diff --git a/Cargo.toml b/Cargo.toml
index 0175f7fb78..04b848f0e8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,6 +11,7 @@ members = [
   "examples/sprites",
   "examples/todolist",
   "fixtures/coverall",
+  "fixtures/callbacks",
   "fixtures/regressions/enum-without-i32-helpers",
   "fixtures/regressions/kotlin-experimental-unsigned-types",
   "fixtures/regressions/cdylib-crate-type-dependency/ffi-crate",
diff --git a/examples/README.md b/examples/README.md
index 7aa76e992f..ab04d0df22 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -14,7 +14,7 @@ Newcomers are recommended to explore them in the following order:
   object-oriented style.
 * [`./todolist`](./todolist/) is a simplistic todo-list that can only add items and show the last item,
   meant to show how interacting with strings works.
-* [`.rondpoint`](./rondpoint/) exercises complex data types by round-tripping them from the foreign-language
+* [`./rondpoint`](./rondpoint/) exercises complex data types by round-tripping them from the foreign-language
   code, through rust and back agian.
 * [`./fxa-client`](./fxa-client/) doesn't work yet, but it contains aspirational example of what the UDL
   might look like for an actual real-world component.
diff --git a/examples/callbacks/Cargo.toml b/examples/callbacks/Cargo.toml
index 2ba8463cf2..29917afaaa 100644
--- a/examples/callbacks/Cargo.toml
+++ b/examples/callbacks/Cargo.toml
@@ -8,7 +8,7 @@ publish = false
 
 [lib]
 crate-type = ["cdylib", "lib"]
-name = "uniffi_callbacks"
+name = "callbacks"
 
 [dependencies]
 uniffi_macros = {path = "../../uniffi_macros"}
diff --git a/examples/callbacks/src/callbacks.udl b/examples/callbacks/src/callbacks.udl
index a8bca47083..0293b14cf1 100644
--- a/examples/callbacks/src/callbacks.udl
+++ b/examples/callbacks/src/callbacks.udl
@@ -9,40 +9,3 @@ callback interface OnCallAnswered {
   void busy();
   void text_received(string text);
 };
-
-/// These objects are implemented by the foreign language and passed 
-/// to Rust. Rust then calls methods on it when it needs to.
-callback interface ForeignGetters {
-  boolean get_bool(boolean v, boolean arg2);
-  string get_string(string v, boolean arg2);
-  string? get_option(string? v, boolean arg2);
-  sequence<i32> get_list(sequence<i32> v, boolean arg2);
-};
-
-/// These objects are implemented in Rust, and call out to `ForeignGetters`
-/// to get the value.
-interface RustGetters {
-  boolean get_bool(ForeignGetters callback, boolean v, boolean arg2);
-  string get_string(ForeignGetters callback, string v, boolean arg2);
-  string? get_option(ForeignGetters callback, string? v, boolean arg2);
-  sequence<i32> get_list(ForeignGetters callback, sequence<i32> v, boolean arg2);
-};
-
-/// These objects are implemented by the foreign language and passed 
-/// to Rust. Rust then calls methods on it when it needs to.
-/// Rust developers need to declare these traits extending `Send` so 
-/// they can be stored in Rust— i.e. not passed in as an argument to
-/// be used immediately.
-callback interface StoredForeignStringifier {
-  string from_simple_type(i32 value);
-  // Test if types are collected from callback interfaces.
-  // kotlinc compile time error if not.
-  string from_complex_type(sequence<f64?>? values); 
-};
-
-/// Rust object that uses the StoredForeignStringifier to produce string representations
-/// of passed arguments.
