1 files changed, 120 insertions, 0 deletions
diff --git a/tvix/eval/src/compiler/import.rs b/tvix/eval/src/compiler/import.rs
new file mode 100644
index 000000000000..9036eec81731
--- /dev/null
+++ b/tvix/eval/src/compiler/import.rs
@@ -0,0 +1,120 @@
+//! This module implements the Nix language's `import` feature, which
+//! is exposed as a builtin in the Nix language.
+//!
+//! This is not a typical builtin, as it needs access to internal
+//! compiler and VM state (such as the [`crate::SourceCode`]
+//! instance, or observers).
+
+use super::GlobalsMap;
+use genawaiter::rc::Gen;
+use std::rc::Weak;
+
+use crate::{
+    builtins::coerce_value_to_path,
+    generators::pin_generator,
+    observer::NoOpObserver,
+    value::{Builtin, Thunk},
+    vm::generators::{self, GenCo},
+    ErrorKind, SourceCode, Value,
+};
+
+async fn import_impl(
+    co: GenCo,
+    globals: Weak<GlobalsMap>,
+    source: SourceCode,
+    mut args: Vec<Value>,
+) -> Result<Value, ErrorKind> {
+    // TODO(sterni): canon_path()?
+    let mut path = match coerce_value_to_path(&co, args.pop().unwrap()).await? {
+        Err(cek) => return Ok(Value::Catchable(Box::new(cek))),
+        Ok(path) => path,
+    };
+
+    if path.is_dir() {
+        path.push("default.nix");
+    }
+
+    if let Some(cached) = generators::request_import_cache_lookup(&co, path.clone()).await {
+        return Ok(cached);
+    }
+
+    let mut reader = generators::request_open_file(&co, path.clone()).await;
+    // We read to a String instead of a Vec<u8> because rnix only supports
+    // string source files.
+    let mut contents = String::new();
+    reader.read_to_string(&mut contents)?;
+
+    let parsed = rnix::ast::Root::parse(&contents);
+    let errors = parsed.errors();
+    let file = source.add_file(path.to_string_lossy().to_string(), contents.to_owned());
+
+    if !errors.is_empty() {
+        return Err(ErrorKind::ImportParseError {
+            path,
+            file,
+            errors: errors.to_vec(),
+        });
+    }
+
+    let result = crate::compiler::compile(
+        &parsed.tree().expr().unwrap(),
+        Some(path.clone()),
+        // The VM must ensure that a strong reference to the globals outlives
+        // any self-references (which are weak) embedded within the globals. If
+        // the expect() below panics, it means that did not happen.
+        globals
+            .upgrade()
+            .expect("globals dropped while still in use"),
+        &source,
+        &file,
+        &mut NoOpObserver::default(),
+    )
+    .map_err(|err| ErrorKind::ImportCompilerError {
+        path: path.clone(),
+        errors: vec![err],
+    })?;
+
+    if !result.errors.is_empty() {
+        return Err(ErrorKind::ImportCompilerError {
+            path,
+            errors: result.errors,
+        });
+    }
+
+    for warning in result.warnings {
+        generators::emit_warning(&co, warning).await;
+    }
+
+    // Compilation succeeded, we can construct a thunk from whatever it spat
+    // out and return that.
+    let res = Value::Thunk(Thunk::new_suspended(
+        result.lambda,
+        generators::request_span(&co).await,
+    ));
+
+    generators::request_import_cache_put(&co, path, res.clone()).await;
+
+    Ok(res)
+}
+
+/// Constructs the `import` builtin. This builtin is special in that
+/// it needs to capture the [crate::SourceCode] structure to correctly
+/// track source code locations while invoking a compiler.
+// TODO: need to be able to pass through a CompilationObserver, too.
+// TODO: can the `SourceCode` come from the compiler?
+pub(super) fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builtin {
+    // This (very cheap, once-per-compiler-startup) clone exists
+    // solely in order to keep the borrow checker happy.  It
+    // resolves the tension between the requirements of
+    // Rc::new_cyclic() and Builtin::new()
+    let globals = globals.clone();
+
+    Builtin::new(
+        "import",
+        Some("Import the given file and return the Nix value it evaluates to"),
+        1,
+        move |args| {
+            Gen::new(|co| pin_generator(import_impl(co, globals.clone(), source.clone(), args)))
+        },
+    )
+}