diff --git a/InterfaceStubGenerator.Roslyn38/InterfaceStubGenerator.Roslyn38.csproj b/InterfaceStubGenerator.Roslyn38/InterfaceStubGenerator.Roslyn38.csproj
index fa3e54b88..1a3a97d7f 100644
--- a/InterfaceStubGenerator.Roslyn38/InterfaceStubGenerator.Roslyn38.csproj
+++ b/InterfaceStubGenerator.Roslyn38/InterfaceStubGenerator.Roslyn38.csproj
@@ -10,6 +10,7 @@
     <IsRoslynComponent>true</IsRoslynComponent>
     <Nullable>enable</Nullable>
     <MsCACSharpVersion>3.8.0</MsCACSharpVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/InterfaceStubGenerator.Roslyn41/InterfaceStubGenerator.Roslyn41.csproj b/InterfaceStubGenerator.Roslyn41/InterfaceStubGenerator.Roslyn41.csproj
index 8911bd287..192b8ebb1 100644
--- a/InterfaceStubGenerator.Roslyn41/InterfaceStubGenerator.Roslyn41.csproj
+++ b/InterfaceStubGenerator.Roslyn41/InterfaceStubGenerator.Roslyn41.csproj
@@ -11,6 +11,7 @@
     <Nullable>enable</Nullable>
     <DefineConstants>$(DefineConstants);ROSLYN_4</DefineConstants>
     <MsCACSharpVersion>4.1.0</MsCACSharpVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/InterfaceStubGenerator.Shared/Emitter.cs b/InterfaceStubGenerator.Shared/Emitter.cs
index a1305fb48..cc6280128 100644
--- a/InterfaceStubGenerator.Shared/Emitter.cs
+++ b/InterfaceStubGenerator.Shared/Emitter.cs
@@ -69,20 +69,18 @@ public static void Initialize()
         addSource("Generated.g.cs", SourceText.From(generatedClassText, Encoding.UTF8));
     }
 
-    public static string EmitInterface(InterfaceModel model)
+    public static SourceText EmitInterface(InterfaceModel model)
     {
-        var source = new StringBuilder();
+        var source = new SourceWriter();
 
         // if nullability is supported emit the nullable directive
         if (model.Nullability != Nullability.None)
         {
-            source.Append("#nullable ");
-            source.Append(model.Nullability == Nullability.Enabled ? "enable" : "disable");
+            source.WriteLine("#nullable " + (model.Nullability == Nullability.Enabled ? "enable" : "disable"));
         }
 
-        source.Append(
-            $@"
-#pragma warning disable
+        source.WriteLine(
+            $@"#pragma warning disable
 namespace Refit.Implementation
 {{
 
@@ -108,8 +106,7 @@ partial class {model.Ns}{model.ClassDeclaration}
         {{
             Client = client;
             this.requestBuilder = requestBuilder;
-        }}
-"
+        }}"
         );
 
         var uniqueNames = new UniqueNameBuilder();
@@ -138,16 +135,15 @@ partial class {model.Ns}{model.ClassDeclaration}
             WriteDisposableMethod(source);
         }
 
-        source.Append(
+        source.WriteLine(
             @"
     }
     }
 }
 
-#pragma warning restore
-"
+#pragma warning restore"
         );
-        return source.ToString();
+        return source.ToSourceText();
     }
 
     /// <summary>
@@ -158,7 +154,7 @@ partial class {model.Ns}{model.ClassDeclaration}
     /// <param name="isTopLevel">True if directly from the type we're generating for, false for methods found on base interfaces</param>
     /// <param name="uniqueNames">Contains the unique member names in the interface scope.</param>
     private static void WriteRefitMethod(
-        StringBuilder source,
+        SourceWriter source,
         MethodModel methodModel,
         bool isTopLevel,
         UniqueNameBuilder uniqueNames
@@ -220,23 +216,23 @@ UniqueNameBuilder uniqueNames
         WriteMethodClosing(source);
     }
 
-    private static void WriteNonRefitMethod(StringBuilder source, MethodModel methodModel)
+    private static void WriteNonRefitMethod(SourceWriter source, MethodModel methodModel)
     {
         WriteMethodOpening(source, methodModel, true);
 
-        source.Append(
+        source.WriteLine(
             @"
-                throw new global::System.NotImplementedException(""Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument."");
-    "
-        );
+            throw new global::System.NotImplementedException(""Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument."");");
 
+        source.Indentation += 1;
         WriteMethodClosing(source);
+        source.Indentation -= 1;
     }
 
     // TODO: This assumes that the Dispose method is a void that takes no parameters.
     // The previous version did not.
     // Does the bool overload cause an issue here.
-    private static void WriteDisposableMethod(StringBuilder source)
+    private static void WriteDisposableMethod(SourceWriter source)
     {
         source.Append(
             """
@@ -252,7 +248,7 @@ private static void WriteDisposableMethod(StringBuilder source)
     }
 
     private static string GenerateTypeParameterExpression(
-        StringBuilder source,
+        SourceWriter source,
         MethodModel methodModel,
         UniqueNameBuilder uniqueNames
     )
@@ -283,7 +279,7 @@ UniqueNameBuilder uniqueNames
     }
 
     private static void WriteMethodOpening(
-        StringBuilder source,
+        SourceWriter source,
         MethodModel methodModel,
         bool isExplicitInterface,
         bool isAsync = false
@@ -324,7 +320,7 @@ private static void WriteMethodOpening(
         );
     }
 