-interface RustStringifier {
-  constructor(StoredForeignStringifier callback);
-  string from_simple_type(i32 value);
-};
diff --git a/examples/callbacks/src/lib.rs b/examples/callbacks/src/lib.rs
index a21464db13..49058d6483 100644
--- a/examples/callbacks/src/lib.rs
+++ b/examples/callbacks/src/lib.rs
@@ -24,65 +24,4 @@ impl Telephone {
     }
 }
 
-trait ForeignGetters {
-    fn get_bool(&self, v: bool, arg2: bool) -> bool;
-    fn get_string(&self, v: String, arg2: bool) -> String;
-    fn get_option(&self, v: Option<String>, arg2: bool) -> Option<String>;
-    fn get_list(&self, v: Vec<i32>, arg2: bool) -> Vec<i32>;
-}
-
-#[derive(Debug, Clone)]
-pub struct RustGetters;
-
-impl RustGetters {
-    pub fn new() -> Self {
-        RustGetters
-    }
-    fn get_bool(&self, callback: Box<dyn ForeignGetters>, v: bool, arg2: bool) -> bool {
-        callback.get_bool(v, arg2)
-    }
-    fn get_string(&self, callback: Box<dyn ForeignGetters>, v: String, arg2: bool) -> String {
-        callback.get_string(v, arg2)
-    }
-    fn get_option(
-        &self,
-        callback: Box<dyn ForeignGetters>,
-        v: Option<String>,
-        arg2: bool,
-    ) -> Option<String> {
-        callback.get_option(v, arg2)
-    }
-    fn get_list(&self, callback: Box<dyn ForeignGetters>, v: Vec<i32>, arg2: bool) -> Vec<i32> {
-        callback.get_list(v, arg2)
-    }
-}
-
-impl Default for RustGetters {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-// Use `Send+Send` because we want to store the callback in an exposed
-// `Send+Sync` object.
-trait StoredForeignStringifier: Send + Sync + std::fmt::Debug {
-    fn from_simple_type(&self, value: i32) -> String;
-    fn from_complex_type(&self, values: Option<Vec<Option<f64>>>) -> String;
-}
-
-#[derive(Debug)]
-pub struct RustStringifier {
-    callback: Box<dyn StoredForeignStringifier>,
-}
-
-impl RustStringifier {
-    fn new(callback: Box<dyn StoredForeignStringifier>) -> Self {
-        RustStringifier { callback }
-    }
-
-    fn from_simple_type(&self, value: i32) -> String {
-        self.callback.from_simple_type(value)
-    }
-}
-
 include!(concat!(env!("OUT_DIR"), "/callbacks.uniffi.rs"));
diff --git a/examples/callbacks/tests/bindings/test_callbacks.kts b/examples/callbacks/tests/bindings/test_callbacks.kts
index 616cdb481b..2007f0f1aa 100644
--- a/examples/callbacks/tests/bindings/test_callbacks.kts
+++ b/examples/callbacks/tests/bindings/test_callbacks.kts
@@ -4,7 +4,7 @@
 
 import uniffi.callbacks.*
 
-// 0. Simple example just to see it work.
+// Simple example just to see it work.
 // Pass in a string, get a string back.
 // Pass in nothing, get unit back.
 class OnCallAnsweredImpl : OnCallAnswered {
@@ -47,67 +47,3 @@ assert(cbObjet2.busyCount == 0) { "yesCount=${cbObjet2.busyCount} (should be 0)"
 assert(cbObjet2.yesCount == 1) { "yesCount=${cbObjet2.yesCount} (should be 1)" }
 
 telephone.destroy()
-
-// A bit more systematic in testing, but this time in English.
-// 
-// 1. Pass in the callback as arguments.
-// Make the callback methods use multiple aruments, with a variety of types, and 
-// with a variety of return types.
