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ccrewrite chokes on async preconditions with using in VS2017 #481

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yaakov-h opened this issue Feb 14, 2017 · 0 comments
Open

ccrewrite chokes on async preconditions with using in VS2017 #481

yaakov-h opened this issue Feb 14, 2017 · 0 comments

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@yaakov-h
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Using Visual Studio 2017 Enterprise RC, 15.0.0-RC.4+26206.0

Output:

error CC1017: Malformed contract section in method 'AsyncContracts.AsyncContracts+<DoTheThingSlowly>d__0.MoveNext'

Code that triggers this bug:

using System;
using System.Diagnostics.Contracts;
using System.IO;
using System.Threading.Tasks;

namespace AsyncContracts
{
	public class AsyncContracts
	{
		public static async Task<string> DoTheThingSlowly(object o)
		{
			Contract.Requires(o != null);

			await Task.Delay(TimeSpan.FromSeconds(1));

			using (var s = new MemoryStream())
			{
				await Task.Delay(TimeSpan.FromSeconds(1));
				await Task.Delay(TimeSpan.FromSeconds(1));
				return o.ToString();
			}
		}
	}
}

IL of the generated async state machine:

.class nested private auto ansi sealed beforefieldinit '<DoTheThingSlowly>d__0'
	extends [mscorlib]System.ValueType
	implements [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine
{
	.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
		01 00 00 00
	)
	// Fields
	.field public int32 '<>1__state'
	.field public valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> '<>t__builder'
	.field public object o
	.field private class [mscorlib]System.IO.MemoryStream '<s>5__1'
	.field private valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter '<>u__1'

	// Methods
	.method private final hidebysig newslot virtual 
		instance void MoveNext () cil managed 
	{
		.override method instance void [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
		// Method begins at RVA 0xbe2c
		// Code size 476 (0x1dc)
		.maxstack 3
		.locals init (
			[0] int32,
			[1] string,
			[2] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[3] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[4] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[5] class [mscorlib]System.Exception
		)

		IL_0000: ldarg.0
		IL_0001: ldfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
		IL_0006: stloc.0
		.try
		{
			IL_0007: ldloc.0
			IL_0008: brfalse.s IL_0066

			IL_000a: ldloc.0
			IL_000b: ldc.i4.1
			IL_000c: sub
			IL_000d: ldc.i4.1
			IL_000e: ble.un IL_009c

			IL_0013: ldarg.0
			IL_0014: ldfld object AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::o
			IL_0019: ldnull
			IL_001a: cgt.un
			IL_001c: call void [mscorlib]System.Diagnostics.Contracts.Contract::Requires(bool)
			IL_0021: ldc.r8 1
			IL_002a: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
			IL_002f: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
			IL_0034: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
			IL_0039: stloc.2
			IL_003a: ldloca.s 2
			IL_003c: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
			IL_0041: brtrue.s IL_0082

			IL_0043: ldarg.0
			IL_0044: ldc.i4.0
			IL_0045: dup
			IL_0046: stloc.0
			IL_0047: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
			IL_004c: ldarg.0
			IL_004d: ldloc.2
			IL_004e: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_0053: ldarg.0
			IL_0054: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
			IL_0059: ldloca.s 2
			IL_005b: ldarg.0
			IL_005c: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
			IL_0061: leave IL_01db

			IL_0066: ldarg.0
			IL_0067: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_006c: stloc.2
			IL_006d: ldarg.0
			IL_006e: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_0073: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
			IL_0079: ldarg.0
			IL_007a: ldc.i4.m1
			IL_007b: dup
			IL_007c: stloc.0
			IL_007d: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

			IL_0082: ldloca.s 2
			IL_0084: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
			IL_0089: ldloca.s 2
			IL_008b: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
			IL_0091: ldarg.0
			IL_0092: newobj instance void [mscorlib]System.IO.MemoryStream::.ctor()
			IL_0097: stfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'

			IL_009c: nop
			.try
			{
				IL_009d: ldloc.0
				IL_009e: ldc.i4.1
				IL_009f: beq.s IL_00ed

				IL_00a1: ldloc.0
				IL_00a2: ldc.i4.2
				IL_00a3: beq IL_015c

				IL_00a8: ldc.r8 1
				IL_00b1: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
				IL_00b6: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
				IL_00bb: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
				IL_00c0: stloc.3
				IL_00c1: ldloca.s 3
				IL_00c3: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
				IL_00c8: brtrue.s IL_0109