-    private static void WriteMethodClosing(StringBuilder source) => source.Append(@"    }");
+    private static void WriteMethodClosing(SourceWriter source) => source.Append(@"    }");
 
     private static string GenerateConstraints(
         ImmutableEquatableArray<TypeConstraint> typeParameters,
diff --git a/InterfaceStubGenerator.Shared/IncrementalValuesProviderExtensions.cs b/InterfaceStubGenerator.Shared/IncrementalValuesProviderExtensions.cs
index 0766c4c93..ef4bd070b 100644
--- a/InterfaceStubGenerator.Shared/IncrementalValuesProviderExtensions.cs
+++ b/InterfaceStubGenerator.Shared/IncrementalValuesProviderExtensions.cs
@@ -60,7 +60,7 @@ IncrementalValuesProvider<InterfaceModel> model
             static (spc, model) =>
             {
                 var mapperText = Emitter.EmitInterface(model);
-                spc.AddSource(model.FileName, SourceText.From(mapperText, Encoding.UTF8));
+                spc.AddSource(model.FileName, mapperText);
             }
         );
     }
diff --git a/InterfaceStubGenerator.Shared/InterfaceStubGenerator.Shared.projitems b/InterfaceStubGenerator.Shared/InterfaceStubGenerator.Shared.projitems
index f4cfc6944..7e8f0ebb5 100644
--- a/InterfaceStubGenerator.Shared/InterfaceStubGenerator.Shared.projitems
+++ b/InterfaceStubGenerator.Shared/InterfaceStubGenerator.Shared.projitems
@@ -23,6 +23,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Models\TypeConstraint.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Models\WellKnownTYpes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Parser.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)SourceWriter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UniqueNameBuilder.cs" />
   </ItemGroup>
 </Project>
\ No newline at end of file
diff --git a/InterfaceStubGenerator.Shared/InterfaceStubGenerator.cs b/InterfaceStubGenerator.Shared/InterfaceStubGenerator.cs
index d33ffc17d..bd6e91cde 100644
--- a/InterfaceStubGenerator.Shared/InterfaceStubGenerator.cs
+++ b/InterfaceStubGenerator.Shared/InterfaceStubGenerator.cs
@@ -63,7 +63,7 @@ out var refitInternalNamespace
                 var interfaceText = Emitter.EmitInterface(interfaceModel);
                 context.AddSource(
                     interfaceModel.FileName,
-                    SourceText.From(interfaceText, Encoding.UTF8)
+                    interfaceText
                 );
             }
 
