nix: teach it to evaluate strings passed over the cli

ofborg/src/nix.rs

@@ -1,5 +1,6 @@
 use ofborg::asynccmd::{AsyncCmd, SpawnedAsyncCmd};
 use ofborg::partition_result;
+use std::collections::HashMap;
 use std::env;
 use std::fmt;
 use std::fs::File;
@@ -13,6 +14,7 @@ use tempfile::tempfile;
 
 #[derive(Clone, Debug)]
 pub enum Operation {
+    Evaluate,
     Instantiate,
     Build,
     QueryPackagesJSON,
@@ -24,6 +26,7 @@ pub enum Operation {
 impl Operation {
     fn command(&self) -> Command {
         match *self {
+            Operation::Evaluate => Command::new("nix-instantiate"),
             Operation::Instantiate => Command::new("nix-instantiate"),
             Operation::Build => Command::new("nix-build"),
             Operation::QueryPackagesJSON => Command::new("nix-env"),
@@ -53,6 +56,9 @@ impl Operation {
             Operation::NoOp { ref operation } => {
                 operation.args(command);
             }
+            Operation::Evaluate => {
+                command.args(&["--eval", "--strict", "--json"]);
+            }
             _ => (),
         };
     }
@@ -67,6 +73,7 @@ impl fmt::Display for Operation {
             Operation::QueryPackagesOutputs => write!(f, "nix-env -qaP --no-name --out-path"),
             Operation::NoOp { ref operation } => operation.fmt(f),
             Operation::Unknown { ref program } => write!(f, "{}", program),
+            Operation::Evaluate => write!(f, "nix-instantiate --strict --json ..."),
         }
     }
 }
@@ -160,6 +167,24 @@ impl Nix {
         self.safe_command(&Operation::Instantiate, nixpkgs, attrargs)
     }
 
+    pub fn safely_evaluate_expr_cmd(
+        &self,
+        nixpkgs: &Path,
+        expr: &str,
+        argstrs: HashMap<&str, &str>,
+    ) -> Command {
+        let mut attrargs: Vec<String> = Vec::with_capacity(2 + (argstrs.len() * 3));
+        attrargs.push("--expr".to_owned());
+        attrargs.push(expr.to_owned());
+        for (argname, argstr) in argstrs {
+            attrargs.push(String::from("--argstr"));
+            attrargs.push(argname.to_owned());
+            attrargs.push(argstr.to_owned());
+        }
+
+        self.safe_command(&Operation::Evaluate, nixpkgs, attrargs)
+    }
+
     pub fn safely_build_attrs(
         &self,
         nixpkgs: &Path,
@@ -646,6 +671,22 @@ mod tests {
         assert_run(ret, Expect::Pass, vec!["-passes-instantiation.drv"]);
     }
 
+    #[test]
+    fn safely_evaluate_expr_success() {
+        let nix = nix();
+
+        let ret: Result<File, File> = nix.run(
+            nix.safely_evaluate_expr_cmd(
+                individual_eval_path().as_path(),
+                r#"{ foo ? "bar" }: "The magic value is ${foo}""#,
+                [("foo", "tux")].iter().cloned().collect(),
+            ),
+            true,
+        );
+
+        assert_run(ret, Expect::Pass, vec!["The magic value is tux"]);
+    }
+
     #[test]
     fn strict_sandboxing() {
         let ret: Result<File, File> = nix().safely_build_attrs(