use yoke crate to erase lifetime of unstructured

cranelift/control/Cargo.toml

@@ -11,6 +11,7 @@ edition.workspace = true
 
 [dependencies]
 arbitrary = { version = "1.3.0" }
+yoke = "0.7.1"
 
 [features]
 

cranelift/control/src/chaos.rs

@@ -1,34 +1,62 @@
 use arbitrary::{Arbitrary, Unstructured};
+use yoke::Yoke;
 
 /// The control plane of chaos mode.
 /// Please see the [crate-level documentation](crate).
-#[derive(Debug, Clone, Default)]
 pub struct ControlPlane {
-    data: Vec<u8>,
-    /// This is used as a little optimization to avoid additional heap
-    /// allocations when using `Unstructured` internally. See the source of
-    /// [`ControlPlane::shuffle`] for an example.
-    tmp: Vec<u8>,
+    // The cart is the vector of pseudo-random data generated by a fuzzer.
+    // The yoke is the slice of unused bytes in that vector.
+    data: Yoke<&'static [u8], Vec<u8>>,
+}
+
+impl Default for ControlPlane {
+    fn default() -> Self {
+        Self::new(Vec::new())
+    }
+}
+
+impl Clone for ControlPlane {
+    fn clone(&self) -> Self {
+        Self::new(self.data.get().to_vec())
+    }
+}
+
+impl std::fmt::Debug for ControlPlane {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("ControlPlane")
+            .field("unused_bytes", &self.data.get())
+            .field("backing_vector", self.data.backing_cart())
+            .finish()
+    }
 }
 
 impl Arbitrary<'_> for ControlPlane {
     fn arbitrary<'a>(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
-        Ok(Self {
-            data: u.arbitrary()?,
-            tmp: Vec::new(),
-        })
+        u.arbitrary().map(Self::new)
     }
 }
 
 impl ControlPlane {
+    fn new(data: Vec<u8>) -> Self {
+        let data = Yoke::attach_to_cart(data, |slice| slice);
+        Self { data }
+    }
+
     /// Returns a pseudo-random boolean if the control plane was constructed
     /// with `arbitrary`.
     ///
     /// The default value `false` will always be returned if the
     /// pseudo-random data is exhausted or the control plane was constructed
     /// with `default`.
     pub fn get_decision(&mut self) -> bool {
-        self.data.pop().unwrap_or_default() & 1 == 1
+        let mut res = false;
+        let data = std::mem::take(self).data.map_project(|unused_bytes, _| {
+            let mut u = Unstructured::new(unused_bytes);
+            res = u.arbitrary().unwrap_or_default();
+            u.take_rest()
+        });
+        *self = Self { data };
+        res
     }
 
     /// Shuffles the items in the slice into a pseudo-random permutation if
@@ -38,25 +66,23 @@ impl ControlPlane {
     /// performed if the pseudo-random data is exhausted or the control
     /// plane was constructed with `default`.
     pub fn shuffle<T>(&mut self, slice: &mut [T]) {
-        let mut u = Unstructured::new(&self.data);
-
-        // adapted from:
-        // https://docs.rs/arbitrary/1.3.0/arbitrary/struct.Unstructured.html#examples-1
-        let mut to_permute = &mut slice[..];
-
-        while to_permute.len() > 1 {
-            if let Ok(idx) = u.choose_index(to_permute.len()) {
-                to_permute.swap(0, idx);
-                to_permute = &mut to_permute[1..];
-            } else {
-                break;
-            }
-        }
+        let data = std::mem::take(self).data.map_project(|unused_bytes, _| {
+            let mut u = Unstructured::new(unused_bytes);
+
+            // adapted from:
+            // https://docs.rs/arbitrary/1.3.0/arbitrary/struct.Unstructured.html#examples-1
+            let mut to_permute = &mut slice[..];
 
-        // take remaining bytes
-        let rest = u.take_rest();
-        self.tmp.resize(rest.len(), 0); // allocates once per control plane
-        self.tmp.copy_from_slice(rest);
-        std::mem::swap(&mut self.data, &mut self.tmp);
+            while to_permute.len() > 1 {
+                if let Ok(idx) = u.choose_index(to_permute.len()) {
+                    to_permute.swap(0, idx);
+                    to_permute = &mut to_permute[1..];
+                } else {
+                    break;
+                }
+            }
+            u.take_rest()
+        });
+        *self = Self { data };
     }
 }