Support basic cases in UseRangeOperator

src/Analyzers/CSharp/Analyzers/UseIndexOrRangeOperator/CSharpUseRangeOperatorDiagnosticAnalyzer.InfoCache.cs

@@ -45,7 +45,7 @@ public InfoCache(Compilation compilation)
                 {
                     var substringMethod = stringType.GetMembers(nameof(string.Substring))
                                                     .OfType<IMethodSymbol>()
-                                                    .FirstOrDefault(m => IsSliceLikeMethod(m));
+                                                    .FirstOrDefault(m => IsTwoArgumentSliceLikeMethod(m));
 
                     _methodToMemberInfo[substringMethod] = ComputeMemberInfo(substringMethod, requireRangeMember: false);
                 }
@@ -54,12 +54,12 @@ public InfoCache(Compilation compilation)
             private static IMethodSymbol GetSliceLikeMethod(INamedTypeSymbol namedType)
                 => namedType.GetMembers()
                             .OfType<IMethodSymbol>()
-                            .Where(m => IsSliceLikeMethod(m))
+                            .Where(m => IsTwoArgumentSliceLikeMethod(m))
                             .FirstOrDefault();
 
             public bool TryGetMemberInfo(IMethodSymbol method, out MemberInfo memberInfo)
             {
-                if (!IsSliceLikeMethod(method))
+                if (!IsTwoArgumentSliceLikeMethod(method))
                 {
                     memberInfo = default;
                     return false;
@@ -69,9 +69,44 @@ public bool TryGetMemberInfo(IMethodSymbol method, out MemberInfo memberInfo)
                 return memberInfo.LengthLikeProperty != null;
             }
 
+            public bool TryGetMemberInfoOneArgument(IMethodSymbol method, out MemberInfo memberInfo)
+            {
+                if (!IsOneArgumentSliceLikeMethod(method))
+                {
+                    memberInfo = default;
+                    return false;
+                }
+
+                if (!_methodToMemberInfo.TryGetValue(method, out memberInfo))
+                {
+                    // Find overload of our method that is a slice-like method with two arguments.
+                    // Computing member info for this method will also check that the containing type
+                    // has an int32 'Length' or 'Count' property, and has a suitable indexer,
+                    // so we don't have to.
+                    var overloadWithTwoArguments = method.ContainingType
+                        .GetMembers(method.Name)
+                        .OfType<IMethodSymbol>()
+                        .FirstOrDefault(s => IsTwoArgumentSliceLikeMethod(s));
+                    if (overloadWithTwoArguments is null)
+                    {
+                        memberInfo = default;
+                        return false;
+                    }
+
+                    // Since the search is expensive, we keep both the original one-argument and
+                    // two-arguments overload as keys in the cache, pointing to the same
+                    // member information object.
+                    var newMemberInfo = _methodToMemberInfo.GetOrAdd(overloadWithTwoArguments, _ => ComputeMemberInfo(overloadWithTwoArguments, requireRangeMember: true));
+                    _methodToMemberInfo.GetOrAdd(method, _ => newMemberInfo);
+                    memberInfo = newMemberInfo;
+                }
+
+                return memberInfo.LengthLikeProperty != null;
+            }
+
             private MemberInfo ComputeMemberInfo(IMethodSymbol sliceLikeMethod, bool requireRangeMember)
             {
-                Debug.Assert(IsSliceLikeMethod(sliceLikeMethod));
+                Debug.Assert(IsTwoArgumentSliceLikeMethod(sliceLikeMethod));
 