				IL_00ca: ldarg.0
				IL_00cb: ldc.i4.1
				IL_00cc: dup
				IL_00cd: stloc.0
				IL_00ce: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
				IL_00d3: ldarg.0
				IL_00d4: ldloc.3
				IL_00d5: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_00da: ldarg.0
				IL_00db: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
				IL_00e0: ldloca.s 3
				IL_00e2: ldarg.0
				IL_00e3: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
				IL_00e8: leave IL_01db

				IL_00ed: ldarg.0
				IL_00ee: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_00f3: stloc.3
				IL_00f4: ldarg.0
				IL_00f5: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_00fa: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0100: ldarg.0
				IL_0101: ldc.i4.m1
				IL_0102: dup
				IL_0103: stloc.0
				IL_0104: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

				IL_0109: ldloca.s 3
				IL_010b: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
				IL_0110: ldloca.s 3
				IL_0112: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0118: ldc.r8 1
				IL_0121: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
				IL_0126: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
				IL_012b: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
				IL_0130: stloc.s 4
				IL_0132: ldloca.s 4
				IL_0134: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
				IL_0139: brtrue.s IL_0179

				IL_013b: ldarg.0
				IL_013c: ldc.i4.2
				IL_013d: dup
				IL_013e: stloc.0
				IL_013f: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
				IL_0144: ldarg.0
				IL_0145: ldloc.s 4
				IL_0147: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_014c: ldarg.0
				IL_014d: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
				IL_0152: ldloca.s 4
				IL_0154: ldarg.0
				IL_0155: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
				IL_015a: leave.s IL_01db

				IL_015c: ldarg.0
				IL_015d: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_0162: stloc.s 4
				IL_0164: ldarg.0
				IL_0165: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_016a: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0170: ldarg.0
				IL_0171: ldc.i4.m1
				IL_0172: dup
				IL_0173: stloc.0
				IL_0174: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

				IL_0179: ldloca.s 4
				IL_017b: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
				IL_0180: ldloca.s 4
				IL_0182: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0188: ldarg.0
				IL_0189: ldfld object AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::o
				IL_018e: callvirt instance string [mscorlib]System.Object::ToString()
				IL_0193: stloc.1
				IL_0194: leave.s IL_01c7
			} // end .try
			finally
			{
				IL_0196: ldloc.0
				IL_0197: ldc.i4.0
				IL_0198: bge.s IL_01ad

				IL_019a: ldarg.0
				IL_019b: ldfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'
				IL_01a0: brfalse.s IL_01ad

				IL_01a2: ldarg.0
				IL_01a3: ldfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'
				IL_01a8: callvirt instance void [mscorlib]System.IDisposable::Dispose()

				IL_01ad: endfinally
			} // end handler
		} // end .try
		catch [mscorlib]System.Exception
		{
			IL_01ae: stloc.s 5
			IL_01b0: ldarg.0
			IL_01b1: ldc.i4.s -2
			IL_01b3: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
			IL_01b8: ldarg.0
			IL_01b9: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
			IL_01be: ldloc.s 5
			IL_01c0: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetException(class [mscorlib]System.Exception)
			IL_01c5: leave.s IL_01db
		} // end handler

		IL_01c7: ldarg.0
		IL_01c8: ldc.i4.s -2
		IL_01ca: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
		IL_01cf: ldarg.0
		IL_01d0: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
		IL_01d5: ldloc.1
		IL_01d6: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetResult(!0)

		IL_01db: ret
	} // end of method '<DoTheThingSlowly>d__0'::MoveNext

	.method private final hidebysig newslot virtual 
		instance void SetStateMachine (
			class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
		) cil managed 
	{
		.custom instance void [mscorlib]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
			01 00 00 00
		)
		.override method instance void [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine)
		// Method begins at RVA 0xc048
		// Code size 13 (0xd)
		.maxstack 8

		IL_0000: ldarg.0
		IL_0001: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
		IL_0006: ldarg.1
		IL_0007: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetStateMachine(class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine)
		IL_000c: ret
	} // end of method '<DoTheThingSlowly>d__0'::SetStateMachine

} // end of class <DoTheThingSlowly>d__0

IL from VS2015, this works with ccrewrite:

