docs: remove references to default-engine feature (#2417)

## What changes are proposed in this pull request?

The `default-engine` feature flag was removed in
#1100 (several months
ago) but the docs were never updated to reflect this. Instead, the
feature flag should have an explicit suffix indicating which tls backend
to use.

This PR just updates the docs to reflect this.

## How was this change tested?
n/a - just docs

Co-authored-by: R. Tyler Croy <rtyler@brokenco.de>

Author: kyli87

CLAUDE.md

@@ -61,11 +61,10 @@ cargo +nightly fmt \
 
 ### Feature Flags
 
-- `default-engine` / `default-engine-rustls` / `default-engine-native-tls` -- async
-  Arrow/Tokio engine (pick one TLS backend)
+- `default-engine-rustls` / `default-engine-native-tls` -- async Arrow/Tokio engine (pick a TLS backend)
 - `arrow`, `arrow-XX`, `arrow-YY` -- Arrow version selection (kernel tracks the latest two
   major Arrow releases; `arrow` defaults to latest). Kernel itself does not depend on Arrow,
-  but default-engine does.
+  but the default engine does.
 - `arrow-conversion`, `arrow-expression` -- Arrow interop (auto-enabled by default engine)
 - `prettyprint` -- enables Arrow pretty-print helpers (primarily test/example oriented)
 - `clustered-table` -- clustered table write support (experimental)

README.md

@@ -55,16 +55,17 @@ consumer's own `Engine` trait, the kernel has a feature flag to enable a default
 delta_kernel = "0.21.0"
 
 # or turn on the default engine, based on latest arrow
-delta_kernel = { version = "0.21.0", features = ["default-engine", "arrow"] }
+delta_kernel = { version = "0.21.0", features = ["default-engine-rustls", "arrow"] }
 ```
 
 ### Feature flags
-There are more feature flags in addition to the `default-engine` flag shown above. Relevant flags
-include:
+There are more feature flags in addition to the `default-engine-rustls` flag shown above. Relevant
+flags include:
 
 | Feature flag  | Description   |
 | ------------- | ------------- |
-| `default-engine`    | Turn on the 'default' engine: async, arrow-based `Engine` implementation  |
+| `default-engine-rustls`    | Turn on the 'default' engine with rustls TLS backend  |
+| `default-engine-native-tls`| Turn on the 'default' engine with native-tls TLS backend  |
 | `arrow-conversion`  | Conversion utilities for arrow/kernel schema interoperation |
 | `arrow-expression`  | Expression system implementation for arrow |
 
@@ -74,8 +75,8 @@ are still unstable. We therefore may break APIs within minor releases (that is,
 we will not break APIs in patch releases (`0.1.0` -> `0.1.1`).
 
 ## Arrow versioning
-If you enable the `default-engine` feature, you get an implementation of the `Engine` trait that
-uses [Arrow] as its data format.
+If you enable a default engine feature (`default-engine-rustls` or `default-engine-native-tls`),
+you get an implementation of the `Engine` trait that uses [Arrow] as its data format.
 
 The [`arrow crate`](https://docs.rs/arrow/latest/arrow/) tends to release new major versions rather
 frequently. To enable engines that already integrate arrow to also integrate kernel and not force

ffi/cbindgen.toml

@@ -11,7 +11,6 @@ namespace = "ffi"
 cpp_compat = true
 
 [defines]
-"feature = default-engine" = "DEFINE_DEFAULT_ENGINE"
 "feature = default-engine-rustls" = "DEFINE_DEFAULT_ENGINE_RUSTLS"
 "feature = default-engine-base" = "DEFINE_DEFAULT_ENGINE_BASE"
 

kernel/Cargo.toml

@@ -117,8 +117,8 @@ arrow-expression = ["need-arrow"]
 # Schema diffing functionality (experimental)
 schema-diff = []
 
-# this is an 'internal' feature flag which has all the shared bits from default-engine and
-# default-engine-rustls
+# this is an 'internal' feature flag which has all the shared bits from default-engine-native-tls
+# and default-engine-rustls
 default-engine-base = [
   "arrow-conversion",
   "arrow-expression",

kernel/src/engine/mod.rs

@@ -1,6 +1,6 @@
 //! Provides an engine implementation that implements the required traits. The engine can optionally
-//! be built into the kernel by setting the `default-engine` feature flag. See the related module
-//! for more information.
+//! be built into the kernel by setting the `default-engine-rustls` or `default-engine-native-tls`
+//! feature flag. See the related module for more information.
 
 #[cfg(feature = "arrow-expression")]
 use crate::parquet::arrow::arrow_reader::ArrowReaderOptions;