make get_entry_exit panic if the entry/exit nodes don't exist

Author: doug-q

src/emit/func.rs

@@ -115,8 +115,6 @@ impl<'c, H: HugrView> EmitFuncContext<'c, H> {
     /// Create a new basic block. When `before` is `Some` the block will be
     /// created immediately before that block, otherwise at the end of the
     /// function.
-    ///
-    /// TODO I think this will be needed for emitting CFGs.
     pub(crate) fn new_basic_block(
         &mut self,
         name: impl AsRef<str>,

src/emit/ops/cfg.rs

@@ -60,7 +60,7 @@ impl<'c, 'd, H: HugrView> CfgEmitter<'c, 'd, H> {
                 }
             })
             .collect::<Result<HashMap<_, _>>>()?;
-        let (entry_node, exit_node) = node.get_entry_exit().unwrap();
+        let (entry_node, exit_node) = node.get_entry_exit();
         Ok(CfgEmitter {
             context,
             bbs,

src/fat.rs

@@ -182,21 +182,22 @@ impl<'c, OT, H: HugrView + ?Sized> FatNode<'c, OT, H> {
 }
 
 impl<'c, H: HugrView> FatNode<'c, CFG, H> {
-    /// TODO it would be reasonable to remove Option and panic on failure here
-    pub fn get_entry_exit(
-        &self,
-    ) -> Option<(FatNode<'c, DataflowBlock, H>, FatNode<'c, ExitBlock, H>)> {
+    /// Returns the entry and exit nodes of a CFG.
+    ///
+    /// These are guaranteed to exist the `Hugr` is valid. Panics if they do not
+    /// exist.
+    pub fn get_entry_exit(&self) -> (FatNode<'c, DataflowBlock, H>, FatNode<'c, ExitBlock, H>) {
         let [i, o] = self
             .hugr
             .children(self.node)
             .take(2)
             .collect_vec()
             .try_into()
-            .ok()?;
-        Some((
-            FatNode::try_new(self.hugr, i)?,
-            FatNode::try_new(self.hugr, o)?,
-        ))
+            .unwrap();
+        (
+            FatNode::try_new(self.hugr, i).unwrap(),
+            FatNode::try_new(self.hugr, o).unwrap(),
+        )
     }
 }