diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3635e9a50d..607101d6e1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,7 +15,9 @@
 [All changes in [[UnreleasedVersion]]](https://github.com/mozilla/uniffi-rs/compare/v0.23.0...HEAD).
 
 ### ⚠️ Breaking Changes ⚠️
-- ABI: Implemented a new callback-interface ABI that significantly improves performance on Python and Kotlin.  UniFFI users will automatically get the benefits of this without any code changes.  This is a breaking change because the new generated code is not compatible with code generated with UniFFI 0.23.x.
+- ABI: Implemented a new callback-interface ABI that significantly improves performance on Python and Kotlin.
+  - UniFFI users will automatically get the benefits of this without any code changes.
+  - External bindings authors will need to update their bindings code.  See PR #1494 for details.
 
 ### What's changed
 
diff --git a/fixtures/benchmarks/README.md b/fixtures/benchmarks/README.md
index 46d234436c..93fb811742 100644
--- a/fixtures/benchmarks/README.md
+++ b/fixtures/benchmarks/README.md
@@ -1,5 +1,10 @@
 This fixture runs a set of benchmark tests, using criterion to test the performance.
 
+- `cargo bench` to run all benchmarks.
+- `cargo bench -- -p` to run all python benchmarks (or -s for swift, -k for kotlin)
+- `cargo bench -- [glob]` to run a subset of the benchmarks
+- `cargo bench -- --help` for more details on the CLI
+
 Benchmarking UniFFI is tricky and involves a bit of ping-pong between Rust and
 the foreign language:
 
diff --git a/fixtures/benchmarks/benches/bindings/run_benchmarks.kts b/fixtures/benchmarks/benches/bindings/run_benchmarks.kts
index d11bfffbf1..9d0b7ef408 100644
--- a/fixtures/benchmarks/benches/bindings/run_benchmarks.kts
+++ b/fixtures/benchmarks/benches/bindings/run_benchmarks.kts
@@ -6,14 +6,14 @@ import uniffi.benchmarks.*
 import kotlin.system.measureNanoTime
 
 class TestCallbackObj : TestCallbackInterface {
-    override fun testMethod(a: Int, b: Int, data: TestData): String {
+    override fun method(a: Int, b: Int, data: TestData): String {
         return data.bar;
     }
 
-    override fun testVoidReturn(a: Int, b: Int, data: TestData) {
+    override fun methodWithVoidReturn(a: Int, b: Int, data: TestData) {
     }
 
-    override fun testNoArgsVoidReturn() {
+    override fun methodWithNoArgsAndVoidReturn() {
     }
 
     override fun runTest(testCase: TestCase, count: ULong): ULong {
diff --git a/fixtures/benchmarks/benches/bindings/run_benchmarks.py b/fixtures/benchmarks/benches/bindings/run_benchmarks.py
index ce2d22ff4d..10063512cd 100644
--- a/fixtures/benchmarks/benches/bindings/run_benchmarks.py
+++ b/fixtures/benchmarks/benches/bindings/run_benchmarks.py
@@ -6,13 +6,13 @@
 import time
 
 class TestCallbackObj:
-    def test_method(self, a, b, data):
+    def method(self, a, b, data):
         return data.bar
 
-    def test_void_return(self, a, b, data):
+    def method_with_void_return(self, a, b, data):
         pass
 
-    def test_no_args_void_return(self):
+    def method_with_no_args_and_void_return(self):
         pass
 
     def run_test(self, test_case, count):
diff --git a/fixtures/benchmarks/benches/bindings/run_benchmarks.swift b/fixtures/benchmarks/benches/bindings/run_benchmarks.swift
index 6404e918df..023e24ee50 100644
--- a/fixtures/benchmarks/benches/bindings/run_benchmarks.swift
+++ b/fixtures/benchmarks/benches/bindings/run_benchmarks.swift
@@ -13,14 +13,14 @@
 #endif
 
 class TestCallbackObj: TestCallbackInterface {
-    func testMethod(a: Int32, b: Int32, data: TestData) -> String {
+    func method(a: Int32, b: Int32, data: TestData) -> String {
         return data.bar
     }
 