-val rustGetters = RustGetters()
-class KotlinGetters(): ForeignGetters {
-    override fun getBool(v: Boolean, arg2: Boolean): Boolean = v xor arg2
-    override fun getString(v: String, arg2: Boolean): String = if (arg2) "1234567890123" else v
-    override fun getOption(v: String?, arg2: Boolean): String? = if (arg2) v?.uppercase() else v
-    override fun getList(v: List<Int>, arg2: Boolean): List<Int> = if (arg2) v else listOf()
-}
-
-val callback = KotlinGetters()
-listOf(true, false).forEach { v -> 
-    val flag = true
-    val expected = callback.getBool(v, flag)
-    val observed = rustGetters.getBool(callback, v, flag)
-    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
-}
-
-listOf(listOf(1,2), listOf(0,1)).forEach { v -> 
-    val flag = true
-    val expected = callback.getList(v, flag)
-    val observed = rustGetters.getList(callback, v, flag)
-    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
-}
-
-listOf("Hello", "world").forEach { v -> 
-    val flag = true
-    val expected = callback.getString(v, flag)
-    val observed = rustGetters.getString(callback, v, flag)
-    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
-}
-
-listOf("Some", null).forEach { v -> 
-    val flag = false
-    val expected = callback.getOption(v, flag)
-    val observed = rustGetters.getOption(callback, v, flag)
-    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
-}
-rustGetters.destroy()
-
-// 2. Pass the callback in as a constructor argument, to be stored on the Object struct.
-// This is crucial if we want to configure a system at startup,
-// then use it without passing callbacks all the time.
-
-class StoredKotlinStringifier: StoredForeignStringifier {
-    override fun fromSimpleType(value: Int): String = "kotlin: $value"
-    // We don't test this, but we're checking that the arg type is included in the minimal list of types used
-    // in the UDL.
-    // If this doesn't compile, then look at TypeResolver.
-    override fun fromComplexType(values: List<Double?>?): String = "kotlin: $values"
-}
-
-val kotlinStringifier = StoredKotlinStringifier()
-val rustStringifier = RustStringifier(kotlinStringifier)
-listOf(1, 2).forEach { v -> 
-    val expected = kotlinStringifier.fromSimpleType(v)
-    val observed = rustStringifier.fromSimpleType(v)
-    assert(expected == observed) { "callback is sent on construction: $expected != $observed" }
-}
-rustStringifier.destroy()
\ No newline at end of file
diff --git a/examples/callbacks/uniffi.toml b/examples/callbacks/uniffi.toml
new file mode 100644
index 0000000000..a2bfcce7b9
--- /dev/null
+++ b/examples/callbacks/uniffi.toml
@@ -0,0 +1,3 @@
+[bindings.kotlin]
+package_name = "uniffi.callbacks"
+cdylib_name = "callbacks"
diff --git a/fixtures/callbacks/Cargo.toml b/fixtures/callbacks/Cargo.toml
new file mode 100644
index 0000000000..c1da936220
--- /dev/null
+++ b/fixtures/callbacks/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "callbacks"
+version = "0.12.0"
+authors = ["Firefox Sync Team <sync-team@mozilla.com>"]
+edition = "2018"
+publish = false
+
+[lib]
+crate-type = ["staticlib", "cdylib"]
+name = "uniffi_callbacks"
+
+[dependencies]
+uniffi_macros = {path = "../../uniffi_macros"}
+uniffi = {path = "../../uniffi", features=["builtin-bindgen"]}
+thiserror = "1.0"
+lazy_static = "1.4"
+
+[build-dependencies]
+uniffi_build = {path = "../../uniffi_build", features=["builtin-bindgen"]}
diff --git a/fixtures/callbacks/README.md b/fixtures/callbacks/README.md
new file mode 100644
index 0000000000..bbb999994a
--- /dev/null
+++ b/fixtures/callbacks/README.md
@@ -0,0 +1,11 @@
+# A "Callbacks" test for uniffi components
+
+This is similar to the `callbacks` example, but it's intended to be contrived and to
+ensure we get good test coverage of all possible options.
+
+It's here so it doesn't even need to make a cursory effort to be a "good"
+example; it intentionally panics, asserts params are certain values, has
+no-op methods etc. If you're trying to get your head around uniffi then the
+"examples" directory will be a much better bet.
