Struct Layer API (#814)

CHANGELOG.md

@@ -9,6 +9,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `libcnb`:
+  - A new API for working with layers has been added. See the `BuildContext::cached_layer` and `BuildContext::uncached_layer` docs for examples of how to use this API. ([#814](https://github.com/heroku/libcnb.rs/pull/814))
+
+### Changed
+
+- `libcnb`:
+  - The `Layer` trait and related types and functions have been deprecated. Please migrate to the new API. ([#814](https://github.com/heroku/libcnb.rs/pull/814))
+  - Errors related to layers have been restructured. While this is technically a breaking change, buildpacks usually don't have to be modified in practice. ([#814](https://github.com/heroku/libcnb.rs/pull/814))
+
 ### Fixed
 
 - `libcnb-data`:

examples/execd/src/main.rs

@@ -1,3 +1,7 @@
+// This example uses the older trait Layer API. The example will be updated to the newer API
+// before the next libcnb.rs release.
+#![allow(deprecated)]
+
 mod layer;
 
 use crate::layer::ExecDLayer;

libcnb/src/error.rs

@@ -1,5 +1,5 @@
 use crate::data::launch::ProcessTypeError;
-use crate::layer::HandleLayerError;
+use crate::layer::LayerError;
 use libcnb_common::toml_file::TomlFileError;
 use std::fmt::Debug;
 
@@ -11,8 +11,8 @@ pub type Result<T, E> = std::result::Result<T, Error<E>>;
 /// An error that occurred during buildpack execution.
 #[derive(thiserror::Error, Debug)]
 pub enum Error<E> {
-    #[error("HandleLayer error: {0}")]
-    HandleLayerError(#[from] HandleLayerError),
+    #[error("Layer error: {0}")]
+    LayerError(#[from] LayerError),
 
     #[error("Process type error: {0}")]
     ProcessTypeError(#[from] ProcessTypeError),

libcnb/src/layer/mod.rs

@@ -1,10 +1,13 @@
 //! Provides types and helpers to work with layers.
 
-mod handling;
-mod public_interface;
+pub(crate) mod shared;
+pub(crate) mod struct_api;
+pub(crate) mod trait_api;
 
-#[cfg(test)]
-mod tests;
+pub use shared::DeleteLayerError;
+pub use shared::LayerError;
+pub use shared::ReadLayerError;
+pub use shared::WriteLayerError;
 
-pub(crate) use handling::*;
-pub use public_interface::*;
+pub use struct_api::*;
+pub use trait_api::*;