feat: introduce layered crate (#189)

Summary

Introduces layered, a modern service abstraction crate for building
composable async services with middleware.

  Key features:
- Service trait using native async fn with &self - no manual Future
types or boxing required
- Layer and Stack for composing middleware using ergonomic tuple syntax:
(layer1, layer2, service).build()
  - Execute helper to convert closures into services
- Intercept middleware for observing/modifying inputs and outputs
(logging, metrics, validation)
  - DynamicService for type erasure when concrete types become unwieldy
  - Bidirectional Tower interoperability via the tower module

Author: martintmk

.spelling

@@ -1,4 +1,5 @@
-297
+299
+→
 0.X.Y
 100k
 10k
@@ -64,6 +65,7 @@ C-NUM-FMT
 codebase
 codebases
 composability
+composable
 config
 const
 contoso

CHANGELOG.md

@@ -10,6 +10,7 @@ Please see each crate's change log below:
 - [`fundle`](./crates/fundle/CHANGELOG.md)
 - [`fundle_macros`](./crates/fundle_macros/CHANGELOG.md)
 - [`fundle_macros_impl`](./crates/fundle_macros_impl/CHANGELOG.md)
+- [`layered`](./crates/layered/CHANGELOG.md)
 - [`ohno`](./crates/ohno/CHANGELOG.md)
 - [`ohno_macros`](./crates/ohno_macros/CHANGELOG.md)
 - [`recoverable`](./crates/recoverable/CHANGELOG.md)

Cargo.lock

@@ -32,6 +32,7 @@ data_privacy_macros_impl = { path = "crates/data_privacy_macros_impl", default-f
 fundle = { path = "crates/fundle", default-features = false, version = "0.3.0" }
 fundle_macros = { path = "crates/fundle_macros", default-features = false, version = "0.3.0" }
 fundle_macros_impl = { path = "crates/fundle_macros_impl", default-features = false, version = "0.3.0" }
+layered = { path = "crates/layered", default-features = false, version = "0.1.0" }
 ohno = { path = "crates/ohno", default-features = false, version = "0.2.0" }
 ohno_macros = { path = "crates/ohno_macros", default-features = false, version = "0.2.0" }
 recoverable = { path = "crates/recoverable", default-features = false, version = "0.1.0" }
@@ -50,6 +51,7 @@ chrono-tz = { version = "0.10.4", default-features = false }
 criterion = { version = "0.7.0", default-features = false }
 derive_more = { version = "2.0.1", default-features = false }
 duct = { version = "1.1.1", default-features = false }
+dynosaur = { version = "0.3.0", default-features = false }
 futures = { version = "0.3.31", default-features = false }
 futures-core = { version = "0.3.31", default-features = false }
 futures-util = { version = "0.3.31", default-features = false }
@@ -83,6 +85,9 @@ thiserror = { version = "2.0.17", default-features = false }
 time = { version = "0.3.37", default-features = false }
 time-tz = { version = "2.0.0", default-features = false }
 tokio = { version = "1.48.0", default-features = false }
+tower = { version = "0.5.2", default-features = false }
+tower-layer = { version = "0.3.3", default-features = false }
+tower-service = { version = "0.3.3", default-features = false }
 tracing = { version = "0.1.41", default-features = false }
 tracing-subscriber = { version = "0.3.20", default-features = false }
 trait-variant = { version = "0.1.2", default-features = false }

Cargo.toml

@@ -32,6 +32,7 @@ These are the crates built out of this repo:
 - [`fundle`](./crates/fundle/README.md) - Compile-time safe dependency injection for Rust.
 - [`fundle_macros`](crates/fundle_macros/README.md) - Macros for the `fundle` crate.
 - [`fundle_macros_impl`](crates/fundle_macros_impl/README.md) - Macros for the `fundle` crate.
+- [`layered`](./crates/layered/README.md) - A foundational service abstraction for building composable, middleware-driven systems.
 - [`ohno`](./crates/ohno/README.md) - High-quality Rust error handling.
 - [`ohno_macros`](./crates/ohno_macros/README.md) - Macros for the `ohno` crate.
 - [`recoverable`](./crates/recoverable/README.md) - Recovery information and classification for resilience patterns.

README.md

@@ -0,0 +1,12 @@
+# Changelog
+
+## [0.1.0] - 2025-12-30
+
+- ✨ Features
+
+  - Introduce the layered crate with `Service` trait and layer composition system
+  - Add `Execute` wrapper for turning async functions into services
+  - Add `Stack` trait for composing layers with tuples
+  - Add `Intercept` middleware for observing and modifying inputs/outputs (`intercept` feature)
+  - Add `DynamicService` for type-erased services (`dynamic-service` feature)
+  - Add Tower interoperability via `Adapter` (`tower-service` feature)

crates/layered/CHANGELOG.md

@@ -0,0 +1,75 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+[package]
+name = "layered"
+description = "Build composable async services with layered middleware."
+version = "0.1.0"
+readme = "README.md"
+keywords = ["oxidizer", "service", "middleware", "layer", "compose"]
+categories = ["data-structures"]
+
+edition.workspace = true
+rust-version.workspace = true
+authors.workspace = true
+license.workspace = true
+homepage.workspace = true
+repository.workspace = true
+
+[package.metadata.docs.rs]
+all-features = true
+
+[package.metadata.cargo_check_external_types]
+allowed_external_types = ["tower_layer::Layer", "tower_service::Service"]
+
+[features]
+default = []
+intercept = []
+dynamic-service = ["dep:dynosaur"]
+tower-service = ["dep:tower-service"]
+
+[dependencies]
+dynosaur = { workspace = true, optional = true }
+tower-layer = { workspace = true }
+tower-service = { workspace = true, optional = true }
+
+[dev-dependencies]
+alloc_tracker = { workspace = true }
+criterion = { workspace = true }
+dynosaur = { workspace = true }
+futures = { workspace = true, features = ["executor"] }
+mutants = { workspace = true }
+pin-project-lite = { workspace = true }
+static_assertions = { workspace = true }
+tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
+tower = { workspace = true, features = ["limit", "util"] }
+tower-service = { workspace = true }
+
+[lints]
+workspace = true
+
+[[example]]
+name = "custom_layer"
+
+[[example]]
+name = "service_basic"
+required-features = ["intercept"]
+
+[[example]]
+name = "tower_compatibility"
+required-features = ["tower-service", "intercept"]
+
+[[bench]]
+harness = false
+name = "dynamic"
+required-features = ["dynamic-service", "intercept"]
+
+[[bench]]
+harness = false
+name = "intercept"
+required-features = ["intercept"]
+
+[[bench]]
+harness = false
+name = "tower"
+required-features = ["intercept", "tower-service"]