differencec between discard and no discard in IL

example.cs

using System.IO;
using System.Threading.Tasks;

public static class C {
    public static async Task A() {
        await File.ReadAllTextAsync("SomeFile.txt");
    }
    
    public static async Task B() {
        _ = await File.ReadAllTextAsync("SomeFile.txt");
    }
}

output.il

.class private auto ansi '<Module>'
{
} // end of class <Module>

.class public auto ansi abstract sealed beforefieldinit C
    extends [System.Private.CoreLib]System.Object
{
    // Nested Types
    .class nested private auto ansi sealed beforefieldinit '<A>d__0'
        extends [System.Private.CoreLib]System.ValueType
        implements [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine
    {
        .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
            01 00 00 00
        )
        // Fields
        .field public int32 '<>1__state'
        .field public valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder '<>t__builder'
        .field private valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> '<>u__1'

        // Methods
        .method private final hidebysig newslot virtual 
            instance void MoveNext () cil managed 
        {
            .override method instance void [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
            // Method begins at RVA 0x20c8
            // Code size 157 (0x9d)
            .maxstack 3
            .locals init (
                [0] int32,
                [1] valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>,
                [2] valuetype [System.Private.CoreLib]System.Threading.CancellationToken,
                [3] class [System.Private.CoreLib]System.Exception
            )

            // sequence point: hidden
            IL_0000: ldarg.0
            IL_0001: ldfld int32 C/'<A>d__0'::'<>1__state'
            IL_0006: stloc.0
            .try
            {
                // sequence point: hidden
                IL_0007: ldloc.0
                IL_0008: brfalse.s IL_004c

                IL_000a: ldstr "SomeFile.txt"
                IL_000f: ldloca.s 2
                IL_0011: initobj [System.Private.CoreLib]System.Threading.CancellationToken
                IL_0017: ldloc.2
                IL_0018: call class [System.Private.CoreLib]System.Threading.Tasks.Task`1<string> [System.IO.FileSystem]System.IO.File::ReadAllTextAsync(string, valuetype [System.Private.CoreLib]System.Threading.CancellationToken)
                IL_001d: callvirt instance valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<!0> class [System.Private.CoreLib]System.Threading.Tasks.Task`1<string>::GetAwaiter()
                IL_0022: stloc.1
                // sequence point: hidden
                IL_0023: ldloca.s 1
                IL_0025: call instance bool valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>::get_IsCompleted()
                IL_002a: brtrue.s IL_0068

                IL_002c: ldarg.0
                IL_002d: ldc.i4.0
                IL_002e: dup
                IL_002f: stloc.0
                IL_0030: stfld int32 C/'<A>d__0'::'<>1__state'
                IL_0035: ldarg.0
                IL_0036: ldloc.1
                IL_0037: stfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<A>d__0'::'<>u__1'
                IL_003c: ldarg.0
                IL_003d: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
                IL_0042: ldloca.s 1
                IL_0044: ldarg.0
                IL_0045: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::AwaitUnsafeOnCompleted<valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>, valuetype C/'<A>d__0'>(!!0&, !!1&)
                IL_004a: leave.s IL_009c

                IL_004c: ldarg.0
                IL_004d: ldfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<A>d__0'::'<>u__1'
                IL_0052: stloc.1
                IL_0053: ldarg.0
                IL_0054: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<A>d__0'::'<>u__1'
                IL_0059: initobj valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>
                IL_005f: ldarg.0
                IL_0060: ldc.i4.m1
                IL_0061: dup
                IL_0062: stloc.0
                IL_0063: stfld int32 C/'<A>d__0'::'<>1__state'

                IL_0068: ldloca.s 1
                IL_006a: call instance !0 valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>::GetResult()
                IL_006f: pop
                IL_0070: leave.s IL_0089
            } // end .try
            catch [System.Private.CoreLib]System.Exception
            {
                // sequence point: hidden
                IL_0072: stloc.3
                IL_0073: ldarg.0
                IL_0074: ldc.i4.s -2
                IL_0076: stfld int32 C/'<A>d__0'::'<>1__state'
                IL_007b: ldarg.0
                IL_007c: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
                IL_0081: ldloc.3
                IL_0082: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::SetException(class [System.Private.CoreLib]System.Exception)
                IL_0087: leave.s IL_009c
            } // end handler

