imp: hidden option to swap eval.nix

- this is useful for custom evaluators which implement custom
  hive or flake schemata.

- as long as they satisfy the `hive` contract (currently a bit implicit)

closes: zhaofengli#81

src/cli.rs

@@ -47,6 +47,17 @@ For more details, read the manual at <{}>.
         format!(r#"If this argument is not specified, Colmena will search upwards from the current working directory for a file named "flake.nix" or "hive.nix". This behavior is disabled if --config/-f is given explicitly.
 
 For a sample configuration, check the manual at <{}>.
+"#, MANUAL_URL)
+    };
+
+    static ref EVAL_HELP: String = {
+        format!(
+          r#"Specify a custom eval.nix file that can consume either a hive.nix or a flake.nix and extract the Hive's contract out of it.
+If this argument is not specified, Colmena will use a builtin eval.nix.
+If this argument is specified, the aspects of the official manual about the colmena nix schema & contracts
+might not be true any more.
+
+For more details, read the manual at <{}>.
 "#, MANUAL_URL)
     };
 }
@@ -111,6 +122,13 @@ pub fn build_cli(include_internal: bool) -> ClapCommand<'static> {
             // is explicitly supplied by the user (occurrences_of > 0).
             .default_value("hive.nix")
             .global(true))
+        .arg(Arg::new("eval")
+            .long("eval")
+            .hide(true)
+            .value_name("EVAL")
+            .help("Path to a custom nix evaluator expression that satisfies the Hive contract")
+            .long_help(Some(EVAL_HELP.as_str()))
+            .global(true))
         .arg(Arg::new("show-trace")
             .long("show-trace")
             .help("Show debug information for Nix commands")

src/nix/hive/mod.rs

@@ -1,5 +1,6 @@
 use std::collections::HashMap;
-use std::io::Write;
+use std::io::{Read, Write};
+use std::fs::File;
 use std::path::{Path, PathBuf};
 use std::convert::AsRef;
 
@@ -53,7 +54,7 @@ pub struct Hive {
     /// or "flake.nix".
     context_dir: Option<PathBuf>,
 
-    /// Path to temporary file containing eval.nix.
+    /// Path to a temporary file containing the builtin eval.nix or to the user supplied eval.nix.
     eval_nix: TempPath,
 
     /// Path to temporary file containing options.nix.
@@ -117,14 +118,24 @@ impl HivePath {
 }
 
 impl Hive {
-    pub fn new(path: HivePath) -> ColmenaResult<Self> {
+    pub fn new(path: HivePath, eval: Option<&Path>) -> ColmenaResult<Self> {
         let mut eval_nix = NamedTempFile::new()?;
         let mut options_nix = NamedTempFile::new()?;
         let mut modules_nix = NamedTempFile::new()?;
-        eval_nix.write_all(HIVE_EVAL).unwrap();
         options_nix.write_all(HIVE_OPTIONS).unwrap();
         modules_nix.write_all(HIVE_MODULES).unwrap();
 
+        if let Some(p) = eval {
+          let mut f = File::open(p)?;
+          let mut buffer = Vec::new();
+
+          // read the whole file
+          f.read_to_end(&mut buffer)?;
+          eval_nix.write_all(&buffer).unwrap();
+        } else {
+          eval_nix.write_all(HIVE_EVAL).unwrap();
+        }
+
         let context_dir = path.context_dir();
 
         Ok(Self {

src/util.rs

@@ -1,5 +1,5 @@
 use std::convert::TryFrom;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::process::Stdio;
 
 use async_trait::async_trait;
@@ -184,6 +184,12 @@ impl CommandExt for CommandExecution {
 }
 
 pub async fn hive_from_args(args: &ArgMatches) -> ColmenaResult<Hive> {
+    let eval = match args.value_of("eval") {
+        Some(s) => {
+          Some(Path::new(s))
+        }
+        None => None,
+    };
     let path = match args.occurrences_of("config") {
         0 => {
             // traverse upwards until we find hive.nix
@@ -227,7 +233,7 @@ pub async fn hive_from_args(args: &ArgMatches) -> ColmenaResult<Hive> {
                 // Treat as flake URI
                 let flake = Flake::from_uri(path).await?;
                 let hive_path = HivePath::Flake(flake);
-                let mut hive = Hive::new(hive_path)?;
+                let mut hive = Hive::new(hive_path, eval)?;
 
                 if args.is_present("show-trace") {
                     hive.set_show_trace(true);
@@ -250,7 +256,7 @@ pub async fn hive_from_args(args: &ArgMatches) -> ColmenaResult<Hive> {
         }
     }
 
-    let mut hive = Hive::new(hive_path)?;
+    let mut hive = Hive::new(hive_path, eval)?;
 
     if args.is_present("show-trace") {
         hive.set_show_trace(true);