diff --git a/InterfaceStubGenerator.Shared/SourceWriter.cs b/InterfaceStubGenerator.Shared/SourceWriter.cs
new file mode 100644
index 000000000..ac93b28e9
--- /dev/null
+++ b/InterfaceStubGenerator.Shared/SourceWriter.cs
@@ -0,0 +1,142 @@
+using System.Diagnostics;
+using System.Text;
+
+using Microsoft.CodeAnalysis.Text;
+
+namespace Refit.Generator;
+
+// From https://github.com/dotnet/runtime/blob/233826c88d2100263fb9e9535d96f75824ba0aea/src/libraries/Common/src/SourceGenerators/SourceWriter.cs#L11
+internal sealed class SourceWriter
+{
+    private const char IndentationChar = ' ';
+    private const int CharsPerIndentation = 4;
+
+    private readonly StringBuilder _sb = new();
+    private int _indentation;
+
+    public int Indentation
+    {
+        get => _indentation;
+        set
+        {
+            if (value < 0)
+            {
+                Throw();
+                static void Throw() => throw new ArgumentOutOfRangeException(nameof(value));
+            }
+
+            _indentation = value;
+        }
+    }
+
+    public void Append(string text)
+    {
+        if (_indentation == 0)
+        {
+            _sb.Append(text);
+            return;
+        }
+
+        bool isFinalLine;
+        ReadOnlySpan<char> remainingText = text.AsSpan();
+        do
+        {
+            ReadOnlySpan<char> nextLine = GetNextLine(ref remainingText, out isFinalLine);
+
+            AddIndentation();
+            AppendSpan(_sb, nextLine);
+            if (!isFinalLine)
+            {
+                _sb.AppendLine();
+            }
+        }
+        while (!isFinalLine);
+    }
+
+    public void WriteLine(char value)
+    {
+        AddIndentation();
+        _sb.Append(value);
+        _sb.AppendLine();
+    }
+
+    public void WriteLine(string text)
+    {
+        if (_indentation == 0)
+        {
+            _sb.AppendLine(text);
+            return;
+        }
+
+        bool isFinalLine;
+        ReadOnlySpan<char> remainingText = text.AsSpan();
+        do
+        {
+            ReadOnlySpan<char> nextLine = GetNextLine(ref remainingText, out isFinalLine);
+
+            AddIndentation();
+            AppendSpan(_sb, nextLine);
+            _sb.AppendLine();
+        }
+        while (!isFinalLine);
+    }
+
+    public void WriteLine() => _sb.AppendLine();
+
+    public SourceText ToSourceText()
+    {
+        Debug.Assert(_indentation == 0 && _sb.Length > 0);
+        return SourceText.From(_sb.ToString(), Encoding.UTF8);
+    }
+
+    public void Reset()
+    {
+        _sb.Clear();
+        _indentation = 0;
+    }
+
+    private void AddIndentation()
+        => _sb.Append(IndentationChar, CharsPerIndentation * _indentation);
+
+    private static ReadOnlySpan<char> GetNextLine(ref ReadOnlySpan<char> remainingText, out bool isFinalLine)
+    {
+        if (remainingText.IsEmpty)
+        {
+            isFinalLine = true;
+            return default;
+        }
+
+        ReadOnlySpan<char> next;
+        ReadOnlySpan<char> rest;
+
+        int lineLength = remainingText.IndexOf('\n');
+        if (lineLength == -1)
+        {
+            lineLength = remainingText.Length;
+            isFinalLine = true;
+            rest = default;
+        }
+        else
+        {
+            rest = remainingText.Slice(lineLength + 1);
+            isFinalLine = false;
+        }
+
+        if ((uint)lineLength > 0 && remainingText[lineLength - 1] == '\r')
+        {
+            lineLength--;
+        }
+
+        next = remainingText.Slice(0, lineLength);
+        remainingText = rest;
+        return next;
+    }
+
+    private static unsafe void AppendSpan(StringBuilder builder, ReadOnlySpan<char> span)
+    {
+        fixed (char* ptr = span)
+        {
+            builder.Append(ptr, span.Length);
+        }
+    }
+}
diff --git a/Refit.GeneratorTests/_snapshots/InterfaceTests.NonRefitMethodShouldRaiseDiagnostic#IGeneratedClient.g.verified.cs b/Refit.GeneratorTests/_snapshots/InterfaceTests.NonRefitMethodShouldRaiseDiagnostic#IGeneratedClient.g.verified.cs
index 1b01c0762..bd40b5712 100644
--- a/Refit.GeneratorTests/_snapshots/InterfaceTests.NonRefitMethodShouldRaiseDiagnostic#IGeneratedClient.g.verified.cs
+++ b/Refit.GeneratorTests/_snapshots/InterfaceTests.NonRefitMethodShouldRaiseDiagnostic#IGeneratedClient.g.verified.cs
@@ -50,7 +50,7 @@ public RefitGeneratorTestIGeneratedClient(global::System.Net.Http.HttpClient cli
         /// <inheritdoc />
         void global::RefitGeneratorTest.IGeneratedClient.NonRefitMethod()
         {
-                throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
+            throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
         }
     }
     }
diff --git a/Refit.GeneratorTests/_snapshots/InterfaceTests.RefitInterfaceDerivedFromBaseTest#IGeneratedInterface.g.verified.cs b/Refit.GeneratorTests/_snapshots/InterfaceTests.RefitInterfaceDerivedFromBaseTest#IGeneratedInterface.g.verified.cs
index e4f43fc5c..ad553c934 100644
--- a/Refit.GeneratorTests/_snapshots/InterfaceTests.RefitInterfaceDerivedFromBaseTest#IGeneratedInterface.g.verified.cs
+++ b/Refit.GeneratorTests/_snapshots/InterfaceTests.RefitInterfaceDerivedFromBaseTest#IGeneratedInterface.g.verified.cs
@@ -50,7 +50,7 @@ public RefitGeneratorTestIGeneratedInterface(global::System.Net.Http.HttpClient
         /// <inheritdoc />
         void global::RefitGeneratorTest.IBaseInterface.NonRefitMethod()
         {
-                throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
+            throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
         }
     }
     }
diff --git a/Refit.GeneratorTests/_snapshots/MethodTests.MethodsWithGenericConstraints#IGeneratedClient.g.verified.cs b/Refit.GeneratorTests/_snapshots/MethodTests.MethodsWithGenericConstraints#IGeneratedClient.g.verified.cs
index 3b8ce9976..349e3867c 100644
--- a/Refit.GeneratorTests/_snapshots/MethodTests.MethodsWithGenericConstraints#IGeneratedClient.g.verified.cs
+++ b/Refit.GeneratorTests/_snapshots/MethodTests.MethodsWithGenericConstraints#IGeneratedClient.g.verified.cs
@@ -61,7 +61,7 @@ public RefitGeneratorTestIGeneratedClient(global::System.Net.Http.HttpClient cli
          where T3 : struct
          where T5 : class
         {
-                throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
+            throw new global::System.NotImplementedException("Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument.");
         }
     }
     }