fix(tvix/value): implement PartialOrd/PartialEq for strings

Instead of comparing the enum variants (which does not yield useful
behaviour), compare &str representations of the string instead.

Change-Id: I5e94b5f6c91b4561e1bc7c36d586f3d23c243764
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6112
Tested-by: BuildkiteCI
Reviewed-by: eta <tvl@eta.st>

tvix/eval/src/value/string.rs

@@ -3,7 +3,7 @@ use std::fmt::Display;
 /// This module implements Nix language strings and their different
 /// backing implementations.
 
-#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Clone, Debug, Hash, Eq, Ord)]
 pub enum NixString {
     Static(&'static str),
     Heap(String),
@@ -18,6 +18,18 @@ impl Display for NixString {
     }
 }
 
+impl PartialEq for NixString {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl PartialOrd for NixString {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.as_str().partial_cmp(other.as_str())
+    }
+}
+
 impl From<&'static str> for NixString {
     fn from(s: &'static str) -> Self {
         NixString::Static(s)