refactor(tvix/eval): generalize EvalIO container r/7407

Don't restrict to a Box<dyn EvalIO>.

There's still one or two places where we do restrict, this will be
solved by b/262.

Change-Id: Ic8d927d6ea81fa12d90b1e4352f35ffaafbd1adf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10639
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

5 files changed, 66 insertions, 43 deletions

diff --git a/tvix/eval/src/lib.rs b/tvix/eval/src/lib.rs
index f852b1a3243d..72cfefee496d 100644
--- a/tvix/eval/src/lib.rs
+++ b/tvix/eval/src/lib.rs
@@ -68,7 +68,7 @@ pub use crate::io::StdIO;
 ///
 /// Public fields are intended to be set by the caller. Setting all
 /// fields is optional.
-pub struct Evaluation<'co, 'ro> {
+pub struct Evaluation<'co, 'ro, IO> {
     /// Source code map used for error reporting.
     source_map: SourceCode,
 
@@ -87,7 +87,7 @@ pub struct Evaluation<'co, 'ro> {
     /// impure builtins.
     ///
     /// Defaults to [`DummyIO`] if not set explicitly.
-    pub io_handle: Box<dyn EvalIO>,
+    pub io_handle: IO,
 
     /// Determines whether the `import` builtin should be made
     /// available. Note that this depends on the `io_handle` being
@@ -131,7 +131,9 @@ pub struct EvaluationResult {
     pub expr: Option<rnix::ast::Expr>,
 }
 
-impl<'co, 'ro> Default for Evaluation<'co, 'ro> {
+/// TODO: this approach of creating the struct, then mutating its values
+/// unnecessarily restricts the type of IO (b/262)
+impl<'co, 'ro> Default for Evaluation<'co, 'ro, Box<dyn EvalIO>> {
     fn default() -> Self {
         let source_map = SourceCode::default();
 
@@ -142,7 +144,7 @@ impl<'co, 'ro> Default for Evaluation<'co, 'ro> {
             source_map,
             builtins,
             src_builtins: vec![],
-            io_handle: Box::new(DummyIO {}),
+            io_handle: Box::new(DummyIO {}) as Box<dyn EvalIO>,
             enable_import: false,
             strict: false,
             nix_path: None,
@@ -152,7 +154,7 @@ impl<'co, 'ro> Default for Evaluation<'co, 'ro> {
     }
 }
 
-impl<'co, 'ro> Evaluation<'co, 'ro> {
+impl<'co, 'ro> Evaluation<'co, 'ro, Box<dyn EvalIO>> {
     #[cfg(feature = "impure")]
     /// Initialise an `Evaluation`, with all impure features turned on by default.
     pub fn new_impure() -> Self {
@@ -166,7 +168,12 @@ impl<'co, 'ro> Evaluation<'co, 'ro> {
 
         eval
     }
+}
 
+impl<'co, 'ro, IO> Evaluation<'co, 'ro, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
     /// Clone the reference to the contained source code map. This is used after
     /// an evaluation for pretty error printing.
     pub fn source_map(&self) -> SourceCode {
@@ -233,7 +240,7 @@ impl<'co, 'ro> Evaluation<'co, 'ro> {
         let compiler_observer = self.compiler_observer.take().unwrap_or(&mut noop_observer);
 
         // Insert a storeDir builtin *iff* a store directory is present.
-        if let Some(store_dir) = self.io_handle.store_dir() {
+        if let Some(store_dir) = self.io_handle.as_ref().store_dir() {
             self.builtins.push(("storeDir", store_dir.into()));
         }
 
diff --git a/tvix/eval/src/nix_search_path.rs b/tvix/eval/src/nix_search_path.rs
index f86cffd6d615..2f6d444475ff 100644
--- a/tvix/eval/src/nix_search_path.rs
+++ b/tvix/eval/src/nix_search_path.rs
@@ -68,11 +68,10 @@ impl NixSearchPathEntry {
     ///
     /// For prefixed path, an entry matches if the prefix does.
     // TODO(tazjin): verify these rules in the C++ impl, seems fishy.
-    fn resolve(
-        &self,
-        io: &mut dyn EvalIO,
-        lookup_path: &Path,
-    ) -> Result<Option<PathBuf>, ErrorKind> {
+    fn resolve<IO>(&self, io: IO, lookup_path: &Path) -> Result<Option<PathBuf>, ErrorKind>
+    where
+        IO: AsRef<dyn EvalIO>,
+    {
         let path = match self {
             NixSearchPathEntry::Path(parent) => canonicalise(parent.join(lookup_path))?,
 
@@ -85,7 +84,7 @@ impl NixSearchPathEntry {
             }
         };
 