                 // Check that the type has an int32 'Length' or 'Count' property. If not, we don't
                 // consider it something indexable.
@@ -106,7 +141,7 @@ private MemberInfo ComputeMemberInfo(IMethodSymbol sliceLikeMethod, bool require
                     var actualSliceMethod =
                         sliceLikeMethod.ContainingType.GetMembers(nameof(Span<int>.Slice))
                                                       .OfType<IMethodSymbol>()
-                                                      .FirstOrDefault(s => IsSliceLikeMethod(s));
+                                                      .FirstOrDefault(s => IsTwoArgumentSliceLikeMethod(s));
                     if (actualSliceMethod != null)
                     {
                         return new MemberInfo(lengthLikeProperty, overloadedMethodOpt: null);

src/Analyzers/CSharp/Analyzers/UseIndexOrRangeOperator/CSharpUseRangeOperatorDiagnosticAnalyzer.Result.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Operations;
@@ -12,7 +14,7 @@ internal partial class CSharpUseRangeOperatorDiagnosticAnalyzer
     {
         public enum ResultKind
         {
-            // like s.Substring(expr, s.Length - expr).  'expr' has to match on both sides.
+            // like s.Substring(expr, s.Length - expr) or s.Substring(expr).  'expr' has to match on both sides.
             Computed,
 
             // like s.Substring(constant1, s.Length - constant2).  the constants don't have to match.
@@ -28,13 +30,17 @@ public readonly struct Result
             public readonly IMethodSymbol SliceLikeMethod;
             public readonly MemberInfo MemberInfo;
             public readonly IOperation Op1;
-            public readonly IOperation Op2;
+
+            /// <summary>
+            /// Can be null, if we are dealing with one-argument call to a slice-like method.
+            /// </summary>
+            public readonly IOperation? Op2;
 
             public Result(
                 ResultKind kind, CodeStyleOption2<bool> option,
                 IInvocationOperation invocationOperation, InvocationExpressionSyntax invocation,
                 IMethodSymbol sliceLikeMethod, MemberInfo memberInfo,
-                IOperation op1, IOperation op2)
+                IOperation op1, IOperation? op2)
             {
                 Kind = kind;
                 Option = option;

src/Analyzers/CSharp/Analyzers/UseIndexOrRangeOperator/CSharpUseRangeOperatorDiagnosticAnalyzer.cs

@@ -111,13 +111,54 @@ private void AnalyzeInvocation(
                 }
             }
 
-            // look for `s.Slice(e1, end - e2)`
-            if (invocation.Instance is null ||
-                invocation.Arguments.Length != 2)
+            // look for `s.Slice(e1, end - e2)` or `s.Slice(e1)`
+            if (invocation.Instance is null)
             {
                 return null;
             }
 
+            return invocation.Arguments.Length switch
+            {
+                1 => AnalyzeOneArgumentInvocation(invocation, infoCache, invocationSyntax, option),
+                2 => AnalyzeTwoArgumentInvocation(invocation, infoCache, invocationSyntax, option),
+                _ => null,
+            };
+        }
+
+        private static Result? AnalyzeOneArgumentInvocation(
+            IInvocationOperation invocation,
+            InfoCache infoCache,
+            InvocationExpressionSyntax invocationSyntax,
+            CodeStyleOption2<bool> option)
+        {
+            var targetMethod = invocation.TargetMethod;
+
+            // We are dealing with a call like `.Substring(expr)`.
+            // Ensure that there is an overload with signature like `Substring(int start, int length)`
+            // and there is a suitable indexer to replace this with `[expr..]`.
+            if (!infoCache.TryGetMemberInfoOneArgument(targetMethod, out var memberInfo))
+            {
+                return null;
+            }
+
+            var startOperation = invocation.Arguments[0].Value;
+            return new Result(
+                ResultKind.Computed,
+                option,
+                invocation,
+                invocationSyntax,
+                targetMethod,
+                memberInfo,
+                op1: startOperation,
+                op2: null); // The range will run to the end.
+        }
+
+        private static Result? AnalyzeTwoArgumentInvocation(
+            IInvocationOperation invocation,
+            InfoCache infoCache,
+            InvocationExpressionSyntax invocationSyntax,
+            CodeStyleOption2<bool> option)
+        {
             // See if the call is to something slice-like.
             var targetMethod = invocation.TargetMethod;
 

