Improve value casting infrastructure

1 files changed, 39 insertions, 0 deletions

diff --git a/src/eval/auto.rs b/src/eval/auto.rs
new file mode 100644
index 00000000..e73b3f33
--- /dev/null
+++ b/src/eval/auto.rs
@@ -0,0 +1,39 @@
+use std::fmt::{self, Debug, Formatter};
+
+use super::{CastInfo, FromValue, IntoValue, Reflect, Value};
+use crate::diag::StrResult;
+
+/// A value that indicates a smart default.
+#[derive(Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct AutoValue;
+
+impl IntoValue for AutoValue {
+    fn into_value(self) -> Value {
+        Value::Auto
+    }
+}
+
+impl FromValue for AutoValue {
+    fn from_value(value: Value) -> StrResult<Self> {
+        match value {
+            Value::Auto => Ok(Self),
+            _ => Err(Self::error(&value)),
+        }
+    }
+}
+
+impl Reflect for AutoValue {
+    fn describe() -> CastInfo {
+        CastInfo::Type("auto")
+    }
+
+    fn castable(value: &Value) -> bool {
+        matches!(value, Value::Auto)
+    }
+}
+
+impl Debug for AutoValue {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.pad("auto")
+    }
+}