-        if io.path_exists(&path).map_err(|e| ErrorKind::IO {
+        if io.as_ref().path_exists(&path).map_err(|e| ErrorKind::IO {
             path: Some(path.clone()),
             error: e.into(),
         })? {
@@ -124,17 +123,18 @@ pub struct NixSearchPath {
 impl NixSearchPath {
     /// Attempt to resolve the given `path` within this [`NixSearchPath`] using the
     /// path resolution rules for `<...>`-style paths
-    pub fn resolve<P>(
+    pub fn resolve<P, IO>(
         &self,
-        io: &mut dyn EvalIO,
+        io: IO,
         path: P,
     ) -> Result<Result<PathBuf, CatchableErrorKind>, ErrorKind>
     where
         P: AsRef<Path>,
+        IO: AsRef<dyn EvalIO>,
     {
         let path = path.as_ref();
         for entry in &self.entries {
-            if let Some(p) = entry.resolve(io, path)? {
+            if let Some(p) = entry.resolve(&io, path)? {
                 return Ok(Ok(p));
             }
         }
@@ -204,8 +204,8 @@ mod tests {
         #[test]
         fn simple_dir() {
             let nix_search_path = NixSearchPath::from_str("./.").unwrap();
-            let mut io = StdIO {};
-            let res = nix_search_path.resolve(&mut io, "src").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "src").unwrap();
             assert_eq!(
                 res.unwrap().to_path_buf(),
                 current_dir().unwrap().join("src").clean()
@@ -215,8 +215,8 @@ mod tests {
         #[test]
         fn failed_resolution() {
             let nix_search_path = NixSearchPath::from_str("./.").unwrap();
-            let mut io = StdIO {};
-            let err = nix_search_path.resolve(&mut io, "nope").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let err = nix_search_path.resolve(&io, "nope").unwrap();
             assert!(
                 matches!(err, Err(CatchableErrorKind::NixPathResolution(..))),
                 "err = {err:?}"
@@ -226,16 +226,16 @@ mod tests {
         #[test]
         fn second_in_path() {
             let nix_search_path = NixSearchPath::from_str("./.:/").unwrap();
-            let mut io = StdIO {};
-            let res = nix_search_path.resolve(&mut io, "etc").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "etc").unwrap();
             assert_eq!(res.unwrap().to_path_buf(), Path::new("/etc"));
         }
 
         #[test]
         fn prefix() {
             let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
-            let mut io = StdIO {};
-            let res = nix_search_path.resolve(&mut io, "tvix/src").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "tvix/src").unwrap();
             assert_eq!(
                 res.unwrap().to_path_buf(),
                 current_dir().unwrap().join("src").clean()
@@ -245,8 +245,8 @@ mod tests {
         #[test]
         fn matching_prefix() {
             let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
-            let mut io = StdIO {};
-            let res = nix_search_path.resolve(&mut io, "tvix").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "tvix").unwrap();
             assert_eq!(res.unwrap().to_path_buf(), current_dir().unwrap().clean());
         }
     }
diff --git a/tvix/eval/src/tests/mod.rs b/tvix/eval/src/tests/mod.rs
index 7509379fdab6..e3d3445a45ac 100644
--- a/tvix/eval/src/tests/mod.rs
+++ b/tvix/eval/src/tests/mod.rs
@@ -1,4 +1,4 @@
-use crate::value::Value;
+use crate::{value::Value, EvalIO};
 use builtin_macros::builtins;
 use pretty_assertions::assert_eq;
 use rstest::rstest;
@@ -119,7 +119,7 @@ fn identity(#[files("src/tests/tvix_tests/identity-*.nix")] code_path: PathBuf)
 
     let eval = crate::Evaluation {
         strict: true,
-        io_handle: Box::new(crate::StdIO),
+        io_handle: Box::new(crate::StdIO) as Box<dyn EvalIO>,
         ..Default::default()
     };
 
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
index f2ce73a0c742..3d2fd9d266c7 100644
--- a/tvix/eval/src/vm/generators.rs
+++ b/tvix/eval/src/vm/generators.rs
@@ -224,7 +224,10 @@ pub fn pin_generator(
     Box::pin(f)
 }
 
-impl<'o> VM<'o> {
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
     /// Helper function to re-enqueue the current generator while it
     /// is awaiting a value.
     fn reenqueue_generator(&mut self, name: &'static str, span: LightSpan, generator: Generator) {
@@ -411,6 +414,7 @@ impl<'o> VM<'o> {
                         VMRequest::PathImport(path) => {
                             let imported = self
                                 .io_handle
+                                .as_ref()
                                 .import_path(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
@@ -424,6 +428,7 @@ impl<'o> VM<'o> {
                         VMRequest::ReadToString(path) => {
                             let content = self
                                 .io_handle
+                                .as_ref()
                                 .read_to_string(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
@@ -437,6 +442,7 @@ impl<'o> VM<'o> {
                         VMRequest::PathExists(path) => {
                             let exists = self
                                 .io_handle
+                                .as_ref()
                                 .path_exists(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
@@ -451,6 +457,7 @@ impl<'o> VM<'o> {
                         VMRequest::ReadDir(path) => {
                             let dir = self
                                 .io_handle
+                                .as_ref()
                                 .read_dir(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 398a02cc78c8..5238fdc0662f 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -48,7 +48,7 @@ trait GetSpan {
     fn get_span(self) -> Span;
 }
 
