feat(tvix/value): add some necessary helpers for strings r/4425

Deriving Ord/Eq is required for the ordered BTreeMaps. Once interning
is implemented this will require some extra magic for the sort order,
but that's fine.

Change-Id: I0c654648eb3609a4a01d84868c25f43a4d35bc2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6089
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

2 files changed, 14 insertions, 4 deletions

diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index ea0c89e6b6c4..56f8620d3a58 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -7,8 +7,8 @@ mod attrs;
 mod string;
 
 use crate::errors::{Error, EvalResult};
-use attrs::NixAttrs;
-use string::NixString;
+pub use attrs::NixAttrs;
+pub use string::NixString;
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum Value {
@@ -49,6 +49,16 @@ impl Value {
             }),
         }
     }
+
+    pub fn as_string(self) -> EvalResult<NixString> {
+        match self {
+            Value::String(s) => Ok(s),
+            other => Err(Error::TypeError {
+                expected: "string",
+                actual: other.type_of(),
+            }),
+        }
+    }
 }
 
 impl Display for Value {
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs
index 7e6f5ecf3bd3..531bcf547b0b 100644
--- a/tvix/eval/src/value/string.rs
+++ b/tvix/eval/src/value/string.rs
@@ -3,8 +3,8 @@ use std::fmt::Display;
 /// This module implements Nix language strings and their different
 /// backing implementations.
 
-#[derive(Clone, Debug, Hash, PartialEq)]
-pub struct NixString(String);
+#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+pub struct NixString(pub String);
 
 impl Display for NixString {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {