support for layered config using a serde Deserializer as a source

This allows a serde Deserializer, representing a config file, to be used
as a value source, via the builder API, which is also new in this commit.

To use the serde feature, you have to put the `#[conf(serde)]` tag on your struct.

We also introduce a `ConfSerde` extension trait, which is implemented
when `#[conf(serde)]` is annotated, and provides implementation details
for this layered config feature on that struct.

The priority order is args > env > serde > defaults

We also support this in subcommands, via the `SubcommandsSerde` extension trait.

To further adjust the serde system, most fields have serde options like
`serde(rename)` and `serde(skip)`. Structs have a `serde(allow_unknown_fields)`
option.

Parameter and Repeat fields have a `serde(use_value_parser)` option, which
can be useful if you just want serde to read a string and then forward it
to the value parser.

.github/workflows/ci-rust.yml

@@ -35,6 +35,7 @@ jobs:
       uses: Swatinem/rust-cache@v2
       with:
         workspaces: . -> target
+        key: default-features
         save-if: ${{ github.ref == 'refs/heads/develop' }}
     - name: Build Rust
       run: cargo check --verbose --locked
@@ -43,6 +44,32 @@ jobs:
     - name: Check dirty git
       uses: ./.github/actions/check-dirty-git
 
+  # builds and tests rust code, with --no-default-features, with --deny warnings. tests for dirty git
+  build-and-test-rust-no-default-features:
+    runs-on: ubuntu-latest
+
+    env:
+      RUSTFLAGS: --deny warnings
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Rustup update
+      run: rustup update
+    - name: Show cargo version
+      run: cargo --version
+    - name: rust build caching
+      uses: Swatinem/rust-cache@v2
+      with:
+        workspaces: . -> target
+        key: no-default-features
+        save-if: ${{ github.ref == 'refs/heads/develop' }}
+    - name: Build Rust
+      run: cargo check --no-default-features --verbose --locked
+    - name: Test Rust
+      run: cargo test --no-default-features --verbose --locked
+    - name: Check dirty git
+      uses: ./.github/actions/check-dirty-git
+
   # lints and checks formatting of rust code
   lint-rust:
     runs-on: ubuntu-latest

Cargo.toml

@@ -21,8 +21,8 @@ include = [
 [package]
 name = "conf"
 version = "0.1.1"
-description = "A derive-based config parser for CLI args and env parameters"
-categories = ["command-line-interface"]
+description = "A derive-based config parser for CLI args, env, and structured config files"
+categories = ["configuration"]
 keywords = [
   "argument",
   "config",
@@ -42,10 +42,15 @@ bench = false
 [dependencies]
 conf_derive = { path = "./conf_derive", version = "0.1.1" }
 clap = { version = "4.5.8", features = ["string"] }
+serde = { version = "1", optional = true }
+
+[features]
+default = ["serde"]
 
 [dev-dependencies]
 assert_matches = "1.5"
 escargot = "0.5"
+figment = { version = "0.10", features = ["json", "toml"] }
+http = { version = "1.1" }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-url = "1"

MOTIVATION.md

@@ -801,6 +801,10 @@ The initial feature set was the features of `clap-derive` I had used most heavil
 
 I ended up adding more features besides this before the first `crates.io` release as I started migrating more of my projects to this, and encountered things that were either harder to migrate, or were just additional features that I realized I wanted and could fit into the framework with relative ease.
 
+In version 0.2, we added support for subcommands.
+
+In version XXX, we added more support for layered config using structured data loaded from files. This was designed as a `serde` integration. This feature was created based on `github` discussions.
+
 ## Testing
 
 If we change the same simple program that we used for testing `clap-derive` to use `conf` instead, we can see that the error handling in these scenarios becomes better.