-impl<'o> GetSpan for &VM<'o> {
+impl<'o, IO> GetSpan for &VM<'o, IO> {
     fn get_span(self) -> Span {
         self.reasonable_span.span()
     }
@@ -75,12 +75,12 @@ impl GetSpan for Span {
 /// Internal helper trait for ergonomically converting from a `Result<T,
 /// ErrorKind>` to a `Result<T, Error>` using the current span of a call frame,
 /// and chaining the VM's frame stack around it for printing a cause chain.
-trait WithSpan<T, S: GetSpan> {
-    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error>;
+trait WithSpan<T, S: GetSpan, IO> {
+    fn with_span(self, top_span: S, vm: &VM<IO>) -> Result<T, Error>;
 }
 
-impl<T, S: GetSpan> WithSpan<T, S> for Result<T, ErrorKind> {
-    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error> {
+impl<T, S: GetSpan, IO> WithSpan<T, S, IO> for Result<T, ErrorKind> {
+    fn with_span(self, top_span: S, vm: &VM<IO>) -> Result<T, Error> {
         match self {
             Ok(something) => Ok(something),
             Err(kind) => {
@@ -149,7 +149,7 @@ impl CallFrame {
 
     /// Construct an error result from the given ErrorKind and the source span
     /// of the current instruction.
-    pub fn error<T>(&self, vm: &VM, kind: ErrorKind) -> Result<T, Error> {
+    pub fn error<T, IO>(&self, vm: &VM<IO>, kind: ErrorKind) -> Result<T, Error> {
         Err(kind).with_span(self, vm)
     }
 
@@ -249,7 +249,7 @@ impl ImportCache {
     }
 }
 
-struct VM<'o> {
+struct VM<'o, IO> {
     /// VM's frame stack, representing the execution contexts the VM is working
     /// through. Elements are usually pushed when functions are called, or
     /// thunks are being forced.
@@ -280,7 +280,7 @@ struct VM<'o> {
 
     /// Implementation of I/O operations used for impure builtins and
     /// features like `import`.
-    io_handle: Box<dyn EvalIO>,
+    io_handle: IO,
 
     /// Runtime observer which can print traces of runtime operations.
     observer: &'o mut dyn RuntimeObserver,
@@ -324,10 +324,13 @@ struct VM<'o> {
     try_eval_frames: Vec<usize>,
 }
 
-impl<'o> VM<'o> {
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
     pub fn new(
         nix_search_path: NixSearchPath,
-        io_handle: Box<dyn EvalIO>,
+        io_handle: IO,
         observer: &'o mut dyn RuntimeObserver,
         globals: Rc<GlobalsMap>,
         reasonable_span: LightSpan,
@@ -858,7 +861,7 @@ impl<'o> VM<'o> {
                     Value::UnresolvedPath(path) => {
                         let resolved = self
                             .nix_search_path
-                            .resolve(&mut *self.io_handle, *path)
+                            .resolve(&self.io_handle, *path)
                             .with_span(&frame, self)?;
                         self.stack.push(resolved.into());
                     }
@@ -914,7 +917,10 @@ impl<'o> VM<'o> {
 }
 
 /// Implementation of helper functions for the runtime logic above.
-impl<'o> VM<'o> {
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
     pub(crate) fn stack_pop(&mut self) -> Value {
         self.stack.pop().expect("runtime stack empty")
     }
@@ -1301,14 +1307,17 @@ async fn final_deep_force(co: GenCo) -> Result<Value, ErrorKind> {
     Ok(generators::request_deep_force(&co, value).await)
 }
 
-pub fn run_lambda(
+pub fn run_lambda<IO>(
     nix_search_path: NixSearchPath,
-    io_handle: Box<dyn EvalIO>,
+    io_handle: IO,
     observer: &mut dyn RuntimeObserver,
     globals: Rc<GlobalsMap>,
     lambda: Rc<Lambda>,
     strict: bool,
-) -> EvalResult<RuntimeResult> {
+) -> EvalResult<RuntimeResult>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
     // Retain the top-level span of the expression in this lambda, as
     // synthetic "calls" in deep_force will otherwise not have a span
     // to fall back to.