feat(tvix/eval): Support builtins.readDir

Co-authored-by: Griffin Smith <root@gws.fyi>
Change-Id: I5ff19efbe87d8f571f22ab0480500505afa624c5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6552
Autosubmit: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -1,5 +1,6 @@
 use crate::spans::ToSpan;
 use crate::value::CoercionKind;
+use std::io;
 use std::path::PathBuf;
 use std::rc::Rc;
 use std::sync::Arc;
@@ -121,6 +122,12 @@ pub enum ErrorKind {
         errors: Vec<Error>,
     },
 
+    /// I/O errors
+    IO {
+        path: Option<PathBuf>,
+        error: Rc<io::Error>,
+    },
+
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, and without which eval can not
     /// proceed.
@@ -142,6 +149,15 @@ impl From<Error> for ErrorKind {
     }
 }
 
+impl From<io::Error> for ErrorKind {
+    fn from(e: io::Error) -> Self {
+        ErrorKind::IO {
+            path: None,
+            error: Rc::new(e),
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Error {
     pub kind: ErrorKind,
@@ -306,6 +322,14 @@ to a missing value in the attribute set(s) included via `with`."#,
                 )
             }
 
+            ErrorKind::IO { path, error } => {
+                write!(f, "I/O error: ")?;
+                if let Some(path) = path {
+                    write!(f, "{}: ", path.display())?;
+                }
+                write!(f, "{error}")
+            }
+
             ErrorKind::NotImplemented(feature) => {
                 write!(f, "feature not yet implemented in Tvix: {}", feature)
             }
@@ -583,6 +607,7 @@ impl Error {
             | ErrorKind::ReadFileError { .. }
             | ErrorKind::ImportParseError { .. }
             | ErrorKind::ImportCompilerError { .. }
+            | ErrorKind::IO { .. }
             | ErrorKind::NotImplemented(_) => return None,
         };
 
@@ -620,6 +645,7 @@ impl Error {
             ErrorKind::ReadFileError { .. } => "E026",
             ErrorKind::ImportParseError { .. } => "E027",
             ErrorKind::ImportCompilerError { .. } => "E028",
+            ErrorKind::IO { .. } => "E029",
 
             // Placeholder error while Tvix is under construction.
             ErrorKind::NotImplemented(_) => "E999",