-    func testVoidReturn(a: Int32, b: Int32, data: TestData) {
+    func methodWithVoidReturn(a: Int32, b: Int32, data: TestData) {
     }
 
-    func testNoArgsVoidReturn() {
+    func methodWithNoArgsAndVoidReturn() {
     }
 
     func runTest(testCase: TestCase, count: UInt64) -> UInt64 {
diff --git a/fixtures/benchmarks/src/benchmarks.udl b/fixtures/benchmarks/src/benchmarks.udl
index 53ea45cc81..c6646d33ee 100644
--- a/fixtures/benchmarks/src/benchmarks.udl
+++ b/fixtures/benchmarks/src/benchmarks.udl
@@ -31,9 +31,9 @@ callback interface TestCallbackInterface {
   // including: popping arguments from the stack, unpacking RustBuffers,
   // pushing return values back to the stack, etc.
 
-  string test_method(i32 a, i32 b, TestData data);  // Should return data.bar
-  void test_void_return(i32 a, i32 b, TestData data);
-  void test_no_args_void_return();
+  string method(i32 a, i32 b, TestData data);  // Should return data.bar
+  void method_with_void_return(i32 a, i32 b, TestData data);
+  void method_with_no_args_and_void_return();
 
   // Run a performance test N times and return the elapsed time in nanoseconds
   u64 run_test(TestCase test_case, u64 count);
diff --git a/fixtures/benchmarks/src/lib.rs b/fixtures/benchmarks/src/lib.rs
index 624f0549a0..34ff5d858b 100644
--- a/fixtures/benchmarks/src/lib.rs
+++ b/fixtures/benchmarks/src/lib.rs
@@ -20,9 +20,9 @@ pub enum TestCase {
 }
 
 pub trait TestCallbackInterface {
-    fn test_method(&self, a: i32, b: i32, data: TestData) -> String;
-    fn test_void_return(&self, a: i32, b: i32, data: TestData);
-    fn test_no_args_void_return(&self);
+    fn method(&self, a: i32, b: i32, data: TestData) -> String;
+    fn method_with_void_return(&self, a: i32, b: i32, data: TestData);
+    fn method_with_no_args_and_void_return(&self);
     fn run_test(&self, test_case: TestCase, count: u64) -> u64;
 }
 
@@ -61,7 +61,7 @@ pub fn run_benchmarks(language: String, cb: Box<dyn TestCallbackInterface>) {
         .noise_threshold(0.05)
         .bench_function(format!("{language}-callbacks-basic"), |b| {
             b.iter(|| {
-                cb.test_method(
+                cb.method(
                     10,
                     100,
                     TestData {
@@ -73,7 +73,7 @@ pub fn run_benchmarks(language: String, cb: Box<dyn TestCallbackInterface>) {
         })
         .bench_function(format!("{language}-callbacks-void-return"), |b| {
             b.iter(|| {
-                cb.test_void_return(
+                cb.method_with_void_return(
                     10,
                     100,
                     TestData {
@@ -84,7 +84,7 @@ pub fn run_benchmarks(language: String, cb: Box<dyn TestCallbackInterface>) {
             })
         })
         .bench_function(format!("{language}-callbacks-no-args-void-return"), |b| {
-            b.iter(|| cb.test_no_args_void_return())
+            b.iter(|| cb.method_with_no_args_and_void_return())
         });
 
     c.final_summary();
diff --git a/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceRuntime.kt b/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceRuntime.kt
index 302702b8bd..0f58da6068 100644
--- a/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceRuntime.kt
+++ b/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceRuntime.kt
@@ -41,6 +41,10 @@ interface ForeignCallback : com.sun.jna.Callback {
 // Magic number for the Rust proxy to call using the same mechanism as every other method,
 // to free the callback once it's dropped by Rust.
 internal const val IDX_CALLBACK_FREE = 0
+// Callback return codes
+internal const val UNIFFI_CALLBACK_SUCCESS = 0
+internal const val UNIFFI_CALLBACK_ERROR = 1
+internal const val UNIFFI_CALLBACK_UNEXPECTED_ERROR = 2
 
