refactor(tvix/eval): simplify NixString representation(s) r/5967

Instead of the two different representations (which we don't really
use much), use a `Box<str>` (which potentially shaves another 8 bytes
off `Value`).

NixString values themselves are immutable anyways (which was a
guarantee we already had with `SmolStr`), so this doesn't change
anything else.

Change-Id: I1d8454c056c21ecb0aebc473cfb3ae06cd70dbb6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8151
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

3 files changed, 37 insertions, 65 deletions

diff --git a/tvix/eval/src/value/attrs.rs b/tvix/eval/src/value/attrs.rs
index 64a1dc035b..2b29c8396f 100644
--- a/tvix/eval/src/value/attrs.rs
+++ b/tvix/eval/src/value/attrs.rs
@@ -8,16 +8,23 @@
 use std::iter::FromIterator;
 
 use imbl::{ordmap, OrdMap};
+use lazy_static::lazy_static;
 use serde::de::{Deserializer, Error, Visitor};
 use serde::ser::SerializeMap;
 use serde::{Deserialize, Serialize};
 
-use crate::errors::ErrorKind;
-
 use super::string::NixString;
 use super::thunk::ThunkSet;
 use super::TotalDisplay;
 use super::Value;
+use crate::errors::ErrorKind;
+
+lazy_static! {
+    static ref NAME_S: NixString = "name".into();
+    static ref NAME_REF: &'static NixString = &NAME_S;
+    static ref VALUE_S: NixString = "value".into();
+    static ref VALUE_REF: &'static NixString = &VALUE_S;
+}
 
 #[cfg(test)]
 mod tests;
@@ -57,8 +64,8 @@ impl AttrsRep {
 
             AttrsRep::KV { name, value } => {
                 *self = AttrsRep::Im(ordmap! {
-                    NixString::NAME => name.clone(),
-                    NixString::VALUE => value.clone()
+                    NAME_S.clone() => name.clone(),
+                    VALUE_S.clone() => value.clone()
                 });
 
                 self.map_mut()
@@ -230,13 +237,13 @@ impl NixAttrs {
         // Slightly more advanced, but still optimised updates
         match (self.0, other.0) {
             (AttrsRep::Im(mut m), AttrsRep::KV { name, value }) => {
-                m.insert(NixString::NAME, name);
-                m.insert(NixString::VALUE, value);
+                m.insert(NAME_S.clone(), name);
+                m.insert(VALUE_S.clone(), value);
                 NixAttrs(AttrsRep::Im(m))
             }
 
             (AttrsRep::KV { name, value }, AttrsRep::Im(mut m)) => {
-                match m.entry(NixString::NAME) {
+                match m.entry(NAME_S.clone()) {
                     imbl::ordmap::Entry::Vacant(e) => {
                         e.insert(name);
                     }
@@ -244,7 +251,7 @@ impl NixAttrs {
                     imbl::ordmap::Entry::Occupied(_) => { /* name from `m` has precedence */ }
                 };
 
-                match m.entry(NixString::VALUE) {
+                match m.entry(VALUE_S.clone()) {
                     imbl::ordmap::Entry::Vacant(e) => {
                         e.insert(value);
                     }
@@ -318,11 +325,7 @@ impl NixAttrs {
         match self.0 {
             AttrsRep::Empty => IntoIter(IntoIterRepr::Empty),
             AttrsRep::KV { name, value } => IntoIter(IntoIterRepr::Finite(
-                vec![
-                    (NixString::NAME_REF.clone(), name),
-                    (NixString::VALUE_REF.clone(), value),
-                ]
-                .into_iter(),
+                vec![(NAME_REF.clone(), name), (VALUE_REF.clone(), value)].into_iter(),
             )),
             AttrsRep::Im(map) => IntoIter(IntoIterRepr::Im(map.into_iter())),
         }
@@ -411,17 +414,9 @@ impl 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 == NixString::NAME && *s2 == NixString::VALUE) =>
-            {
-                (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 == NixString::VALUE && *s2 == NixString::NAME) =>
-            {
-                (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
@@ -502,12 +497,12 @@ impl<'a> Iterator for Iter<KeyValue<'a>> {
             KeyValue::KV { name, value, at } => match at {
                 IterKV::Name => {
                     at.next();
-                    Some((NixString::NAME_REF, name))
+                    Some((&NAME_REF, name))
                 }
 
                 IterKV::Value => {
                     at.next();
-                    Some((NixString::VALUE_REF, value))
+                    Some((&VALUE_REF, value))
                 }
 
