refactor(tvix/eval): rm From<(T, Option<Box<NixContext>>)> for NixString

This conversion was a bit too magic, and we can just use
`NixString::new_context_from` without having to worry about the
distinction between an empty context or no context, as
NixString::new_context_from already deals with that internally.

Change-Id: I3e5d57ecfa0f7456aa6c526863e49f2523afaec3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12754
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>
Autosubmit: flokli <flokli@flokli.de>

tvix/eval/src/builtins/mod.rs

@@ -1498,17 +1498,7 @@ mod pure_builtins {
         let mut buf: Vec<u8> = vec![];
         let context = to_xml::value_to_xml(&mut buf, &value)?;
 
-        Ok((
-            buf,
-            // FUTUREWORK: We have a distinction between an empty context, and
-            // no context at all. Fix this.
-            if !context.is_empty() {
-                Some(Box::new(context))
-            } else {
-                None
-            },
-        )
-            .into())
+        Ok(NixString::new_context_from(context, buf).into())
     }
 
     #[builtin("trace")]

tvix/eval/src/value/string/mod.rs

@@ -455,15 +455,6 @@ impl From<String> for NixString {
     }
 }
 
-impl<T> From<(T, Option<Box<NixContext>>)> for NixString
-where
-    NixString: From<T>,
-{
-    fn from((s, ctx): (T, Option<Box<NixContext>>)) -> Self {
-        Self::new(NixString::from(s).as_ref(), ctx)
-    }
-}
-
 impl From<Box<str>> for NixString {
     fn from(s: Box<str>) -> Self {
         s.into_boxed_bytes().into()