feature: Provide ParameterInfo with error ParseResult (#1355)

Currently, when handling parsing errors, there is no way to know what
parameter caused the error. This change makes the CommandParser create
the parsing error with the current parameter info when ParseAsync()
fails. It is then available through the ErrorParameter of the
ParseResult.

src/Discord.Net.Commands/CommandParser.cs

@@ -103,7 +103,7 @@ namespace Discord.Commands
                             argBuilder.Append(c);
                             continue;
                         }
-                        
+
                         if (IsOpenQuote(aliasMap, c))
                         {
                             curPart = ParserPart.QuotedParameter;
@@ -136,7 +136,7 @@ namespace Discord.Commands
                     else
                         argBuilder.Append(c);
                 }
-                                
+
                 if (argString != null)
                 {
                     if (curParam == null)
@@ -149,7 +149,7 @@ namespace Discord.Commands
 
                     var typeReaderResult = await curParam.ParseAsync(context, argString, services).ConfigureAwait(false);
                     if (!typeReaderResult.IsSuccess && typeReaderResult.Error != CommandError.MultipleMatches)
-                        return ParseResult.FromError(typeReaderResult);
+                        return ParseResult.FromError(typeReaderResult, curParam);
 
                     if (curParam.IsMultiple)
                     {
@@ -172,7 +172,7 @@ namespace Discord.Commands
             {
                 var typeReaderResult = await curParam.ParseAsync(context, argBuilder.ToString(), services).ConfigureAwait(false);
                 if (!typeReaderResult.IsSuccess)
-                    return ParseResult.FromError(typeReaderResult);
+                    return ParseResult.FromError(typeReaderResult, curParam);
                 argList.Add(typeReaderResult);
             }
 
@@ -180,7 +180,7 @@ namespace Discord.Commands
                 return ParseResult.FromError(CommandError.ParseFailed, "Input text may not end on an incomplete escape.");
             if (curPart == ParserPart.QuotedParameter)
                 return ParseResult.FromError(CommandError.ParseFailed, "A quoted parameter is incomplete.");
-            
+
             //Add missing optionals
             for (int i = argList.Count; i < command.Parameters.Count; i++)
             {
@@ -191,7 +191,7 @@ namespace Discord.Commands
                     return ParseResult.FromError(CommandError.BadArgCount, "The input text has too few parameters.");
                 argList.Add(TypeReaderResult.FromSuccess(param.DefaultValue));
             }
-            
+
             return ParseResult.FromSuccess(argList.ToImmutable(), paramList.ToImmutable());
         }
     }

src/Discord.Net.Commands/Results/ParseResult.cs

@@ -18,30 +18,40 @@ namespace Discord.Commands
         /// <inheritdoc/>
         public string ErrorReason { get; }
 
+        /// <summary>
+        ///     Provides information about the parameter that caused the parsing error.
+        /// </summary>
+        /// <returns>
+        ///     A <see cref="ParameterInfo" /> indicating the parameter info of the error that may have occurred during parsing; 
+        ///     <c>null</c> if the parsing was successful or the parsing error is not specific to a single parameter.
+        /// </returns>
+        public ParameterInfo ErrorParameter { get; }
+
         /// <inheritdoc/>
         public bool IsSuccess => !Error.HasValue;
 
-        private ParseResult(IReadOnlyList<TypeReaderResult> argValues, IReadOnlyList<TypeReaderResult> paramValues, CommandError? error, string errorReason)
+        private ParseResult(IReadOnlyList<TypeReaderResult> argValues, IReadOnlyList<TypeReaderResult> paramValues, CommandError? error, string errorReason, ParameterInfo errorParamInfo)
         {
             ArgValues = argValues;
             ParamValues = paramValues;
             Error = error;
             ErrorReason = errorReason;
+            ErrorParameter = errorParamInfo;
         }
-        
+
         public static ParseResult FromSuccess(IReadOnlyList<TypeReaderResult> argValues, IReadOnlyList<TypeReaderResult> paramValues)
         {
             for (int i = 0; i < argValues.Count; i++)
             {
                 if (argValues[i].Values.Count > 1)
-                    return new ParseResult(argValues, paramValues, CommandError.MultipleMatches, "Multiple matches found.");
+                    return new ParseResult(argValues, paramValues, CommandError.MultipleMatches, "Multiple matches found.", null);
             }
             for (int i = 0; i < paramValues.Count; i++)
             {
                 if (paramValues[i].Values.Count > 1)
-                    return new ParseResult(argValues, paramValues, CommandError.MultipleMatches, "Multiple matches found.");
+                    return new ParseResult(argValues, paramValues, CommandError.MultipleMatches, "Multiple matches found.", null);
             }
-            return new ParseResult(argValues, paramValues, null, null);
+            return new ParseResult(argValues, paramValues, null, null, null);
         }
         public static ParseResult FromSuccess(IReadOnlyList<TypeReaderValue> argValues, IReadOnlyList<TypeReaderValue> paramValues)
         {
@@ -55,15 +65,19 @@ namespace Discord.Commands
                 for (int i = 0; i < paramValues.Count; i++)
                     paramList[i] = TypeReaderResult.FromSuccess(paramValues[i]);
             }
-            return new ParseResult(argList, paramList, null, null);
+            return new ParseResult(argList, paramList, null, null, null);
         }
 
         public static ParseResult FromError(CommandError error, string reason)
-            => new ParseResult(null, null, error, reason);
+            => new ParseResult(null, null, error, reason, null);
+        public static ParseResult FromError(CommandError error, string reason, ParameterInfo parameterInfo)
+            => new ParseResult(null, null, error, reason, parameterInfo);
         public static ParseResult FromError(Exception ex)
             => FromError(CommandError.Exception, ex.Message);
         public static ParseResult FromError(IResult result)
-            => new ParseResult(null, null, result.Error, result.ErrorReason);
+            => new ParseResult(null, null, result.Error, result.ErrorReason, null);
+        public static ParseResult FromError(IResult result, ParameterInfo parameterInfo)
+            => new ParseResult(null, null, result.Error, result.ErrorReason, parameterInfo);
 
         public override string ToString() => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
         private string DebuggerDisplay => IsSuccess ? $"Success ({ArgValues.Count}{(ParamValues.Count > 0 ? $" +{ParamValues.Count} Values" : "")})" : $"{Error}: {ErrorReason}";