                 IterKV::Done => None,
@@ -542,11 +537,11 @@ impl<'a> Iterator for Keys<'a> {
             KeysInner::Empty => None,
             KeysInner::KV(at @ IterKV::Name) => {
                 at.next();
-                Some(NixString::NAME_REF)
+                Some(&NAME_REF)
             }
             KeysInner::KV(at @ IterKV::Value) => {
                 at.next();
-                Some(NixString::VALUE_REF)
+                Some(&VALUE_REF)
             }
             KeysInner::KV(IterKV::Done) => None,
             KeysInner::Im(m) => m.next(),
diff --git a/tvix/eval/src/value/attrs/tests.rs b/tvix/eval/src/value/attrs/tests.rs
index 39ac55b679..473592c519 100644
--- a/tvix/eval/src/value/attrs/tests.rs
+++ b/tvix/eval/src/value/attrs/tests.rs
@@ -84,10 +84,10 @@ fn test_kv_attrs_iter() {
         .into_iter()
         .map(|(k, v)| (k, v));
     let (k, v) = iter.next().unwrap();
-    assert!(k == NixString::NAME_REF);
+    assert!(k == *NAME_REF);
     assert!(v.to_str().unwrap() == meaning_val.to_str().unwrap());
     let (k, v) = iter.next().unwrap();
-    assert!(k == NixString::VALUE_REF);
+    assert!(k == *VALUE_REF);
     assert!(v.as_int().unwrap() == forty_two_val.as_int().unwrap());
     assert!(iter.next().is_none());
 }
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs
index 5962c94ea5..8bb41a7825 100644
--- a/tvix/eval/src/value/string.rs
+++ b/tvix/eval/src/value/string.rs
@@ -1,5 +1,8 @@
-//! This module implements Nix language strings and their different
-//! backing implementations.
+//! This module implements Nix language strings.
+//!
+//! Nix language strings never need to be modified on the language
+//! level, allowing us to shave off some memory overhead and only
+//! paying the cost when creating new strings.
 use rnix::ast;
 use smol_str::SmolStr;
 use std::ffi::OsStr;
@@ -11,16 +14,9 @@ use std::{borrow::Cow, fmt::Display, str::Chars};
 use serde::de::{Deserializer, Visitor};
 use serde::{Deserialize, Serialize};
 
-#[derive(Clone, Debug, Serialize)]
-#[serde(untagged)]
-enum StringRepr {
-    Smol(SmolStr),
-    Heap(String),
-}
-
 #[repr(transparent)]
 #[derive(Clone, Debug, Serialize)]
-pub struct NixString(StringRepr);
+pub struct NixString(Box<str>);
 
 impl PartialEq for NixString {
     fn eq(&self, other: &Self) -> bool {
@@ -44,19 +40,19 @@ impl Ord for NixString {
 
 impl From<&str> for NixString {
     fn from(s: &str) -> Self {
-        NixString(StringRepr::Smol(SmolStr::new(s)))
+        NixString(Box::from(s))
     }
 }
 
 impl From<String> for NixString {
     fn from(s: String) -> Self {
-        NixString(StringRepr::Heap(s))
+        NixString(s.into_boxed_str())
     }
 }
 
 impl From<SmolStr> for NixString {
     fn from(s: SmolStr) -> Self {
-        NixString(StringRepr::Smol(s))
+        NixString(Box::from(s.as_str()))
     }
 }
 
@@ -109,7 +105,6 @@ impl<'de> Deserialize<'de> for NixString {
 mod arbitrary {
     use super::*;
     use proptest::prelude::{any_with, Arbitrary};
-    use proptest::prop_oneof;
     use proptest::strategy::{BoxedStrategy, Strategy};
 
     impl Arbitrary for NixString {
@@ -118,29 +113,14 @@ mod arbitrary {
         type Strategy = BoxedStrategy<Self>;
 
         fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
-            prop_oneof![
-                // Either generate `StringRepr::Heap`...
-                any_with::<String>(args).prop_map(Self::from),
-                // ...or generate `StringRepr::Smol` (which `impl From<&str> for NixString` returns)
-                any_with::<String>(args).prop_map(|s| Self::from(s.as_str())),
-            ]
-            .boxed()
+            any_with::<String>(args).prop_map(Self::from).boxed()
         }
     }
 }
 
 impl NixString {
-    pub const NAME: Self = NixString(StringRepr::Smol(SmolStr::new_inline("name")));
-    pub const NAME_REF: &'static Self = &Self::NAME;
-
-    pub const VALUE: Self = NixString(StringRepr::Smol(SmolStr::new_inline("value")));
-    pub const VALUE_REF: &'static Self = &Self::VALUE;
-
     pub fn as_str(&self) -> &str {
-        match &self.0 {
-            StringRepr::Smol(s) => s.as_str(),
-            StringRepr::Heap(s) => s,
-        }
+        &self.0
     }
 
     /// Return a displayable representation of the string as an
@@ -172,14 +152,11 @@ impl NixString {
     pub fn concat(&self, other: &Self) -> Self {
         let mut s = self.as_str().to_owned();
         s.push_str(other.as_str());
-        NixString(StringRepr::Heap(s))
+        NixString(s.into_boxed_str())
     }
 
     pub fn chars(&self) -> Chars<'_> {
-        match &self.0 {
-            StringRepr::Heap(h) => h.chars(),
-            StringRepr::Smol(s) => s.chars(),
-        }
+        self.0.chars()
     }
 }