Support struct field type remapping via --with-type

sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.Naming.cs

@@ -793,6 +793,14 @@ private string GetRemappedTypeName(Cursor? cursor, Cursor? context, Type type, o
 
                     WithType(enumDecl, ref remappedName, ref nativeTypeName);
                 }
+                else if (cursor is FieldDecl fieldDecl)
+                {
+                    // A field can have its emitted type overridden via `--with-type Struct.field=Type`.
+                    // The `*` catch-all is intentionally not honored here, as rewriting every field to
+                    // a single type is meaningless; the qualified `Struct.field` key must be explicit.
+
+                    WithType(fieldDecl, ref remappedName, ref nativeTypeName, matchStar: false);
+                }
             }
         }
 

sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs

@@ -2399,9 +2399,9 @@ private void WithTestAssertTrue(string actual)
         }
     }
 
-    private void WithType(NamedDecl namedDecl, ref string integerTypeName, ref string nativeTypeName)
+    private void WithType(NamedDecl namedDecl, ref string integerTypeName, ref string nativeTypeName, bool matchStar = true)
     {
-        if (TryGetRemappedValue(namedDecl, _config._withTypes, out var type, matchStar: true))
+        if (TryGetRemappedValue(namedDecl, _config._withTypes, out var type, matchStar))
         {
             if (string.IsNullOrWhiteSpace(nativeTypeName))
             {

sources/ClangSharpPInvokeGenerator/Program.Options.cs

@@ -84,7 +84,7 @@ internal static partial class Program
     private static readonly CommandLineOption s_withSetLastErrors = Multi(s_withSetLastErrorOptionAliases, "Add the SetLastError=true modifier or SetsSystemLastError attribute to a given DllImport or UnmanagedFunctionPointer. Supports wildcards.");
     private static readonly CommandLineOption s_withSuppressGCTransitions = Multi(s_withSuppressGCTransitionOptionAliases, "Add the SuppressGCTransition calling convention to a given DllImport or UnmanagedFunctionPointer. Supports wildcards.");
     private static readonly CommandLineOption s_withTransparentStructNameValuePairs = Multi(s_withTransparentStructOptionAliases, "A remapped type name to be treated as a transparent wrapper during binding generation. Supports wildcards.");
-    private static readonly CommandLineOption s_withTypeNameValuePairs = Multi(s_withTypeOptionAliases, "A type to be used for the given enum declaration during binding generation. Supports wildcards.");
+    private static readonly CommandLineOption s_withTypeNameValuePairs = Multi(s_withTypeOptionAliases, "A type to be used for the given enum declaration, macro constant, or struct field (using the qualified `Type.field`) during binding generation. Supports wildcards.");
     private static readonly CommandLineOption s_withUsingNameValuePairs = Multi(s_withUsingOptionAliases, "A using directive to be included for the given remapped declaration name during binding generation. Supports wildcards.");
     private static readonly CommandLineOption s_helpOption = Flag(s_helpOptionAliases, "Show help and usage information");
 

tests/ClangSharp.PInvokeGenerator.UnitTests/WithTypeFieldTest.cs

@@ -0,0 +1,47 @@
+// Copyright (c) .NET Foundation and Contributors. All Rights Reserved. Licensed under the MIT License (MIT). See License.md in the repository root for more information.
+
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace ClangSharp.UnitTests;
+
+/// <summary>
+/// Regression tests for https://github.com/dotnet/ClangSharp/issues/559.
+/// The <c>--with-type</c> option is extended to accept a field-qualified key (<c>Struct.field=Type</c>)
+/// so the emitted type of an individual struct field can be overridden. The original Clang type is
+/// preserved as a <c>[NativeTypeName]</c> attribute and unrelated fields are left untouched.
+/// </summary>
+[Platform("win")]
+public sealed class WithTypeFieldTest : PInvokeGeneratorTest
+{
+    [Test]
+    public Task OverridesFieldType()
+    {
+        var inputContents = @"struct Something
+{
+    int type;
+    int other;
+};
+";
+
+        var expectedOutputContents = @"namespace ClangSharp.Test
+{
+    public partial struct Something
+    {
+        [NativeTypeName(""int"")]
+        public SomethingType type;
+
+        public int other;
+    }
+}
+";
+
+        var withTypes = new Dictionary<string, string>
+        {
+            ["Something.type"] = "SomethingType",
+        };
+
+        return ValidateGeneratedCSharpLatestWindowsBindingsAsync(inputContents, expectedOutputContents, withTypes: withTypes);
+    }
+}