 public abstract class FfiConverterCallbackInterface<CallbackInterface>(
     protected val foreignCallback: ForeignCallback
diff --git a/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceTemplate.kt b/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceTemplate.kt
index d1fb106c12..7ea7fe25d8 100644
--- a/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceTemplate.kt
+++ b/uniffi_bindgen/src/bindings/kotlin/templates/CallbackInterfaceTemplate.kt
@@ -27,15 +27,15 @@ internal class {{ foreign_callback }} : ForeignCallback {
         return when (method) {
             IDX_CALLBACK_FREE -> {
                 {{ ffi_converter_name }}.drop(handle)
-                // No return value.
-                // See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
-                0
+                // Successful return
+                // See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
+                UNIFFI_CALLBACK_SUCCESS
             }
             {% for meth in cbi.methods() -%}
             {% let method_name = format!("invoke_{}", meth.name())|fn_name -%}
             {{ loop.index }} -> {
                 // Call the method, write to outBuf and return a status code
-                // See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs` for info
+                // See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs` for info
                 try {
                     this.{{ method_name }}(cb, argsData, argsLen, outBuf)
                 } catch (e: Throwable) {
@@ -46,20 +46,20 @@ internal class {{ foreign_callback }} : ForeignCallback {
                     } catch (e: Throwable) {
                         // If that fails, then it's time to give up and just return
                     }
-                    -1
+                    UNIFFI_CALLBACK_UNEXPECTED_ERROR
                 }
             }
             {% endfor %}
             else -> {
                 // An unexpected error happened.
-                // See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
+                // See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
                 try {
                     // Try to serialize the error into a string
                     outBuf.setValue({{ Type::String.borrow()|ffi_converter_name }}.lower("Invalid Callback index"))
                 } catch (e: Throwable) {
                     // If that fails, then it's time to give up and just return
                 }
-                -1
+                UNIFFI_CALLBACK_UNEXPECTED_ERROR
             }
         }
     }
@@ -84,7 +84,7 @@ internal class {{ foreign_callback }} : ForeignCallback {
                 {%- endfor %}
             )
             outBuf.setValue({{ return_type|ffi_converter_name }}.lowerIntoRustBuffer(returnValue))
-            return 1
+            return UNIFFI_CALLBACK_SUCCESS
         }
         {%- when None %}
         fun makeCall() : Int {
@@ -94,7 +94,7 @@ internal class {{ foreign_callback }} : ForeignCallback {
                 {%- if !loop.last %}, {% endif %}
                 {%- endfor %}
             )
-            return 1
+            return UNIFFI_CALLBACK_SUCCESS
         }
         {%- endmatch %}
 
@@ -107,7 +107,7 @@ internal class {{ foreign_callback }} : ForeignCallback {
         } catch (e: {{ error_type|type_name }}) {
             // Expected error, serialize it into outBuf
             outBuf.setValue({{ error_type|ffi_converter_name }}.lowerIntoRustBuffer(e))
-            -2
+            UNIFFI_CALLBACK_ERROR
         }
         {%- endmatch %}
 
diff --git a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
index e8a0ec56d9..4d23701689 100644
--- a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
+++ b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
@@ -40,6 +40,10 @@ def remove(self, handle):
 # Magic number for the Rust proxy to call using the same mechanism as every other method,
 # to free the callback once it's dropped by Rust.
 IDX_CALLBACK_FREE = 0
+# Return codes for callback calls
+UNIFFI_CALLBACK_SUCCESS = 0
+UNIFFI_CALLBACK_ERROR = 1
+UNIFFI_CALLBACK_UNEXPECTED_ERROR = 2
 
 class FfiConverterCallbackInterface:
     _handle_map = ConcurrentHandleMap()
diff --git a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceTemplate.py b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceTemplate.py
index 5702736de4..b847f04971 100644
--- a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceTemplate.py
@@ -40,7 +40,7 @@ def makeCallAndHandleReturn():
             {%- when None %}
             makeCall()
             {%- endmatch %}
-            return 1
+            return UNIFFI_CALLBACK_SUCCESS
 
