2 files changed, 135 insertions, 0 deletions
diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index b93bbd99f87a..7614353f5bbb 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -20,6 +20,7 @@ use crate::{
 
 use self::versions::{VersionPart, VersionPartsIter};
 
+mod to_xml;
 mod versions;
 
 #[cfg(feature = "impure")]
@@ -918,6 +919,14 @@ mod pure_builtins {
             .map(Value::String)
     }
 
+    #[builtin("toXML")]
+    fn builtin_to_xml(vm: &mut VM, value: Value) -> Result<Value, ErrorKind> {
+        value.deep_force(vm, &mut Default::default())?;
+        let mut buf: Vec<u8> = vec![];
+        to_xml::value_to_xml(&mut buf, &value)?;
+        Ok(String::from_utf8(buf)?.into())
+    }
+
     #[builtin("placeholder")]
     fn builtin_placeholder(vm: &mut VM, #[lazy] _: Value) -> Result<Value, ErrorKind> {
         // TODO(amjoseph)
diff --git a/tvix/eval/src/builtins/to_xml.rs b/tvix/eval/src/builtins/to_xml.rs
new file mode 100644
index 000000000000..f2b98a7e3168
--- /dev/null
+++ b/tvix/eval/src/builtins/to_xml.rs
@@ -0,0 +1,126 @@
+//! This module implements `builtins.toXML`, which is a serialisation
+//! of value information as well as internal tvix state that several
+//! things in nixpkgs rely on.
+
+use std::{io::Write, rc::Rc};
+use xml::writer::events::XmlEvent;
+use xml::writer::EmitterConfig;
+use xml::writer::EventWriter;
+
+use crate::{ErrorKind, Value};
+
+/// Recursively serialise a value to XML. The value *must* have been
+/// deep-forced before being passed to this function.
+pub(super) fn value_to_xml<W: Write>(mut writer: W, value: &Value) -> Result<(), ErrorKind> {
+    let config = EmitterConfig {
+        perform_indent: true,
+        pad_self_closing: true,
+
+        // Nix uses single-quotes *only* in the document declaration,
+        // so we need to write it manually.
+        write_document_declaration: false,
+        ..Default::default()
+    };
+
+    // Write a literal document declaration, using C++-Nix-style
+    // single quotes.
+    write!(writer, "<?xml version='1.0' encoding='utf-8'?>\n")?;
+
+    let mut writer = EventWriter::new_with_config(writer, config);
+
+    writer.write(XmlEvent::start_element("expr"))?;
+    value_variant_to_xml(&mut writer, value)?;
+    writer.write(XmlEvent::end_element())?;
+
+    // Unwrap the writer to add the final newline that C++ Nix adds.
+    write!(writer.into_inner(), "\n")?;
+
+    Ok(())
+}
+
+fn write_typed_value<W: Write, V: ToString>(
+    w: &mut EventWriter<W>,
+    name: &str,
+    value: V,
+) -> Result<(), ErrorKind> {
+    w.write(XmlEvent::start_element(name).attr("value", &value.to_string()))?;
+    w.write(XmlEvent::end_element())?;
+    Ok(())
+}
+
+fn value_variant_to_xml<W: Write>(w: &mut EventWriter<W>, value: &Value) -> Result<(), ErrorKind> {
+    match value {
+        Value::Thunk(t) => return value_variant_to_xml(w, &t.value()),
+
+        Value::Null => {
+            w.write(XmlEvent::start_element("null"))?;
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Bool(b) => return write_typed_value(w, "bool", b),
+        Value::Integer(i) => return write_typed_value(w, "int", i),
+        Value::Float(f) => return write_typed_value(w, "float", f),
+        Value::String(s) => return write_typed_value(w, "string", s.as_str()),
+        Value::Path(p) => return write_typed_value(w, "path", p.to_string_lossy()),
+
+        Value::List(list) => {
+            w.write(XmlEvent::start_element("list"))?;
+
+            for elem in list.into_iter() {
+                value_variant_to_xml(w, &elem)?;
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Attrs(attrs) => {
+            w.write(XmlEvent::start_element("attrs"))?;
+
+            for elem in attrs.iter() {
+                w.write(XmlEvent::start_element("attr").attr("name", elem.0.as_str()))?;
+                value_variant_to_xml(w, &elem.1)?;
+                w.write(XmlEvent::end_element())?;
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Closure(c) => {
+            w.write(XmlEvent::start_element("function"))?;
+
+            match &c.lambda.formals {
+                Some(formals) => {
+                    if formals.ellipsis {
+                        w.write(XmlEvent::start_element("attrspat").attr("ellipsis", "1"))?;
+                        w.write(XmlEvent::end_element())?;
+                    }
+
+                    for arg in formals.arguments.iter() {
+                        w.write(XmlEvent::start_element("attr").attr("name", arg.0.as_str()))?;
+                        w.write(XmlEvent::end_element())?;
+                    }
+                }
+                None => todo!("we don't persist the arg name ..."),
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Builtin(_) => {
+            w.write(XmlEvent::start_element("unevaluated"))?;
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::AttrNotFound
+        | Value::Blueprint(_)
+        | Value::DeferredUpvalue(_)
+        | Value::UnresolvedPath(_) => {
+            return Err(ErrorKind::TvixBug {
+                msg: "internal value variant encountered in builtins.toXML",
+                metadata: Some(Rc::new(value.clone())),
+            })
+        }
+    }?;
+
+    Ok(())
+}