+
+This is its own crate, because the callback mechanism is not implemented for all backends yet.
diff --git a/fixtures/callbacks/build.rs b/fixtures/callbacks/build.rs
new file mode 100644
index 0000000000..a98aacddb3
--- /dev/null
+++ b/fixtures/callbacks/build.rs
@@ -0,0 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+fn main() {
+    uniffi_build::generate_scaffolding("./src/callbacks.udl").unwrap();
+}
diff --git a/fixtures/callbacks/src/callbacks.udl b/fixtures/callbacks/src/callbacks.udl
new file mode 100644
index 0000000000..db61326e66
--- /dev/null
+++ b/fixtures/callbacks/src/callbacks.udl
@@ -0,0 +1,38 @@
+namespace callbacks {};
+
+/// These objects are implemented by the foreign language and passed
+/// to Rust. Rust then calls methods on it when it needs to.
+callback interface ForeignGetters {
+  boolean get_bool(boolean v, boolean arg2);
+  string get_string(string v, boolean arg2);
+  string? get_option(string? v, boolean arg2);
+  sequence<i32> get_list(sequence<i32> v, boolean arg2);
+};
+
+/// These objects are implemented in Rust, and call out to `ForeignGetters`
+/// to get the value.
+interface RustGetters {
+  boolean get_bool(ForeignGetters callback, boolean v, boolean arg2);
+  string get_string(ForeignGetters callback, string v, boolean arg2);
+  string? get_option(ForeignGetters callback, string? v, boolean arg2);
+  sequence<i32> get_list(ForeignGetters callback, sequence<i32> v, boolean arg2);
+};
+
+/// These objects are implemented by the foreign language and passed
+/// to Rust. Rust then calls methods on it when it needs to.
+/// Rust developers need to declare these traits extending `Send` so
+/// they can be stored in Rust— i.e. not passed in as an argument to
+/// be used immediately.
+callback interface StoredForeignStringifier {
+  string from_simple_type(i32 value);
+  // Test if types are collected from callback interfaces.
+  // kotlinc compile time error if not.
+  string from_complex_type(sequence<f64?>? values);
+};
+
+/// Rust object that uses the StoredForeignStringifier to produce string representations
+/// of passed arguments.
+interface RustStringifier {
+  constructor(StoredForeignStringifier callback);
+  string from_simple_type(i32 value);
+};
diff --git a/fixtures/callbacks/src/lib.rs b/fixtures/callbacks/src/lib.rs
new file mode 100644
index 0000000000..8f41583614
--- /dev/null
+++ b/fixtures/callbacks/src/lib.rs
@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+trait ForeignGetters {
+    fn get_bool(&self, v: bool, arg2: bool) -> bool;
+    fn get_string(&self, v: String, arg2: bool) -> String;
+    fn get_option(&self, v: Option<String>, arg2: bool) -> Option<String>;
+    fn get_list(&self, v: Vec<i32>, arg2: bool) -> Vec<i32>;
+}
+
+#[derive(Debug, Clone)]
+pub struct RustGetters;
+
+impl RustGetters {
+    pub fn new() -> Self {
+        RustGetters
+    }
+    fn get_bool(&self, callback: Box<dyn ForeignGetters>, v: bool, arg2: bool) -> bool {
+        callback.get_bool(v, arg2)
+    }
+    fn get_string(&self, callback: Box<dyn ForeignGetters>, v: String, arg2: bool) -> String {
+        callback.get_string(v, arg2)
+    }
+    fn get_option(
+        &self,
+        callback: Box<dyn ForeignGetters>,
+        v: Option<String>,
+        arg2: bool,
+    ) -> Option<String> {
+        callback.get_option(v, arg2)
+    }
+    fn get_list(&self, callback: Box<dyn ForeignGetters>, v: Vec<i32>, arg2: bool) -> Vec<i32> {
+        callback.get_list(v, arg2)
+    }
+}
+
+impl Default for RustGetters {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// Use `Send+Send` because we want to store the callback in an exposed
+// `Send+Sync` object.
