feat(tvix/value): introduce string representation with &'static str r/4457

For cases where the strings are statically known (such as the
oft-occuring name/value), this can be a useful optimisation.

It's also much more convenient in tests.

Change-Id: Ie462b684805bd4986ea5e85ca4bff663bc2d3c3c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6111
Tested-by: BuildkiteCI
Reviewed-by: eta <tvl@eta.st>

2 files changed, 37 insertions, 19 deletions

diff --git a/tvix/eval/src/value/attrs.rs b/tvix/eval/src/value/attrs.rs
index 75ade6a89a9f..7e3b5f231d66 100644
--- a/tvix/eval/src/value/attrs.rs
+++ b/tvix/eval/src/value/attrs.rs
@@ -71,14 +71,8 @@ impl NixAttrs {
 
             NixAttrs::KV { name, value } => {
                 *self = NixAttrs::Map(BTreeMap::from([
-                    (
-                        NixString("name".into()),
-                        std::mem::replace(name, Value::Blackhole),
-                    ),
-                    (
-                        NixString("value".into()),
-                        std::mem::replace(value, Value::Blackhole),
-                    ),
+                    ("name".into(), std::mem::replace(name, Value::Blackhole)),
+                    ("value".into(), std::mem::replace(value, Value::Blackhole)),
                 ]));
                 self.map_mut()
             }
@@ -155,14 +149,14 @@ impl NixAttrs {
 fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
     let (name_idx, value_idx) = {
         match (&slice[2], &slice[0]) {
-            (Value::String(NixString(s1)), Value::String(NixString(s2)))
-                if (s1 == "name" && s2 == "value") =>
+            (Value::String(s1), Value::String(s2))
+                if (*s1 == NixString::NAME && *s2 == NixString::VALUE) =>
             {
                 (3, 1)
             }
 
-            (Value::String(NixString(s1)), Value::String(NixString(s2)))
-                if (s1 == "value" && s2 == "name") =>
+            (Value::String(s1), Value::String(s2))
+                if (*s1 == NixString::VALUE && *s2 == NixString::NAME) =>
             {
                 (1, 3)
             }
@@ -189,7 +183,7 @@ fn set_attr(attrs: &mut NixAttrs, key: NixString, value: Value) -> EvalResult<()
     match entry {
         std::collections::btree_map::Entry::Occupied(entry) => {
             return Err(Error::DuplicateAttrsKey {
-                key: entry.key().0.clone(),
+                key: entry.key().as_str().to_string(),
             })
         }
 
@@ -254,7 +248,7 @@ fn set_nested_attr(
 
             _ => {
                 return Err(Error::DuplicateAttrsKey {
-                    key: entry.key().0.clone(),
+                    key: entry.key().as_str().to_string(),
                 })
             }
         },
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs
index d4776caea403..47661e03add4 100644
--- a/tvix/eval/src/value/string.rs
+++ b/tvix/eval/src/value/string.rs
@@ -4,16 +4,40 @@ use std::fmt::Display;
 /// backing implementations.
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
-pub struct NixString(pub String);
+pub enum NixString {
+    Static(&'static str),
+    Heap(String),
+}
 
 impl Display for NixString {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(self.0.as_str())
+        match self {
+            NixString::Static(s) => f.write_str(s),
+            NixString::Heap(s) => f.write_str(s),
+        }
+    }
+}
+
+impl From<&'static str> for NixString {
+    fn from(s: &'static str) -> Self {
+        NixString::Static(s)
     }
 }
 
-impl From<&str> for NixString {
-    fn from(s: &str) -> Self {
-        NixString(s.to_string())
+impl From<String> for NixString {
+    fn from(s: String) -> Self {
+        NixString::Heap(s)
+    }
+}
+
+impl NixString {
+    pub const NAME: Self = NixString::Static("name");
+    pub const VALUE: Self = NixString::Static("value");
+
+    pub fn as_str(&self) -> &str {
+        match self {
+            NixString::Static(s) => s,
+            NixString::Heap(s) => s,
+        }
     }
 }