feat(tvix/serde): add an example using nixpkgs/lib

This displays how users can configure an impure evaluation for
tvix-serde, which makes it possible to use e.g. `nixpkgs/lib`.

We might want to add an example showing how the full Nix-glue
compatibility stuff can be added here, too.

Change-Id: I2224a3fc66e739969d4c723c3d9d8127a046b6fd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10994
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>

tvix/serde/examples/nixpkgs.rs

@@ -0,0 +1,34 @@
+//! This program demonstrates deserialising some configuration
+//! structure from Nix code that makes use of nixpkgs.lib
+//!
+//! This example does not add the full set of Nix features (i.e.
+//! builds & derivations).
+
+use serde::Deserialize;
+
+#[derive(Debug, Deserialize)]
+struct Config {
+    host: String,
+    port: usize,
+}
+
+fn main() {
+    let code = r#"
+    let
+       lib = import <nixpkgs/lib>;
+       host = lib.strings.concatStringsSep "." ["foo" "example" "com"];
+    in {
+      inherit host;
+      port = 4242;
+    }
+    "#;
+
+    let result = tvix_serde::from_str_with_config::<Config, _>(code, |eval| {
+        eval.enable_impure(None);
+    });
+
+    match result {
+        Ok(cfg) => println!("Config says: {}:{}", cfg.host, cfg.port),
+        Err(e) => eprintln!("{:?} / {}", e, e),
+    }
+}