+trait StoredForeignStringifier: Send + Sync + std::fmt::Debug {
+    fn from_simple_type(&self, value: i32) -> String;
+    fn from_complex_type(&self, values: Option<Vec<Option<f64>>>) -> String;
+}
+
+#[derive(Debug)]
+pub struct RustStringifier {
+    callback: Box<dyn StoredForeignStringifier>,
+}
+
+impl RustStringifier {
+    fn new(callback: Box<dyn StoredForeignStringifier>) -> Self {
+        RustStringifier { callback }
+    }
+
+    fn from_simple_type(&self, value: i32) -> String {
+        self.callback.from_simple_type(value)
+    }
+}
+
+include!(concat!(env!("OUT_DIR"), "/callbacks.uniffi.rs"));
diff --git a/fixtures/callbacks/tests/bindings/test_callbacks.kts b/fixtures/callbacks/tests/bindings/test_callbacks.kts
new file mode 100644
index 0000000000..863b531cbb
--- /dev/null
+++ b/fixtures/callbacks/tests/bindings/test_callbacks.kts
@@ -0,0 +1,69 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import uniffi.callbacks.*
+
+// A bit more systematic in testing, but this time in English.
+//
+// 1. Pass in the callback as arguments.
+// Make the callback methods use multiple aruments, with a variety of types, and
+// with a variety of return types.
+val rustGetters = RustGetters()
+class KotlinGetters(): ForeignGetters {
+    override fun getBool(v: Boolean, arg2: Boolean): Boolean = v xor arg2
+    override fun getString(v: String, arg2: Boolean): String = if (arg2) "1234567890123" else v
+    override fun getOption(v: String?, arg2: Boolean): String? = if (arg2) v?.uppercase() else v
+    override fun getList(v: List<Int>, arg2: Boolean): List<Int> = if (arg2) v else listOf()
+}
+
+val callback = KotlinGetters()
+listOf(true, false).forEach { v ->
+    val flag = true
+    val expected = callback.getBool(v, flag)
+    val observed = rustGetters.getBool(callback, v, flag)
+    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
+}
+
+listOf(listOf(1,2), listOf(0,1)).forEach { v ->
+    val flag = true
+    val expected = callback.getList(v, flag)
+    val observed = rustGetters.getList(callback, v, flag)
+    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
+}
+
+listOf("Hello", "world").forEach { v ->
+    val flag = true
+    val expected = callback.getString(v, flag)
+    val observed = rustGetters.getString(callback, v, flag)
+    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
+}
+
+listOf("Some", null).forEach { v ->
+    val flag = false
+    val expected = callback.getOption(v, flag)
+    val observed = rustGetters.getOption(callback, v, flag)
+    assert(expected == observed) { "roundtripping through callback: $expected != $observed" }
+}
+rustGetters.destroy()
+
+// 2. Pass the callback in as a constructor argument, to be stored on the Object struct.
+// This is crucial if we want to configure a system at startup,
+// then use it without passing callbacks all the time.
+
+class StoredKotlinStringifier: StoredForeignStringifier {
+    override fun fromSimpleType(value: Int): String = "kotlin: $value"
+    // We don't test this, but we're checking that the arg type is included in the minimal list of types used
+    // in the UDL.
+    // If this doesn't compile, then look at TypeResolver.
+    override fun fromComplexType(values: List<Double?>?): String = "kotlin: $values"
+}
+
+val kotlinStringifier = StoredKotlinStringifier()
+val rustStringifier = RustStringifier(kotlinStringifier)
+listOf(1, 2).forEach { v ->
+    val expected = kotlinStringifier.fromSimpleType(v)
+    val observed = rustStringifier.fromSimpleType(v)
+    assert(expected == observed) { "callback is sent on construction: $expected != $observed" }
+}
+rustStringifier.destroy()
diff --git a/fixtures/callbacks/tests/test_generated_bindings.rs b/fixtures/callbacks/tests/test_generated_bindings.rs
new file mode 100644
index 0000000000..95d37265f3
--- /dev/null
+++ b/fixtures/callbacks/tests/test_generated_bindings.rs
@@ -0,0 +1,4 @@
+uniffi_macros::build_foreign_language_testcases!(
+    "src/callbacks.udl",
+    ["tests/bindings/test_callbacks.kts"]
+);