.class nested private auto ansi sealed beforefieldinit '<DoTheThingSlowly>d__0'
	extends [mscorlib]System.ValueType
	implements [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine
{
	.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
		01 00 00 00
	)
	// Fields
	.field public int32 '<>1__state'
	.field public valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> '<>t__builder'
	.field public object o
	.field private class [mscorlib]System.IO.MemoryStream '<s>5__1'
	.field private valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter '<>u__1'

	// Methods
	.method private final hidebysig newslot virtual 
		instance void MoveNext () cil managed 
	{
		.override method instance void [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
		// Method begins at RVA 0xbe2c
		// Code size 482 (0x1e2)
		.maxstack 3
		.locals init (
			[0] int32,
			[1] string,
			[2] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[3] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[4] valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter,
			[5] class [mscorlib]System.Exception
		)

		IL_0000: ldarg.0
		IL_0001: ldfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
		IL_0006: stloc.0
		.try
		{
			IL_0007: ldloc.0
			IL_0008: switch (IL_006c, IL_00a2, IL_00a2)

			IL_0019: ldarg.0
			IL_001a: ldfld object AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::o
			IL_001f: ldnull
			IL_0020: cgt.un
			IL_0022: call void [mscorlib]System.Diagnostics.Contracts.Contract::Requires(bool)
			IL_0027: ldc.r8 1
			IL_0030: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
			IL_0035: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
			IL_003a: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
			IL_003f: stloc.2
			IL_0040: ldloca.s 2
			IL_0042: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
			IL_0047: brtrue.s IL_0088

			IL_0049: ldarg.0
			IL_004a: ldc.i4.0
			IL_004b: dup
			IL_004c: stloc.0
			IL_004d: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
			IL_0052: ldarg.0
			IL_0053: ldloc.2
			IL_0054: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_0059: ldarg.0
			IL_005a: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
			IL_005f: ldloca.s 2
			IL_0061: ldarg.0
			IL_0062: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
			IL_0067: leave IL_01e1

			IL_006c: ldarg.0
			IL_006d: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_0072: stloc.2
			IL_0073: ldarg.0
			IL_0074: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
			IL_0079: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
			IL_007f: ldarg.0
			IL_0080: ldc.i4.m1
			IL_0081: dup
			IL_0082: stloc.0
			IL_0083: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

			IL_0088: ldloca.s 2
			IL_008a: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
			IL_008f: ldloca.s 2
			IL_0091: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
			IL_0097: ldarg.0
			IL_0098: newobj instance void [mscorlib]System.IO.MemoryStream::.ctor()
			IL_009d: stfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'

			IL_00a2: nop
			.try
			{
				IL_00a3: ldloc.0
				IL_00a4: ldc.i4.1
				IL_00a5: beq.s IL_00f3

				IL_00a7: ldloc.0
				IL_00a8: ldc.i4.2
				IL_00a9: beq IL_0162

				IL_00ae: ldc.r8 1
				IL_00b7: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
				IL_00bc: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
				IL_00c1: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
				IL_00c6: stloc.3
				IL_00c7: ldloca.s 3
				IL_00c9: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
				IL_00ce: brtrue.s IL_010f

				IL_00d0: ldarg.0
				IL_00d1: ldc.i4.1
				IL_00d2: dup
				IL_00d3: stloc.0
				IL_00d4: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
				IL_00d9: ldarg.0
				IL_00da: ldloc.3
				IL_00db: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_00e0: ldarg.0
				IL_00e1: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
				IL_00e6: ldloca.s 3
				IL_00e8: ldarg.0
				IL_00e9: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
				IL_00ee: leave IL_01e1

				IL_00f3: ldarg.0
				IL_00f4: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_00f9: stloc.3
				IL_00fa: ldarg.0
				IL_00fb: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_0100: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0106: ldarg.0
				IL_0107: ldc.i4.m1
				IL_0108: dup
				IL_0109: stloc.0
				IL_010a: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

				IL_010f: ldloca.s 3
				IL_0111: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
				IL_0116: ldloca.s 3
				IL_0118: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_011e: ldc.r8 1
				IL_0127: call valuetype [mscorlib]System.TimeSpan [mscorlib]System.TimeSpan::FromSeconds(float64)
				IL_012c: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::Delay(valuetype [mscorlib]System.TimeSpan)
				IL_0131: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter [mscorlib]System.Threading.Tasks.Task::GetAwaiter()
				IL_0136: stloc.s 4
				IL_0138: ldloca.s 4
				IL_013a: call instance bool [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::get_IsCompleted()
				IL_013f: brtrue.s IL_017f

