)
+    {
+        ReadMemory() = await t;
+    }
+
+    private ref long ReadMemory() => throw null;
+}
+";
+            var expected = new[]
+            {
+                // (8,9): error CS8178: 'await' cannot be used in an expression containing a call to 'IntCode.ReadMemory()' because it returns by reference
+                //         ReadMemory() = await t;
+                Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, "ReadMemory()").WithArguments("IntCode.ReadMemory()").WithLocation(8, 9)
+            };
+            var comp = CreateCompilation(source, options: TestOptions.DebugDll);
+            comp.VerifyEmitDiagnostics(expected);
+            comp = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            comp.VerifyEmitDiagnostics(expected);
+        }
     }
 }