feat(tvix/eval): Implement builtins.fromJSON

Using `serde_json` for parsing JSON here, plus an `impl FromJSON for
Value`. The latter is primarily to stay "dependency light" for now -
likely going with an actual serde `Deserialize` impl in the future is
going to be way better as it allows saving significantly on intermediary
allocations.

Change-Id: I152a0448ff7c87cf7ebaac927c38912b99de1c18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6920
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/errors.rs

@@ -129,6 +129,9 @@ pub enum ErrorKind {
         error: Rc<io::Error>,
     },
 
+    /// Errors converting JSON to a value
+    FromJsonError(String),
+
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, and without which eval can not
     /// proceed.
@@ -176,6 +179,13 @@ impl ErrorKind {
     }
 }
 
+impl From<serde_json::Error> for ErrorKind {
+    fn from(err: serde_json::Error) -> Self {
+        // Can't just put the `serde_json::Error` in the ErrorKind since it doesn't impl `Clone`
+        Self::FromJsonError(format!("Error parsing JSON: {err}"))
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Error {
     pub kind: ErrorKind,
@@ -343,6 +353,10 @@ to a missing value in the attribute set(s) included via `with`."#,
                 write!(f, "{error}")
             }
 
+            ErrorKind::FromJsonError(msg) => {
+                write!(f, "Error converting JSON to a Nix value: {msg}")
+            }
+
             ErrorKind::NotImplemented(feature) => {
                 write!(f, "feature not yet implemented in Tvix: {}", feature)
             }
@@ -621,6 +635,7 @@ impl Error {
             | ErrorKind::ImportParseError { .. }
             | ErrorKind::ImportCompilerError { .. }
             | ErrorKind::IO { .. }
+            | ErrorKind::FromJsonError(_)
             | ErrorKind::NotImplemented(_) => return None,
         };
 
@@ -659,6 +674,7 @@ impl Error {
             ErrorKind::ImportParseError { .. } => "E027",
             ErrorKind::ImportCompilerError { .. } => "E028",
             ErrorKind::IO { .. } => "E029",
+            ErrorKind::FromJsonError { .. } => "E030",
 
             // Placeholder error while Tvix is under construction.
             ErrorKind::NotImplemented(_) => "E999",