Fix #4019: Recognize with expressions whose initializer awaits

Roslyn spills the receiver of an initializer assignment that precedes an
await into a hoisted temporary, so the clone of a with expression (and the
object of an object initializer) reaches the initializer transform through
a copy it rejects. Only copies between two hoisted compiler temporaries are
inlined: folding copies of hoisted user locals as well would turn a user's
local into an initializer.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/AsyncAwaitPatterns.cs

@@ -16,9 +16,9 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-// Await shapes that still decompile to code that does not compile are tracked as #4017 (type
-// parameter with an interface constraint), #4018 (static dynamic call) and #4019 (with
-// expression); they are absent here because they have no correct output to pin yet.
+// An await shape that still decompiles to code that does not compile is tracked as #4017 (type
+// parameter with an interface constraint); it is absent here because it has no correct output to
+// pin yet.
 
 #pragma warning disable 1998
 using System;

ICSharpCode.Decompiler.Tests/TestCases/Pretty/InitializerTests.cs

@@ -22,6 +22,9 @@ using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
 using System.Threading;
+#if ROSLYN2
+using System.Threading.Tasks;
+#endif
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.InitializerTests
 {
@@ -1233,6 +1236,23 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.InitializerTests
 				Property = 42
 			};
 		}
+
+#if ROSLYN2
+		public static async Task<C> ObjectInitializerWithAwait(Task<int> task)
+		{
+			return new C {
+				Z = await task
+			};
+		}
+
+		public static async Task<C> ObjectInitializerWithAwaitAfterConstant(Task<S> task)
+		{
+			return new C {
+				Z = 42,
+				Y = await task
+			};
+		}
+#endif
 		#endregion
 
 		#region Collection initializer

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Records.cs

@@ -3,6 +3,7 @@ using System;
 using System.Runtime.InteropServices;
 using System.Text;
 #endif
+using System.Threading.Tasks;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
@@ -129,6 +130,19 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 					}
 				};
 			}
+			public async Task<PrimaryCtor> WithAwait(PrimaryCtor input)
+			{
+				return input with {
+					A = await Task.FromResult(42)
+				};
+			}
+			public async Task<Fields> WithAwaitAfterConstant(Fields input)
+			{
+				return input with {
+					A = 42,
+					B = await Task.FromResult(3.141)
+				};
+			}
 		}
 
 		public abstract record WithNestedRecords

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -668,12 +668,17 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				case OpCode.SwitchInstruction:
 					return true;
 				case OpCode.LdLoc:
-					if (v.StateMachineField == null && ((LdLoc)inlinedExpression).Variable.StateMachineField != null)
+					var copiedVariable = ((LdLoc)inlinedExpression).Variable;
+					if (v.StateMachineField == null && copiedVariable.StateMachineField != null)
 					{
 						// Roslyn likes to put the result of fetching a state machine field into a temporary variable,
 						// so inline more aggressively in such cases.
 						return true;
 					}
+					if (IsHoistedCompilerTemporary(v) && IsHoistedCompilerTemporary(copiedVariable))
+					{
+						return true;
+					}
 					break;
 			}
 			if (inlinedExpression.ResultType == StackType.Ref)
@@ -773,6 +778,13 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
+		static bool IsHoistedCompilerTemporary(ILVariable v)
+		{
+			string name = v.StateMachineField?.Name;
+			return name != null
+				&& (name.StartsWith("<>7__wrap", StringComparison.Ordinal) || name.StartsWith("<>s__", StringComparison.Ordinal));
+		}
+
 		/// <summary>
 		/// Gets whether 'expressionBeingMoved' can be inlined into 'expr'.
 		/// </summary>