            IL_0089: ldarg.0
            IL_008a: ldc.i4.s -2
            IL_008c: stfld int32 C/'<A>d__0'::'<>1__state'
            // sequence point: hidden
            IL_0091: ldarg.0
            IL_0092: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
            IL_0097: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::SetResult()

            IL_009c: ret
        } // end of method '<A>d__0'::MoveNext

        .method private final hidebysig newslot virtual 
            instance void SetStateMachine (
                class [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
            ) cil managed 
        {
            .custom instance void [System.Private.CoreLib]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
                01 00 00 00
            )
            .override method instance void [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine)
            // Method begins at RVA 0x2184
            // Code size 13 (0xd)
            .maxstack 8

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

    } // end of class <A>d__0

    .class nested private auto ansi sealed beforefieldinit '<B>d__1'
        extends [System.Private.CoreLib]System.ValueType
        implements [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine
    {
        .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
            01 00 00 00
        )
        // Fields
        .field public int32 '<>1__state'
        .field public valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder '<>t__builder'
        .field private valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> '<>u__1'

        // Methods
        .method private final hidebysig newslot virtual 
            instance void MoveNext () cil managed 
        {
            .override method instance void [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
            // Method begins at RVA 0x2194
            // Code size 157 (0x9d)
            .maxstack 3
            .locals init (
                [0] int32,
                [1] valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>,
                [2] valuetype [System.Private.CoreLib]System.Threading.CancellationToken,
                [3] class [System.Private.CoreLib]System.Exception
            )

            // sequence point: hidden
            IL_0000: ldarg.0
            IL_0001: ldfld int32 C/'<B>d__1'::'<>1__state'
            IL_0006: stloc.0
            .try
            {
                // sequence point: hidden
                IL_0007: ldloc.0
                IL_0008: brfalse.s IL_004c

                IL_000a: ldstr "SomeFile.txt"
                IL_000f: ldloca.s 2
                IL_0011: initobj [System.Private.CoreLib]System.Threading.CancellationToken
                IL_0017: ldloc.2
                IL_0018: call class [System.Private.CoreLib]System.Threading.Tasks.Task`1<string> [System.IO.FileSystem]System.IO.File::ReadAllTextAsync(string, valuetype [System.Private.CoreLib]System.Threading.CancellationToken)
                IL_001d: callvirt instance valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<!0> class [System.Private.CoreLib]System.Threading.Tasks.Task`1<string>::GetAwaiter()
                IL_0022: stloc.1
                // sequence point: hidden
                IL_0023: ldloca.s 1
                IL_0025: call instance bool valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>::get_IsCompleted()
                IL_002a: brtrue.s IL_0068

                IL_002c: ldarg.0
                IL_002d: ldc.i4.0
                IL_002e: dup
                IL_002f: stloc.0
                IL_0030: stfld int32 C/'<B>d__1'::'<>1__state'
                IL_0035: ldarg.0
                IL_0036: ldloc.1
                IL_0037: stfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<B>d__1'::'<>u__1'
                IL_003c: ldarg.0
                IL_003d: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
                IL_0042: ldloca.s 1
                IL_0044: ldarg.0
                IL_0045: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::AwaitUnsafeOnCompleted<valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>, valuetype C/'<B>d__1'>(!!0&, !!1&)
                IL_004a: leave.s IL_009c

                IL_004c: ldarg.0
                IL_004d: ldfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<B>d__1'::'<>u__1'
                IL_0052: stloc.1
                IL_0053: ldarg.0
                IL_0054: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string> C/'<B>d__1'::'<>u__1'
                IL_0059: initobj valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>
                IL_005f: ldarg.0
                IL_0060: ldc.i4.m1
                IL_0061: dup
                IL_0062: stloc.0
                IL_0063: stfld int32 C/'<B>d__1'::'<>1__state'

