compiler: Add FramePointer::ratchet

compiler/rustc_target/src/spec/mod.rs

@@ -1413,6 +1413,20 @@ pub enum FramePointer {
     MayOmit,
 }
 
+impl FramePointer {
+    /// It is intended that the "force frame pointer" transition is "one way"
+    /// so this convenience assures such if used
+    #[inline]
+    pub fn ratchet(&mut self, rhs: FramePointer) -> FramePointer {
+        *self = match (*self, rhs) {
+            (FramePointer::Always, _) | (_, FramePointer::Always) => FramePointer::Always,
+            (FramePointer::NonLeaf, _) | (_, FramePointer::NonLeaf) => FramePointer::NonLeaf,
+            _ => FramePointer::MayOmit,
+        };
+        *self
+    }
+}
+
 impl FromStr for FramePointer {
     type Err = ();
     fn from_str(s: &str) -> Result<Self, ()> {