Pinned-VReg mechanism.

src/lib.rs

@@ -741,6 +741,15 @@ pub trait Function {
         &[]
     }
 
+    /// Is the given VReg pinned permanently to a PReg? Note that the
+    /// input program must not contain constraints that contradict
+    /// this (e.g., using another VReg with a fixed-reg policy to a
+    /// given preg at the same time as using a VReg pinned to that
+    /// preg) or else allocation may be impossible.
+    fn is_pinned_vreg(&self, _: VReg) -> Option<PReg> {
+        None
+    }
+
     // --------------
     // Spills/reloads
     // --------------