fix(tvix/eval): len_without_is_empty clippy warn

This CL addresses clippy warning len_without_is_empty
which expects `.is_empty()` method to be present when
implementing `.len()` method for an item.

Change-Id: I8878db630b9ef5853649a906b764a33299bb5dc8
Signed-off-by: Aaqa Ishtyaq <aaqaishtyaq@gmail.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7806
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -898,7 +898,7 @@ mod pure_builtins {
     fn builtin_tail(_: &mut VM, list: Value) -> Result<Value, ErrorKind> {
         let xs = list.to_list()?;
 
-        if xs.len() == 0 {
+        if xs.is_empty() {
             Err(ErrorKind::TailEmptyList)
         } else {
             let output = xs.into_iter().skip(1).collect::<Vec<_>>();

tvix/eval/src/value/attrs.rs

@@ -301,6 +301,14 @@ impl NixAttrs {
         }
     }
 
+    pub fn is_empty(&self) -> bool {
+        match &self.0 {
+            AttrsRep::Im(map) => map.is_empty(),
+            AttrsRep::Empty => true,
+            AttrsRep::KV { .. } => false,
+        }
+    }
+
     /// Select a value from an attribute set by key.
     pub fn select(&self, key: &str) -> Option<&Value> {
         self.0.select(key)

tvix/eval/src/value/list.rs

@@ -67,6 +67,10 @@ impl NixList {
         self.0.get(i)
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     pub fn construct(count: usize, stack_slice: Vec<Value>) -> Self {
         debug_assert!(
             count == stack_slice.len(),