         {%- match meth.throws_type() %}
         {%- when None %}
@@ -53,7 +53,7 @@ def makeCallAndHandleReturn():
             with RustBuffer.allocWithBuilder() as builder:
                 {{ err|write_fn }}(e, builder)
                 buf_ptr[0] = builder.finalize()
-            return -2
+            return UNIFFI_CALLBACK_ERROR
         {%- endmatch %}
 
     {% endfor %}
@@ -64,15 +64,15 @@ def makeCallAndHandleReturn():
 
     if method == IDX_CALLBACK_FREE:
         {{ ffi_converter_name }}.drop(handle)
-        # No return value.
-        # See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
-        return 0
+        # Successfull return
+        # See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
+        return UNIFFI_CALLBACK_SUCCESS
 
     {% for meth in cbi.methods() -%}
     {% let method_name = format!("invoke_{}", meth.name())|fn_name -%}
     if method == {{ loop.index }}:
         # Call the method and handle any errors
-        # See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs` for details
+        # See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs` for details
         try:
             return {{ method_name }}(cb, RustBufferStream(args_data, args_len), buf_ptr)
         except BaseException as e:
@@ -83,7 +83,7 @@ def makeCallAndHandleReturn():
             except:
                 # If that fails, just give up
                 pass
-            return -1
+            return UNIFFI_CALLBACK_UNEXPECTED_ERROR
     {% endfor %}
 
     # This should never happen, because an out of bounds method index won't
@@ -91,8 +91,8 @@ def makeCallAndHandleReturn():
     # https://github.com/mozilla/uniffi-rs/issues/351
 
     # An unexpected error happened.
-    # See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
-    return -1
+    # See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
+    return UNIFFI_CALLBACK_UNEXPECTED_ERROR
 
 # We need to keep this function reference alive:
 # if they get GC'd while in use then UniFFI internals could attempt to call a function
diff --git a/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceRuntime.swift b/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceRuntime.swift
index 9ab9bdbe48..9ae62d1667 100644
--- a/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceRuntime.swift
+++ b/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceRuntime.swift
@@ -58,3 +58,7 @@ fileprivate class UniFFICallbackHandleMap<T> {
 // Magic number for the Rust proxy to call using the same mechanism as every other method,
 // to free the callback once it's dropped by Rust.
 private let IDX_CALLBACK_FREE: Int32 = 0
+// Callback return codes
+private let UNIFFI_CALLBACK_SUCCESS: Int32 = 0
+private let UNIFFI_CALLBACK_ERROR: Int32 = 1
+private let UNIFFI_CALLBACK_UNEXPECTED_ERROR: Int32 = 2
diff --git a/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceTemplate.swift b/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceTemplate.swift
index 4e4267b7f4..53e347c189 100644
--- a/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceTemplate.swift
+++ b/uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceTemplate.swift
@@ -37,7 +37,7 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
             var writer = [UInt8]()
             {{ return_type|write_fn }}(result, into: &writer)
             out_buf.pointee = RustBuffer(bytes: writer)
-            return 1
+            return UNIFFI_CALLBACK_SUCCESS
         }
         {%- when None %}
         func makeCall() throws -> Int32 {
@@ -47,7 +47,7 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
                     {%- if !loop.last %}, {% endif %}
                     {% endfor -%}
                 )
-            return 1
+            return UNIFFI_CALLBACK_SUCCESS
         }
         {%- endmatch %}
 
