List patterns: Slice value is assumed to be never null

src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder.cs

@@ -421,8 +421,12 @@ private static void MakeCheckNotNull(
  ArrayBuilder<Tests> tests)
  {
  // Add a null test if needed
- if (input.Type.CanContainNull())
+ if (input.Type.CanContainNull() &&
+ // The slice value is assumed to be never null
+ input.Source is not BoundDagSliceEvaluation)
+ {
  tests.Add(new Tests.One(new BoundDagNonNullTest(syntax, isExplicitTest, input)));
+ }
  }
 
  /// <summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6730,9 +6730,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
  <data name="IDS_FeatureListPattern" xml:space="preserve">
  <value>list pattern</value>
  </data>
- <data name="IDS_FeatureLengthPattern" xml:space="preserve">
- <value>length pattern</value>
- </data>
  <data name="ERR_UnsupportedTypeForListPattern" xml:space="preserve">
  <value>List patterns may not be used for a value of type '{0}'.</value>
  </data>
@@ -6893,4 +6890,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
  <data name="IDS_FeatureNewLinesInInterpolations" xml:space="preserve">
  <value>newlines in interpolations</value>
  </data>
+ <data name="WRN_AnnotationOnSliceReturnType" xml:space="preserve">
+ <value>The annotation on the output type '{0}' is ignored.</value>
+ </data>
+ <data name="WRN_AnnotationOnSliceReturnType_Title" xml:space="preserve">
+ <value>The annotation on the output type is ignored.</value>
+ </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2006,6 +2006,8 @@ internal enum ErrorCode
  ERR_UnsupportedTypeForSlicePattern,
  ERR_MisplacedSlicePattern,
 
+ WRN_AnnotationOnSliceReturnType = 8978,
+
  #endregion
 
  // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -80,6 +80,7 @@ static ErrorFacts()
  nullableWarnings.Add(GetId(ErrorCode.WRN_ParameterDisallowsNull));
  nullableWarnings.Add(GetId(ErrorCode.WRN_ParameterNotNullIfNotNull));
  nullableWarnings.Add(GetId(ErrorCode.WRN_ReturnNotNullIfNotNull));
+ nullableWarnings.Add(GetId(ErrorCode.WRN_AnnotationOnSliceReturnType));
 
  NullableWarnings = nullableWarnings.ToImmutable();
  }
@@ -486,6 +487,7 @@ internal static int GetWarningLevel(ErrorCode code)
  case ErrorCode.WRN_InterpolatedStringHandlerArgumentAttributeIgnoredOnLambdaParameters:
  case ErrorCode.WRN_CompileTimeCheckedOverflow:
  case ErrorCode.WRN_MethGrpToNonDel:
+ case ErrorCode.WRN_AnnotationOnSliceReturnType:
  return 1;
  default:
  return 0;

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -567,6 +567,7 @@ public PossiblyConditionalState Clone()
  var output = new BoundDagTemp(e.Syntax, type.Type, e);
  var outputSlot = makeDagTempSlot(type, output);
  Debug.Assert(outputSlot > 0);
+ ignoreAnnotationAndReport(outputSlot, type, ref this.State, e);
  addToTempMap(output, outputSlot, type.Type);
  break;
  }
@@ -719,6 +720,18 @@ public PossiblyConditionalState Clone()
  nodeStateMap.Free();
  return labelStateMap;
 
+ void ignoreAnnotationAndReport(int outputSlot, TypeWithAnnotations outputType, ref LocalState state, BoundDagSliceEvaluation e)
+ {
+ if (PossiblyNullableType(outputType.Type))
+ {
+ if (state[outputSlot] == NullableFlowState.MaybeNull)
+ {
+ ReportDiagnostic(ErrorCode.WRN_AnnotationOnSliceReturnType, e.Syntax, outputType);
+ state[outputSlot] = NullableFlowState.NotNull;
+ }
+ }
+ }
+
  void learnFromNonNullTest(int inputSlot, ref LocalState state)
  {
  if (stateWhenNotNullOpt is { } stateWhenNotNull && inputSlot == originalInputSlot)