Merge #2026

2026: expose trap error from RuntimeError r=syrusakbary a=ailisp

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# Description
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Internal of RuntimeError is private, user can know UserError via `downcast`, but user isn't able to tell the trap code if it's a `Trap`. This PR expose trap code via `RuntimeError#to_trap`. (And, if both downcast returns error and to_trap returns None, user will know it's a `RuntimeError::Generic`, which can be shown by `error.message()`, so there's no need to expose `RuntimeError::Generic` case)

# Review

- [] Add a short description of the the change to the CHANGELOG.md file


Co-authored-by: Bo Yao <[email protected]>

lib/engine/src/trap/error.rs

@@ -207,6 +207,15 @@ impl RuntimeError {
         }
     }
 
+    /// Returns trap code, if it's a Trap
+    pub fn to_trap(self) -> Option<TrapCode> {
+        if let RuntimeErrorSource::Trap(trap_code) = self.inner.source {
+            Some(trap_code)
+        } else {
+            None
+        }
+    }
+
     /// Returns true if the `RuntimeError` is the same as T
     pub fn is<T: Error + 'static>(&self) -> bool {
         match &self.inner.source {