feat(tvix/eval): track pattern binding names

These need to be preserved at least for builtins.toXML.

Also, we incorrectly only wrote an <attrspat> in case ellipsis was true,
but that's not the case.

Change-Id: I6bff9c47c2922f878d5c43e48280cda9c9ddb692
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10686
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: aspen <root@gws.fyi>

tvix/eval/src/builtins/to_xml.rs

@@ -90,15 +90,22 @@ fn value_variant_to_xml<W: Write>(w: &mut EventWriter<W>, value: &Value) -> Resu
 
             match &c.lambda.formals {
                 Some(formals) => {
+                    let mut attrspat = XmlEvent::start_element("attrspat");
                     if formals.ellipsis {
-                        w.write(XmlEvent::start_element("attrspat").attr("ellipsis", "1"))?;
-                        w.write(XmlEvent::end_element())?;
+                        attrspat = attrspat.attr("ellipsis", "1");
                     }
+                    if let Some(ref name) = &formals.name {
+                        attrspat = attrspat.attr("name", name.as_str());
+                    }
+
+                    w.write(attrspat)?;
 
                     for arg in formals.arguments.iter() {
                         w.write(XmlEvent::start_element("attr").attr("name", arg.0.as_str()))?;
                         w.write(XmlEvent::end_element())?;
                     }
+
+                    w.write(XmlEvent::end_element())?;
                 }
                 None => {
                     // TODO(tazjin): tvix does not currently persist function

tvix/eval/src/compiler/mod.rs

@@ -973,9 +973,16 @@ impl Compiler<'_> {
     ///    cases.
     fn compile_param_pattern(&mut self, pattern: &ast::Pattern) -> (Formals, CodeIdx) {
         let span = self.span_for(pattern);
-        let set_idx = match pattern.pat_bind() {
-            Some(name) => self.declare_local(&name, name.ident().unwrap().to_string()),
-            None => self.scope_mut().declare_phantom(span, true),
+
+        let (set_idx, pat_bind_name) = match pattern.pat_bind() {
+            Some(name) => {
+                let pat_bind_name = name.ident().unwrap().to_string();
+                (
+                    self.declare_local(&name, pat_bind_name.clone()),
+                    Some(pat_bind_name),
+                )
+            }
+            None => (self.scope_mut().declare_phantom(span, true), None),
         };
 
         // At call time, the attribute set is already at the top of the stack.
@@ -1126,6 +1133,7 @@ impl Compiler<'_> {
                 arguments,
                 ellipsis,
                 span,
+                name: pat_bind_name,
             }),
             throw_idx,
         )

tvix/eval/src/value/function.rs

@@ -18,6 +18,10 @@ pub(crate) struct Formals {
 
     /// The span of the formals themselves, to use to emit errors
     pub(crate) span: Span,
+
+    /// Optionally tracks a name for all function arguments (args@ style).
+    /// Used by toXML.
+    pub(crate) name: Option<String>,
 }
 
 impl Formals {