refactor(eval): add keys_sorted() to NixAttrs

builtins.toXML relies on attr keys being sorted, which is an
implementation detail.
Cppnix creates sorted xml, and tests expect it too, so changed that
iter call site.
builtins.attrNames and attrValues should also returns sorted lists, so
changed that too.

Change-Id: Ic706afc6855d5d28932ade7482449c965b0ae94f
Reviewed-on: https://cl.snix.dev/c/snix/+/30307
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii

snix/eval/src/builtins/mod.rs

@@ -150,7 +150,7 @@ mod pure_builtins {
         let xs = set.to_attrs()?;
         let mut output = Vec::with_capacity(xs.len());
 
-        for (key, _val) in xs.iter() {
+        for (key, _val) in xs.iter_sorted() {
             output.push(Value::from(key.clone()));
         }
 
@@ -162,7 +162,7 @@ mod pure_builtins {
         let xs = set.to_attrs()?;
         let mut output = Vec::with_capacity(xs.len());
 
-        for (_key, val) in xs.iter() {
+        for (_key, val) in xs.iter_sorted() {
             output.push(val.clone());
         }
 
@@ -789,13 +789,13 @@ mod pure_builtins {
         if left_set.is_empty() {
             return Ok(Value::attrs(NixAttrs::empty()));
         }
-        let mut left_keys = left_set.keys();
+        let mut left_keys = left_set.keys_sorted();
 
         let right_set = y.to_attrs()?;
         if right_set.is_empty() {
             return Ok(Value::attrs(NixAttrs::empty()));
         }
-        let mut right_keys = right_set.keys();
+        let mut right_keys = right_set.keys_sorted();
 
         let mut out: BTreeMap<NixString, Value> = BTreeMap::new();
 

snix/eval/src/builtins/to_xml.rs

@@ -67,7 +67,7 @@ fn value_variant_to_xml<W: Write>(w: &mut XmlEmitter<W>, value: &Value) -> Resul
         Value::Attrs(attrs) => {
             w.write_open_tag("attrs", &[])?;
 
-            for elem in attrs.iter() {
+            for elem in attrs.iter_sorted() {
                 w.write_open_tag("attr", &[("name", &elem.0.to_str_lossy())])?;
                 value_variant_to_xml(w, elem.1)?;
                 w.write_closing_tag("attr")?;

snix/eval/src/value/attrs.rs

@@ -318,6 +318,12 @@ impl NixAttrs {
         })
     }
 
+    /// Same as [Self::keys], but marks call sites which rely on the
+    /// iteration being lexicographic.
+    pub fn keys_sorted(&self) -> Keys {
+        self.keys()
+    }
+
     /// Implement construction logic of an attribute set, to encapsulate
     /// logic about attribute set optimisations inside of this module.
     pub fn construct(