@@ -59,7 +59,7 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
             return try makeCall()
         } catch let error as {{ error_type|type_name }} {
             out_buf.pointee = {{ error_type|lower_fn }}(error)
-            return -2
+            return UNIFFI_CALLBACK_ERROR
         }
         {%- endmatch %}
     }
@@ -69,9 +69,9 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
     switch method {
         case IDX_CALLBACK_FREE:
             {{ ffi_converter_name }}.drop(handle: handle)
-            // No return value.
-            // See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
-            return 0
+            // Sucessful return
+            // See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
+            return UNIFFI_CALLBACK_SUCCESS
         {% for meth in cbi.methods() -%}
         {% let method_name = format!("invoke_{}", meth.name())|fn_name -%}
         case {{ loop.index }}:
@@ -80,13 +80,13 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
                 cb = try {{ ffi_converter_name }}.lift(handle)
             } catch {
                 out_buf.pointee = {{ Type::String.borrow()|lower_fn }}("{{ cbi.name() }}: Invalid handle")
-                return -1
+                return UNIFFI_CALLBACK_UNEXPECTED_ERROR
             }
             do {
                 return try {{ method_name }}(cb, argsData, argsLen, out_buf)
             } catch let error {
                 out_buf.pointee = {{ Type::String.borrow()|lower_fn }}(String(describing: error))
-                return -1
+                return UNIFFI_CALLBACK_UNEXPECTED_ERROR
             }
         {% endfor %}
         // This should never happen, because an out of bounds method index won't
@@ -94,8 +94,8 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
         // https://github.com/mozilla/uniffi-rs/issues/351
         default:
             // An unexpected error happened.
-            // See docs of ForeignCallback in `uniffi/src/ffi/foreigncallbacks.rs`
-            return -1
+            // See docs of ForeignCallback in `uniffi_core/src/ffi/foreigncallbacks.rs`
+            return UNIFFI_CALLBACK_UNEXPECTED_ERROR
     }
 }
 
diff --git a/uniffi_core/src/ffi/foreigncallbacks.rs b/uniffi_core/src/ffi/foreigncallbacks.rs
index 40a51ecafc..9fd50508a7 100644
--- a/uniffi_core/src/ffi/foreigncallbacks.rs
+++ b/uniffi_core/src/ffi/foreigncallbacks.rs
@@ -126,8 +126,8 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 /// * The `handle` is the key into a handle map on the other side of the FFI used to look up the foreign language object
 ///   that implements the callback interface/trait.
 /// * The `method` selector specifies the method that will be called on the object, by looking it up in a list of methods from
-///   the IDL. The index is 1 indexed. Note that the list of methods is generated by at uniffi from the IDL and used in all
-///   bindings: so we can rely on the method list being stable within the same run of uniffi.
+///   the IDL. The list is 1 indexed. Note that the list of methods is generated by UniFFI from the IDL and used in all
+///   bindings, so we can rely on the method list being stable within the same run of UniFFI.
 /// * `args_data` and `args_len` represents a serialized buffer of arguments to the function. The scaffolding code
 ///   writes the callback arguments to this buffer, in order, using `FfiConverter.write()`. The bindings code reads the
 ///   arguments from the buffer and passes them to the user's callback.
@@ -151,6 +151,9 @@ pub type ForeignCallback = unsafe extern "C" fn(
 /// The method index used by the Drop trait to communicate to the foreign language side that Rust has finished with it,
 /// and it can be deleted from the handle map.
 pub const IDX_CALLBACK_FREE: u32 = 0;
+pub const CALLBACK_SUCCESS: i32 = 0;
+pub const CALLBACK_ERROR: i32 = 1;
+pub const CALLBACK_UNEXPECTED_ERROR: i32 = 2;
 
 // Overly-paranoid sanity checking to ensure that these types are
 // convertible between each-other. `transmute` actually should check this for
@@ -206,6 +209,8 @@ impl ForeignCallbackInternals {
     ) -> c_int {
         let ptr_value = self.callback_ptr.load(Ordering::SeqCst);
         unsafe {
+            // SAFETY: `callback_ptr` was set in `set_callback` from a ForeignCallback pointer, so
+            // it's safe to transmute it back here.
             let callback = std::mem::transmute::<usize, Option<ForeignCallback>>(ptr_value)
                 .expect("Callback interface handler not set");
             callback(
@@ -218,6 +223,8 @@ impl ForeignCallbackInternals {
         }
     }
 
