feat(tvix/eval): Implement builtins.tryEval r/5089

With asserts compiled using conditional jumps, this ends up being quite
straightforward - the only real tricky bit is that we have to know
whether an error can or can't be handled.

Change-Id: I75617da73b7a9c5cdd888c0e26ae81d2c5c0d714
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6924
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

4 files changed, 32 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 08b8299e48..1fc8336417 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -431,6 +431,21 @@ fn pure_builtins() -> Vec<Builtin> {
         Builtin::new("throw", &[true], |args: Vec<Value>, _: &mut VM| {
             Err(ErrorKind::Throw(args[0].to_str()?.to_string()))
         }),
+        Builtin::new("tryEval", &[false], |args: Vec<Value>, vm: &mut VM| {
+            let mut res = BTreeMap::new();
+            match args[0].force(vm) {
+                Ok(value) => {
+                    res.insert("value".into(), (*value).clone());
+                    res.insert("success".into(), true.into());
+                }
+                Err(e) if e.is_catchable() => {
+                    res.insert("value".into(), false.into());
+                    res.insert("success".into(), false.into());
+                }
+                Err(e) => return Err(e),
+            }
+            Ok(Value::attrs(NixAttrs::from_map(res)))
+        }),
         // coerce_to_string forces for us
         Builtin::new("toString", &[false], |args: Vec<Value>, vm: &mut VM| {
             args[0]
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index a7d54d51bb..3ba3404a8d 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -158,6 +158,17 @@ impl From<io::Error> for ErrorKind {
     }
 }
 
+impl ErrorKind {
+    /// Returns `true` if this error can be caught by `builtins.tryEval`
+    pub fn is_catchable(&self) -> bool {
+        match self {
+            Self::Throw(_) | Self::AssertionFailed => true,
+            Self::ThunkForce(err) => err.kind.is_catchable(),
+            _ => false,
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Error {
     pub kind: ErrorKind,
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp
new file mode 100644
index 0000000000..2b2e6fa711
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp
@@ -0,0 +1 @@
+{ x = { success = true; value = "x"; }; y = { success = false; value = false; }; z = { success = false; value = false; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix
new file mode 100644
index 0000000000..629bc440a8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix
@@ -0,0 +1,5 @@
+{
+  x = builtins.tryEval "x";
+  y = builtins.tryEval (assert false; "y");
+  z = builtins.tryEval (throw "bla");
+}