Add more info to TypeInfo (#178)

Adding FieldCount allows users to enumerate the field info for each
field in the TypeInfo. Retrieving the System.Type is an experimental API
needed for the command line Serde implementation. This API might become
permanent if it's cheap enough and there are no better options.

src/serde/TypeInfo.cs

@@ -2,6 +2,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
@@ -18,8 +19,23 @@ namespace Serde;
 /// </summary>
 public sealed class TypeInfo
 {
+    // The field names are sorted by the Utf8 representation of the field name.
+    private readonly ImmutableArray<(ReadOnlyMemory<byte> Utf8Name, int Index)> _nameToIndex;
+    private readonly ImmutableArray<PrivateFieldInfo> _indexToInfo;
+
+    /// <summary>
+    /// Holds information for a field or property in the given type.
+    /// </summary>
+    private readonly record struct PrivateFieldInfo(
+        string StringName,
+        int Utf8NameIndex,
+        IList<CustomAttributeData> CustomAttributesData,
+        Type FieldType);
+
     public string TypeName { get; }
 
+    public int FieldCount => _nameToIndex.Length;
+
     public enum TypeKind
     {
         Primitive,
@@ -43,6 +59,9 @@ public int TryGetIndex(Utf8Span utf8FieldName)
         return mapIndex < 0 ? IDeserializeType.IndexNotFound : _nameToIndex[mapIndex].Index;
     }
 
+    [Experimental("SerdeExperimentalFieldType")]
+    public Type GetFieldType(int index) => _indexToInfo[index].FieldType;
+
     public IList<CustomAttributeData> GetCustomAttributeData(int index)
     {
         return _indexToInfo[index].CustomAttributesData;
@@ -72,13 +91,19 @@ public static TypeInfo Create(
         for (int index = 0; index < fields.Length; index++)
         {
             var (serializeName, memberInfo) = fields[index];
-            if (memberInfo is null)
+            var fieldType = memberInfo switch
             {
-                throw new ArgumentNullException(serializeName);
-            }
+                FieldInfo info => info.FieldType,
+                PropertyInfo propertyInfo => propertyInfo.PropertyType,
+                _ => throw new ArgumentException("MemberInfo must be a FieldInfo or PropertyInfo", nameof(memberInfo))
+            };
 
             nameToIndexBuilder.Add((s_utf8.GetBytes(serializeName), index));
-            var fieldInfo = new PrivateFieldInfo(serializeName, -1, memberInfo.GetCustomAttributesData());
+            var fieldInfo = new PrivateFieldInfo(
+                serializeName,
+                -1,
+                memberInfo.GetCustomAttributesData(),
+                fieldType);
             indexToInfoBuilder.Add(fieldInfo);
         }
 
@@ -94,20 +119,6 @@ public static TypeInfo Create(
         return new TypeInfo(typeName, typeKind, nameToIndexBuilder.ToImmutable(), indexToInfoBuilder.ToImmutable());
     }
 
-    #region Private implementation details
-
-    // The field names are sorted by the Utf8 representation of the field name.
-    private readonly ImmutableArray<(ReadOnlyMemory<byte> Utf8Name, int Index)> _nameToIndex;
-    private readonly ImmutableArray<PrivateFieldInfo> _indexToInfo;
-
-    /// <summary>
-    /// Holds information for a field or property in the given type.
-    /// </summary>
-    private readonly record struct PrivateFieldInfo(
-        string StringName,
-        int Utf8NameIndex,
-        IList<CustomAttributeData> CustomAttributesData);
-
     private static readonly UTF8Encoding s_utf8 = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
 
     private TypeInfo(
@@ -165,6 +176,4 @@ private static int BinarySearch(ref (ReadOnlyMemory<byte> Utf8Name, int) spanSta
         // is `lo` at this point.
         return ~lo;
     }
-
-    #endregion  // Private implementation details
 }