nix: Add NixExpression abstraction

This decouples expression generation from evaluation. `NixExpression`s
that evaluate to a set of derivations can be fed to `DrvSetEvaluator`s
which may be able to parallelize evaluation.

src/nix/hive/mod.rs

@@ -15,6 +15,7 @@ use super::{
     NodeName,
     NodeConfig,
     NodeFilter,
+    NixExpression,
     ProfileDerivation,
     StorePath,
 };
@@ -74,6 +75,12 @@ struct SerializedNixExpression {
     json_file: TempPath,
 }
 
+/// An expression to evaluate the system profiles of selected nodes.
+struct EvalSelectedExpression<'hive> {
+    hive: &'hive Hive,
+    nodes_expr: SerializedNixExpression,
+}
+
 impl HivePath {
     pub async fn from_path<P: AsRef<Path>>(path: P) -> ColmenaResult<Self> {
         let path = path.as_ref();
@@ -290,6 +297,16 @@ impl Hive {
             .collect()
     }
 
+    /// Returns the expression to evaluate selected nodes.
+    pub fn eval_selected_expr(&self, nodes: &[NodeName]) -> ColmenaResult<impl NixExpression + '_> {
+        let nodes_expr = SerializedNixExpression::new(nodes)?;
+
+        Ok(EvalSelectedExpression {
+            hive: self,
+            nodes_expr,
+        })
+    }
+
     /// Evaluates an expression using values from the configuration.
     pub async fn introspect(&self, expression: String, instantiate: bool) -> ColmenaResult<String> {
         if instantiate {
@@ -419,8 +436,24 @@ impl SerializedNixExpression {
             json_file: tmp.into_temp_path(),
         })
     }
+}
 
-    pub fn expression(&self) -> String {
+impl NixExpression for SerializedNixExpression {
+    fn expression(&self) -> String {
         format!("(builtins.fromJSON (builtins.readFile {}))", self.json_file.to_str().unwrap())
     }
 }
+
+impl<'hive> NixExpression for EvalSelectedExpression<'hive> {
+    fn expression(&self) -> String {
+        format!(
+            "{} hive.evalSelected {}",
+            self.hive.get_base_expression(),
+            self.nodes_expr.expression(),
+        )
+    }
+
+    fn requires_flakes(&self) -> bool {
+        self.hive.is_flake()
+    }
+}

src/nix/mod.rs

@@ -97,6 +97,17 @@ pub struct NixOptions {
     builders: Option<String>,
 }
 
+/// A Nix expression.
+pub trait NixExpression : Send + Sync {
+    /// Returns the full Nix expression to be evaluated.
+    fn expression(&self) -> String;
+
+    /// Returns whether this expression requires the use of flakes.
+    fn requires_flakes(&self) -> bool {
+        false
+    }
+}
+
 impl NodeName {
     /// Returns the string.
     pub fn as_str(&self) -> &str {
@@ -203,6 +214,12 @@ impl NixOptions {
     }
 }
 
+impl NixExpression for String {
+    fn expression(&self) -> String {
+        self.clone()
+    }
+}
+
 fn validate_keys(keys: &HashMap<String, Key>) -> Result<(), ValidationErrorType> {
     // Bad secret names:
     // - /etc/passwd