feat(config): generate the settings registry from the spec

Cargo.toml

@@ -3,6 +3,7 @@ resolver = "2"
 members = [
     "argv",
     "config",
+    "config-build",
     "derive",
     "clap_usage",
     "cli",

config-build/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "usage-config-build"
+description = "Generates a usage-config settings registry from a usage spec, at build time"
+version = "5.1.0"
+edition = "2021"
+rust-version = "1.80.0"
+homepage = { workspace = true }
+documentation = { workspace = true }
+repository = { workspace = true }
+authors = { workspace = true }
+license = { workspace = true }
+
+# Runs in a build script, so its dependencies cost an adopter's *build* and never ship: the KDL
+# parser lives here and `usage-config` carries none of it.
+[dependencies]
+usage-lib = { workspace = true }
+
+# The generated code is compiled and run in the tests, which is the only way to know it is code
+# rather than plausible text.
+[dev-dependencies]
+usage-config = { workspace = true, features = ["toml"] }
+
+[package.metadata.release]
+shared-version = true
+release = true

config-build/examples/gen.rs

@@ -0,0 +1,20 @@
+//! Regenerates the checked-in registry the tests compile.
+//!
+//! ```sh
+//! cargo run -p usage-config-build --example gen > config-build/tests/golden/settings.rs
+//! ```
+//!
+//! An example rather than a build script, because the point of checking the file in is that a
+//! human reads the diff: generated code that nobody looks at is where a generator's mistakes live.
+
+const SPEC: &str = "config-build/tests/fixtures/hk.usage.kdl";
+
+fn main() {
+    match usage_config_build::source(SPEC) {
+        Ok(source) => print!("{source}"),
+        Err(err) => {
+            eprintln!("{SPEC}: {err}");
+            std::process::exit(1);
+        }
+    }
+}