feat(tvix/eval): add configuration of Nix search path to public API

This is required for passing through NIX_PATH from the CLI.

Change-Id: If129df79ef9c3ffab31408adb85679909276c4f0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7544
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -37,6 +37,7 @@ mod tests;
 
 use std::path::PathBuf;
 use std::rc::Rc;
+use std::str::FromStr;
 use std::sync::Arc;
 
 // Re-export the public interface used by other crates.
@@ -47,7 +48,7 @@ pub use crate::pretty_ast::pretty_print_expr;
 pub use crate::source::SourceCode;
 pub use crate::value::Value;
 pub use crate::vm::run_lambda;
-pub use crate::warnings::EvalWarning;
+pub use crate::warnings::{EvalWarning, WarningKind};
 
 /// Internal-only parts of `tvix-eval`, exported for use in macros, but not part of the public
 /// interface of the crate.
@@ -65,6 +66,9 @@ pub(crate) fn unwrap_or_clone_rc<T: Clone>(rc: Rc<T>) -> T {
 /// An `Evaluation` represents how a piece of Nix code is evaluated. It can be
 /// instantiated and configured directly, or it can be accessed through the
 /// various simplified helper methods available below.
+///
+/// Public fields are intended to be set by the caller. Setting all
+/// fields is optional.
 #[derive(Clone)]
 pub struct Evaluation<'a> {
     /// The Nix source code to be evaluated.
@@ -83,6 +87,10 @@ pub struct Evaluation<'a> {
 
     /// Root expression of the Nix code after parsing.
     expr: Option<rnix::ast::Expr>,
+
+    /// (optional) Nix search path, e.g. the value of `NIX_PATH` used
+    /// for resolving items on the search path (such as `<nixpkgs>`).
+    pub nix_path: Option<String>,
 }
 
 /// Result of evaluating a piece of Nix code. If evaluation succeeded, a value
@@ -121,6 +129,7 @@ impl<'a> Evaluation<'a> {
             source_map,
             file,
             expr: None,
+            nix_path: None,
         }
     }
 
@@ -177,8 +186,24 @@ impl<'a> Evaluation<'a> {
 
         // If there were no errors during compilation, the resulting bytecode is
         // safe to execute.
+
+        let nix_path = self
+            .nix_path
+            .as_ref()
+            .and_then(|s| match nix_search_path::NixSearchPath::from_str(s) {
+                Ok(path) => Some(path),
+                Err(err) => {
+                    result.warnings.push(EvalWarning {
+                        kind: WarningKind::InvalidNixPath(err.to_string()),
+                        span: self.file.span,
+                    });
+                    None
+                }
+            })
+            .unwrap_or_else(|| Default::default());
+
         let vm_result = run_lambda(
-            Default::default(), // TODO: add nix search path to `Evaluation`
+            nix_path,
             &mut observer::NoOpObserver::default(), // TODO: runtime observer
             compiler_result.lambda,
         );

tvix/eval/src/warnings.rs

@@ -12,6 +12,7 @@ pub enum WarningKind {
     UnusedBinding,
     ShadowedGlobal(&'static str),
     DeprecatedLegacyLet,
+    InvalidNixPath(String),
 
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, but do not cause runtime failures.
@@ -80,6 +81,10 @@ impl EvalWarning {
                 "legacy `let` syntax used, please rewrite this as `let .. in ...`".to_string()
             }
 
+            WarningKind::InvalidNixPath(ref err) => {
+                format!("invalid NIX_PATH resulted in a parse error: {}", err)
+            }
+
             WarningKind::NotImplemented(what) => {
                 format!("feature not yet implemented in tvix: {}", what)
             }
@@ -95,6 +100,7 @@ impl EvalWarning {
             WarningKind::UnusedBinding => "W003",
             WarningKind::ShadowedGlobal(_) => "W004",
             WarningKind::DeprecatedLegacyLet => "W005",
+            WarningKind::InvalidNixPath(_) => "W006",
             WarningKind::NotImplemented(_) => "W999",
         }
     }