+    // TODO: Refactor the callback code so that we don't need to have 3 different functions here
+
     /// Invoke a callback interface method that can't throw on the foreign side and returns
     /// non-Result value on the Rust side
     pub fn invoke_callback<R, UniFfiTag>(&self, handle: u64, method: u32, args: RustBuffer) -> R
@@ -227,23 +234,15 @@ impl ForeignCallbackInternals {
         let mut ret_rbuf = RustBuffer::new();
         let callback_result = self.call_callback(handle, method, args, &mut ret_rbuf);
         match callback_result {
-            1 => {
-                // 1 indicates success with the return value written to the RustBuffer for
-                //   non-void calls.
+            CALLBACK_SUCCESS => {
                 let vec = ret_rbuf.destroy_into_vec();
                 let mut ret_buf = vec.as_slice();
                 R::try_read(&mut ret_buf).unwrap()
             }
-            -2 => {
-                panic!("Callback return -2, but no Error was expected")
-            }
-            // 0 is a deprecated method to indicates success for void returns
-            0 => {
-                log::error!("UniFFI: Callback interface returned 0. Please update the bindings code to return 1 for all successful calls");
-                panic!("Callback returned 0 when we were expecting a return value");
+            CALLBACK_ERROR => {
+                panic!("Callback return 1, but no Error was expected")
             }
-            // -1 indicates an unexpected error
-            -1 => {
+            CALLBACK_UNEXPECTED_ERROR => {
                 let reason = if !ret_rbuf.is_empty() {
                     match <String as FfiConverter<UniFfiTag>>::try_lift(ret_rbuf) {
                         Ok(s) => s,
@@ -263,8 +262,8 @@ impl ForeignCallbackInternals {
         }
     }
 
-    /// Invoke a callback interface method that can throw on the foreign side / returnns a Result<>
-    /// on the Rust side
+    /// Invoke a callback interface method that can throw on the foreign side / returns a
+    /// `Result<>` on the Rust side
     pub fn invoke_callback_with_error<R, E, UniFfiTag>(
         &self,
         handle: u64,
@@ -279,26 +278,17 @@ impl ForeignCallbackInternals {
         let mut ret_rbuf = RustBuffer::new();
         let callback_result = self.call_callback(handle, method, args, &mut ret_rbuf);
         match callback_result {
-            1 => {
-                // 1 indicates success with the return value written to the RustBuffer for
-                //   non-void calls.
+            CALLBACK_SUCCESS => {
                 let vec = ret_rbuf.destroy_into_vec();
                 let mut ret_buf = vec.as_slice();
                 Ok(R::try_read(&mut ret_buf).unwrap())
             }
-            -2 => {
-                // -2 indicates an error written to the RustBuffer
+            CALLBACK_ERROR => {
                 let vec = ret_rbuf.destroy_into_vec();
                 let mut ret_buf = vec.as_slice();
                 Err(E::try_read(&mut ret_buf).unwrap())
             }
-            // 0 is a deprecated method to indicates success for void returns
-            0 => {
-                log::error!("UniFFI: Callback interface returned 0. Please update the bindings code to return 1 for all successful calls");
-                panic!("Callback returned 0 when we were expecting a return value");
-            }
-            // -1 indicates an unexpected error
-            -1 => {
+            CALLBACK_UNEXPECTED_ERROR => {
                 let reason = if !ret_rbuf.is_empty() {
                     match <String as FfiConverter<UniFfiTag>>::try_lift(ret_rbuf) {
                         Ok(s) => s,
@@ -323,17 +313,9 @@ impl ForeignCallbackInternals {
         let mut ret_rbuf = RustBuffer::new();
         let callback_result = self.call_callback(handle, method, args, &mut ret_rbuf);
         match callback_result {
-            // 1 indicates success
-            1 => {}
-            -2 => {
-                panic!("Callback return -2, but no Error was expected")
-            }
-            // 0 is a deprecated method to indicates success for void returns
-            0 => {
-                log::error!("UniFFI: Callback interface returned 0. Please update the bindings code to return 1 for all successful calls");
-            }
-            // -1 indicates an unexpected error
-            -1 => {
+            CALLBACK_SUCCESS => {}
+            CALLBACK_ERROR => panic!("Callback return -2, but no Error was expected"),
+            CALLBACK_UNEXPECTED_ERROR => {
                 let reason = if !ret_rbuf.is_empty() {
                     match <String as FfiConverter<crate::UniFfiTag>>::try_lift(ret_rbuf) {
                         Ok(s) => s,