				IL_0141: ldarg.0
				IL_0142: ldc.i4.2
				IL_0143: dup
				IL_0144: stloc.0
				IL_0145: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
				IL_014a: ldarg.0
				IL_014b: ldloc.s 4
				IL_014d: stfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_0152: ldarg.0
				IL_0153: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
				IL_0158: ldloca.s 4
				IL_015a: ldarg.0
				IL_015b: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::AwaitUnsafeOnCompleted<valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter, valuetype AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'>(!!0&, !!1&)
				IL_0160: leave.s IL_01e1

				IL_0162: ldarg.0
				IL_0163: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_0168: stloc.s 4
				IL_016a: ldarg.0
				IL_016b: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>u__1'
				IL_0170: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_0176: ldarg.0
				IL_0177: ldc.i4.m1
				IL_0178: dup
				IL_0179: stloc.0
				IL_017a: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'

				IL_017f: ldloca.s 4
				IL_0181: call instance void [mscorlib]System.Runtime.CompilerServices.TaskAwaiter::GetResult()
				IL_0186: ldloca.s 4
				IL_0188: initobj [mscorlib]System.Runtime.CompilerServices.TaskAwaiter
				IL_018e: ldarg.0
				IL_018f: ldfld object AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::o
				IL_0194: callvirt instance string [mscorlib]System.Object::ToString()
				IL_0199: stloc.1
				IL_019a: leave.s IL_01cd
			} // end .try
			finally
			{
				IL_019c: ldloc.0
				IL_019d: ldc.i4.0
				IL_019e: bge.s IL_01b3

				IL_01a0: ldarg.0
				IL_01a1: ldfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'
				IL_01a6: brfalse.s IL_01b3

				IL_01a8: ldarg.0
				IL_01a9: ldfld class [mscorlib]System.IO.MemoryStream AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<s>5__1'
				IL_01ae: callvirt instance void [mscorlib]System.IDisposable::Dispose()

				IL_01b3: endfinally
			} // end handler
		} // end .try
		catch [mscorlib]System.Exception
		{
			IL_01b4: stloc.s 5
			IL_01b6: ldarg.0
			IL_01b7: ldc.i4.s -2
			IL_01b9: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
			IL_01be: ldarg.0
			IL_01bf: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
			IL_01c4: ldloc.s 5
			IL_01c6: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetException(class [mscorlib]System.Exception)
			IL_01cb: leave.s IL_01e1
		} // end handler

		IL_01cd: ldarg.0
		IL_01ce: ldc.i4.s -2
		IL_01d0: stfld int32 AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>1__state'
		IL_01d5: ldarg.0
		IL_01d6: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
		IL_01db: ldloc.1
		IL_01dc: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetResult(!0)

		IL_01e1: ret
	} // end of method '<DoTheThingSlowly>d__0'::MoveNext

	.method private final hidebysig newslot virtual 
		instance void SetStateMachine (
			class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
		) cil managed 
	{
		.custom instance void [mscorlib]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
			01 00 00 00
		)
		.override method instance void [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine)
		// Method begins at RVA 0xc050
		// Code size 13 (0xd)
		.maxstack 8

		IL_0000: ldarg.0
		IL_0001: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string> AsyncContracts.AsyncContracts/'<DoTheThingSlowly>d__0'::'<>t__builder'
		IL_0006: ldarg.1
		IL_0007: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<string>::SetStateMachine(class [mscorlib]System.Runtime.CompilerServices.IAsyncStateMachine)
		IL_000c: ret
	} // end of method '<DoTheThingSlowly>d__0'::SetStateMachine

} // end of class <DoTheThingSlowly>d__0

It appears that this block of IL, which exists before the calls to Contract.Requires, is confusing the rewriter. The rewriter seems to understand a switch statement and a simple condition, but not this subtraction-based condition.

IL_0007: ldloc.0
IL_0008: brfalse.s IL_0066

IL_000a: ldloc.0
IL_000b: ldc.i4.1
IL_000c: sub
IL_000d: ldc.i4.1
IL_000e: ble.un IL_009c

(Related to VS2017, so tagging #451 here).

@SergeyTeplyakov Is this something that can be easily fixed?
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@yaakov-h