src/Analyzers/CSharp/Analyzers/UseIndexOrRangeOperator/Helpers.cs

@@ -80,16 +80,35 @@ public static bool IsIntIndexingMethod(IMethodSymbol method)
         /// names of the parameters are checked to ensure they are appropriate slice-like.  These
         /// names were picked by examining the patterns in the BCL for slicing members.
         /// </summary>
-        public static bool IsSliceLikeMethod(IMethodSymbol method)
-            => method != null &&
-               IsPublicInstance(method) &&
-               method.Parameters.Length == 2 &&
-               // From: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#decisions-made-during-implementation
-               //
-               // When looking for the pattern members, we look for original definitions, not
-               // constructed members
-               IsSliceFirstParameter(method.OriginalDefinition.Parameters[0]) &&
-               IsSliceSecondParameter(method.OriginalDefinition.Parameters[1]);
+        public static bool IsTwoArgumentSliceLikeMethod(IMethodSymbol method)
+        {
+            // From: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#decisions-made-during-implementation
+            //
+            // When looking for the pattern members, we look for original definitions, not
+            // constructed members
+            return method != null &&
+                IsPublicInstance(method) &&
+                method.Parameters.Length == 2 &&
+                IsSliceFirstParameter(method.OriginalDefinition.Parameters[0]) &&
+                IsSliceSecondParameter(method.OriginalDefinition.Parameters[1]);
+        }
+
+        /// <summary>
+        /// Look for methods like "SomeType MyType.Slice(int start)".  Note that the
+        /// name of the parameter is checked to ensure it is appropriate slice-like.
+        /// This name was picked by examining the patterns in the BCL for slicing members.
+        /// </summary>
+        public static bool IsOneArgumentSliceLikeMethod(IMethodSymbol method)
+        {
+            // From: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#decisions-made-during-implementation
+            //
+            // When looking for the pattern members, we look for original definitions, not
+            // constructed members
+            return method != null &&
+                IsPublicInstance(method) &&
+                method.Parameters.Length == 1 &&
+                IsSliceFirstParameter(method.OriginalDefinition.Parameters[0]);
+        }
 
         private static bool IsSliceFirstParameter(IParameterSymbol parameter)
             => parameter.Type.SpecialType == SpecialType.System_Int32 &&

src/Analyzers/CSharp/CodeFixes/UseIndexOrRangeOperator/CSharpUseRangeOperatorCodeFixProvider.cs

@@ -150,16 +150,22 @@ private static RangeExpressionSyntax CreateComputedRange(Result result)
             var startFromEnd = IsFromEnd(lengthLikeProperty, instance, ref startOperation);
             var startExpr = (ExpressionSyntax)startOperation.Syntax;
 
-            // Similarly, if our end-op is actually equivalent to `expr.Length - val`, then just
-            // change our end-op to be `val` and record that we should emit it as `^val`.
-            var endFromEnd = IsFromEnd(lengthLikeProperty, instance, ref endOperation);
-            var endExpr = (ExpressionSyntax)endOperation.Syntax;
-
-            // If the range operation goes to 'expr.Length' then we can just leave off the end part
-            // of the range.  i.e. `start..`
-            if (IsInstanceLengthCheck(lengthLikeProperty, instance, endOperation))
+            var endFromEnd = false;
+            ExpressionSyntax endExpr = null;
+
+            if (!(endOperation is null))
             {
-                endExpr = null;
+                // We need to do the same for the second argument, since it's present.
+                // Similarly, if our end-op is actually equivalent to `expr.Length - val`, then just
+                // change our end-op to be `val` and record that we should emit it as `^val`.
+                endFromEnd = IsFromEnd(lengthLikeProperty, instance, ref endOperation);
+
+                // Check if the range goes to 'expr.Length'; if it does, we leave off
+                // the end part of the range, i.e. `start..`.
+                if (!IsInstanceLengthCheck(lengthLikeProperty, instance, endOperation))
+                {
+                    endExpr = (ExpressionSyntax)endOperation.Syntax;
+                }
             }
 
             // If we're starting the range operation from 0, then we can just leave off the start of