feat(tvix/eval): teach builtins.toXML context

XmlEmitter gains a NixContext field, and `write_typed_value` extends it
with all context elements present in the passed value.

Once all serialization is done, a into_context() function returns the
collected context, so we can construct a NixString with context.

Tests for this live in tvix-glue, as we use builtins.derivation, which
is not present in the tvix-eval crate.

Fixes b/398.

Change-Id: I85feaaa17b753885f8a017a54e419ec4e602af21
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11704
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Alyssa Ross <hi@alyssa.is>

tvix/eval/src/builtins/mod.rs

@@ -1504,8 +1504,19 @@ mod pure_builtins {
         }
 
         let mut buf: Vec<u8> = vec![];
-        to_xml::value_to_xml(&mut buf, &value)?;
+        let context = to_xml::value_to_xml(&mut buf, &value)?;
-        Ok(buf.into())
+
+        Ok((
+            buf,
+            // FUTUREWORK: We have a distinction between an empty context, and
+            // no context at all. Fix this.
+            if !context.is_empty() {
+                Some(Box::new(context))
+            } else {
+                None
+            },
+        )
+            .into())
     }
 
     #[builtin("placeholder")]

tvix/eval/src/builtins/to_xml.rs

@@ -6,11 +6,12 @@ use bstr::ByteSlice;
 use std::borrow::Cow;
 use std::{io::Write, rc::Rc};
 
-use crate::{ErrorKind, Value};
+use crate::{ErrorKind, NixContext, NixContextElement, Value};
 
 /// Recursively serialise a value to XML. The value *must* have been
 /// deep-forced before being passed to this function.
-pub fn value_to_xml<W: Write>(mut writer: W, value: &Value) -> Result<(), ErrorKind> {
+/// On success, returns the NixContext.
+pub fn value_to_xml<W: Write>(mut writer: W, value: &Value) -> Result<NixContext, ErrorKind> {
     // Write a literal document declaration, using C++-Nix-style
     // single quotes.
     writeln!(writer, "<?xml version='1.0' encoding='utf-8'?>")?;
@@ -21,7 +22,7 @@ pub fn value_to_xml<W: Write>(mut writer: W, value: &Value) -> Result<(), ErrorK
     value_variant_to_xml(&mut emitter, value)?;
     emitter.write_closing_tag("expr")?;
 
-    Ok(())
+    Ok(emitter.into_context())
 }
 
 fn write_typed_value<W: Write, V: ToString>(
@@ -45,7 +46,12 @@ fn value_variant_to_xml<W: Write>(w: &mut XmlEmitter<W>, value: &Value) -> Resul
         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.to_str()?),
+        Value::String(s) => {
+            if let Some(context) = s.context() {
+                w.extend_context(context.iter().cloned());
+            }
+            return write_typed_value(w, "string", s.to_str()?);
+        }
         Value::Path(p) => return write_typed_value(w, "path", p.to_string_lossy()),
 
         Value::List(list) => {
@@ -137,6 +143,7 @@ struct XmlEmitter<W> {
     /// The current indentation
     cur_indent: usize,
     writer: W,
+    context: NixContext,
 }
 
 impl<W: Write> XmlEmitter<W> {
@@ -144,6 +151,7 @@ impl<W: Write> XmlEmitter<W> {
         XmlEmitter {
             cur_indent: 0,
             writer,
+            context: Default::default(),
         }
     }
 
@@ -245,6 +253,19 @@ impl<W: Write> XmlEmitter<W> {
             _ => None,
         }
     }
+
+    /// Extends the existing context with more context elements.
+    fn extend_context<T>(&mut self, iter: T)
+    where
+        T: IntoIterator<Item = NixContextElement>,
+    {
+        self.context.extend(iter)
+    }
+
+    /// Consumes [Self] and returns the [NixContext] collected.
+    fn into_context(self) -> NixContext {
+        self.context
+    }
 }
 
 #[cfg(test)]

tvix/glue/src/tests/tvix_tests/eval-okay-toxml-context.exp

@@ -0,0 +1 @@
+[ { "/nix/store/y1s2fiq89v2h9vkb38w508ir20dwv6v2-test.drv" = { allOutputs = true; }; } false ]

tvix/glue/src/tests/tvix_tests/eval-okay-toxml-context.nix

@@ -0,0 +1,14 @@
+[
+  # builtins.toXML retains context where there is.
+  (builtins.getContext (builtins.toXML {
+    inherit (derivation {
+      name = "test";
+      builder = "/bin/sh";
+      system = builtins.currentSystem;
+    }) drvPath;
+  }))
+
+  # this should have no context.
+  (builtins.hasContext
+    (builtins.toXML { }))
+]