refactor(tvix/eval): Box Value::String

NixString is *quite* large - like 80 bytes - because of the extra
capacity value for BString and because of the context. We want to keep
Value small since we're passing it around a lot, so let's box the
NixString inside Value::String to save on some memory, and make cloning
ostensibly a little cheaper

Change-Id: I343c8b4e7f61dc3dcbbaba4382efb3b3e5bbabb2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10729
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/arbitrary.rs

@@ -91,7 +91,7 @@ fn leaf_value() -> impl Strategy<Value = Value> {
         any::<bool>().prop_map(Bool),
         any::<i64>().prop_map(Integer),
         any::<f64>().prop_map(Float),
-        any::<NixString>().prop_map(String),
+        any::<Box<NixString>>().prop_map(String),
         any::<OsString>().prop_map(|s| Path(PathBuf::from(s).into_boxed_path())),
     ]
 }

tvix/eval/src/value/attrs.rs

@@ -360,7 +360,7 @@ impl NixAttrs {
             let key = stack_slice.pop().unwrap();
 
             match key {
-                Value::String(ks) => set_attr(&mut attrs, ks, value)?,
+                Value::String(ks) => set_attr(&mut attrs, *ks, value)?,
 
                 Value::Null => {
                     // This is in fact valid, but leads to the value
@@ -414,9 +414,13 @@ impl IntoIterator for NixAttrs {
 fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
     let (name_idx, value_idx) = {
         match (&slice[2], &slice[0]) {
-            (Value::String(s1), Value::String(s2)) if (*s1 == *NAME_S && *s2 == *VALUE_S) => (3, 1),
+            (Value::String(s1), Value::String(s2)) if (**s1 == *NAME_S && **s2 == *VALUE_S) => {
+                (3, 1)
+            }
 
-            (Value::String(s1), Value::String(s2)) if (*s1 == *VALUE_S && *s2 == *NAME_S) => (1, 3),
+            (Value::String(s1), Value::String(s2)) if (**s1 == *VALUE_S && **s2 == *NAME_S) => {
+                (1, 3)
+            }
 
             // Technically this branch lets type errors pass,
             // but they will be caught during normal attribute

tvix/eval/src/value/attrs/tests.rs

@@ -14,11 +14,8 @@ fn test_empty_attrs() {
 
 #[test]
 fn test_simple_attrs() {
-    let attrs = NixAttrs::construct(
-        1,
-        vec![Value::String("key".into()), Value::String("value".into())],
-    )
-    .expect("simple attr construction should succeed");
+    let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
+        .expect("simple attr construction should succeed");
 
     assert!(
         matches!(attrs, NixAttrs(AttrsRep::Im(_))),
@@ -28,9 +25,9 @@ fn test_simple_attrs() {
 
 #[test]
 fn test_kv_attrs() {
-    let name_val = Value::String("name".into());
-    let value_val = Value::String("value".into());
-    let meaning_val = Value::String("meaning".into());
+    let name_val = Value::from("name");
+    let value_val = Value::from("value");
+    let meaning_val = Value::from("meaning");
     let forty_two_val = Value::Integer(42);
 
     let kv_attrs = NixAttrs::construct(
@@ -64,9 +61,9 @@ fn test_empty_attrs_iter() {
 
 #[test]
 fn test_kv_attrs_iter() {
-    let name_val = Value::String("name".into());
-    let value_val = Value::String("value".into());
-    let meaning_val = Value::String("meaning".into());
+    let name_val = Value::from("name");
+    let value_val = Value::from("value");
+    let meaning_val = Value::from("meaning");
     let forty_two_val = Value::Integer(42);
 
     let kv_attrs = NixAttrs::construct(
@@ -92,11 +89,8 @@ fn test_kv_attrs_iter() {
 
 #[test]
 fn test_map_attrs_iter() {
-    let attrs = NixAttrs::construct(
-        1,
-        vec![Value::String("key".into()), Value::String("value".into())],
-    )
-    .expect("simple attr construction should succeed");
+    let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
+        .expect("simple attr construction should succeed");
 
     let mut iter = attrs.iter().collect::<Vec<_>>().into_iter();
     let (k, v) = iter.next().unwrap();

tvix/eval/src/value/mod.rs

@@ -47,7 +47,7 @@ pub enum Value {
     Bool(bool),
     Integer(i64),
     Float(f64),
-    String(NixString),
+    String(Box<NixString>),
 
     #[serde(skip)]
     Path(Box<Path>),
@@ -186,7 +186,7 @@ where
     T: Into<NixString>,
 {
     fn from(t: T) -> Self {
-        Self::String(t.into())
+        Self::String(Box::new(t.into()))
     }
 }
 
@@ -327,7 +327,9 @@ impl Value {
             let value = if let Some(v) = vals.pop() {
                 v.force(co, span.clone()).await?
             } else {
-                return Ok(Value::String(NixString::new_context_from(context, result)));
+                return Ok(Value::String(Box::new(NixString::new_context_from(
+                    context, result,
+                ))));
             };
             let coerced: Result<BString, _> = match (value, kind) {
                 // coercions that are always done
@@ -335,7 +337,7 @@ impl Value {
                     if let Some(ctx) = s.context_mut() {
                         context = context.join(ctx);
                     }
-                    Ok(s.into())
+                    Ok((*s).into())
                 }
 
                 // TODO(sterni): Think about proper encoding handling here. This needs
@@ -692,7 +694,7 @@ impl Value {
     /// everytime you want a string.
     pub fn to_str(&self) -> Result<NixString, ErrorKind> {
         match self {
-            Value::String(s) if !s.has_context() => Ok(s.clone()),
+            Value::String(s) if !s.has_context() => Ok((**s).clone()),
             Value::Thunk(thunk) => Self::to_str(&thunk.value()),
             other => Err(type_error("contextless strings", other)),
         }
@@ -703,7 +705,7 @@ impl Value {
         NixString,
         "contextful string",
         Value::String(s),
-        s.clone()
+        (**s).clone()
     );
     gen_cast!(to_path, Box<Path>, "path", Value::Path(p), p.clone());
     gen_cast!(to_attrs, Box<NixAttrs>, "set", Value::Attrs(a), a.clone());