                IL_0068: ldloca.s 1
                IL_006a: call instance !0 valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.TaskAwaiter`1<string>::GetResult()
                IL_006f: pop
                IL_0070: leave.s IL_0089
            } // end .try
            catch [System.Private.CoreLib]System.Exception
            {
                // sequence point: hidden
                IL_0072: stloc.3
                IL_0073: ldarg.0
                IL_0074: ldc.i4.s -2
                IL_0076: stfld int32 C/'<B>d__1'::'<>1__state'
                IL_007b: ldarg.0
                IL_007c: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
                IL_0081: ldloc.3
                IL_0082: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::SetException(class [System.Private.CoreLib]System.Exception)
                IL_0087: leave.s IL_009c
            } // end handler

            IL_0089: ldarg.0
            IL_008a: ldc.i4.s -2
            IL_008c: stfld int32 C/'<B>d__1'::'<>1__state'
            // sequence point: hidden
            IL_0091: ldarg.0
            IL_0092: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
            IL_0097: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::SetResult()

            IL_009c: ret
        } // end of method '<B>d__1'::MoveNext

        .method private final hidebysig newslot virtual 
            instance void SetStateMachine (
                class [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
            ) cil managed 
        {
            .custom instance void [System.Private.CoreLib]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
                01 00 00 00
            )
            .override method instance void [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine)
            // Method begins at RVA 0x2250
            // Code size 13 (0xd)
            .maxstack 8

            IL_0000: ldarg.0
            IL_0001: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
            IL_0006: ldarg.1
            IL_0007: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::SetStateMachine(class [System.Private.CoreLib]System.Runtime.CompilerServices.IAsyncStateMachine)
            IL_000c: ret
        } // end of method '<B>d__1'::SetStateMachine

    } // end of class <B>d__1


    // Methods
    .method public hidebysig static 
        class [System.Private.CoreLib]System.Threading.Tasks.Task A () cil managed 
    {
        .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncStateMachineAttribute::.ctor(class [System.Private.CoreLib]System.Type) = (
            01 00 09 43 2b 3c 41 3e 64 5f 5f 30 00 00
        )
        // Method begins at RVA 0x2050
        // Code size 47 (0x2f)
        .maxstack 2
        .locals init (
            [0] valuetype C/'<A>d__0'
        )

        IL_0000: ldloca.s 0
        IL_0002: call valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Create()
        IL_0007: stfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
        IL_000c: ldloca.s 0
        IL_000e: ldc.i4.m1
        IL_000f: stfld int32 C/'<A>d__0'::'<>1__state'
        IL_0014: ldloca.s 0
        IL_0016: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
        IL_001b: ldloca.s 0
        IL_001d: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Start<valuetype C/'<A>d__0'>(!!0&)
        IL_0022: ldloca.s 0
        IL_0024: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<A>d__0'::'<>t__builder'
        IL_0029: call instance class [System.Private.CoreLib]System.Threading.Tasks.Task [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::get_Task()
        IL_002e: ret
    } // end of method C::A

    .method public hidebysig static 
        class [System.Private.CoreLib]System.Threading.Tasks.Task B () cil managed 
    {
        .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncStateMachineAttribute::.ctor(class [System.Private.CoreLib]System.Type) = (
            01 00 09 43 2b 3c 42 3e 64 5f 5f 31 00 00
        )
        // Method begins at RVA 0x208c
        // Code size 47 (0x2f)
        .maxstack 2
        .locals init (
            [0] valuetype C/'<B>d__1'
        )

        IL_0000: ldloca.s 0
        IL_0002: call valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Create()
        IL_0007: stfld valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
        IL_000c: ldloca.s 0
        IL_000e: ldc.i4.m1
        IL_000f: stfld int32 C/'<B>d__1'::'<>1__state'
        IL_0014: ldloca.s 0
        IL_0016: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
        IL_001b: ldloca.s 0
        IL_001d: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Start<valuetype C/'<B>d__1'>(!!0&)
        IL_0022: ldloca.s 0
        IL_0024: ldflda valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder C/'<B>d__1'::'<>t__builder'
        IL_0029: call instance class [System.Private.CoreLib]System.Threading.Tasks.Task [System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder::get_Task()
        IL_002e: ret
    } // end of method C::B

} // end of class C