diff --git a/src/lib.rs b/src/lib.rs
index 7f2bac1..aa1c140 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -225,6 +225,7 @@
 
 #![deny(missing_docs, missing_debug_implementations)]
 
+use std::any::{Any, TypeId};
 use std::collections::HashMap;
 use std::env::VarError;
 use std::fmt::Debug;
@@ -818,6 +819,153 @@ macro_rules! fail_point {
     ($name:expr, $cond:expr, $e:expr) => {{}};
 }
 
+#[derive(Clone)]
+struct SyncMutCallback1(Arc<Mutex<dyn FnMut(&mut dyn Any) + Send + Sync + 'static>>);
+
+impl Debug for SyncMutCallback1 {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("SyncMutCallback1()")
+    }
+}
+
+impl PartialEq for SyncMutCallback1 {
+    #[allow(clippy::vtable_address_comparisons)]
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.0, &other.0)
+    }
+}
+
+impl SyncMutCallback1 {
+    fn new(f: Box<dyn FnMut(&mut dyn Any) + Send + Sync>) -> SyncMutCallback1 {
+        SyncMutCallback1(Arc::new(Mutex::new(f)))
+    }
+
+    fn run(&mut self, var: &mut dyn Any) {
+        let callback = &mut self.0.lock().unwrap();
+        callback(var);
+    }
+}
+
+struct MapEntry {
+    type_id: TypeId,
+    cb: SyncMutCallback1,
+}
+
+impl MapEntry {
+    fn new(type_id: TypeId, cb: Box<dyn FnMut(&mut dyn Any) + Send + Sync>) -> MapEntry {
+        MapEntry {
+            type_id: type_id,
+            cb: SyncMutCallback1::new(cb),
+        }
+    }
+}
+
+lazy_static::lazy_static! {
+    static ref TESTVALUE_REGISTRY: RwLock<HashMap<String, MapEntry>>= Default::default();
+}
+
+/// Set the callback for a test value adjustment.
+///
+/// Usage:
+///
+/// ```rust
+/// use fail::{adjust, ScopedCallback};
+///
+/// fn production_code() {
+/// 	let mut var = 1;
+/// 	adjust!("adjust_this_var", &mut var);
+/// }
+///
+/// fn test_code() {
+///     let _raii = ScopedCallback::new("adjust_this_var", |var| {
+/// 	    *var = 2; 
+///     });
+/// }
+/// ```
+///
+pub fn set_callback<S, T, F>(name: S, mut f: F) -> Result<(), String>
+where
+    S: Into<String>,
+    T: Any,
+    F: FnMut(&mut T) + Send + Sync + 'static,
+{
+    let mut registry = TESTVALUE_REGISTRY.write().unwrap();
+    registry.insert(
+        name.into(),
+        MapEntry::new(
+            TypeId::of::<T>(),
+            Box::new(move |var| {
+                if let Some(var) = var.downcast_mut::<T>() {
+                    f(var);
+                } else {
+                    panic!("Type mismtach");
+                }
+            }),
+        ),
+    );
+    Ok(())
+}
+
+/// Set a scoped callback using RAII
+#[derive(Debug)]
+pub struct ScopedCallback {
+    name: String,
+}
+
+impl ScopedCallback {
+    /// Creates a RAII instance.
+    pub fn new<S, T, F>(name: S, f: F) -> Self
+    where
+        S: Into<String> + Copy,
+        T: Any,
+        F: FnMut(&mut T) + Send + Sync + 'static,
+    {
+        set_callback(name.clone(), f).unwrap();
+        ScopedCallback { name: name.into() }
+    }
+}
+
+impl Drop for ScopedCallback {
+    fn drop(&mut self) {
+        let mut registry = TESTVALUE_REGISTRY.write().unwrap();
+        registry.remove(&self.name);
+    }
+}
+
+#[doc(hidden)]
+pub fn internal_adjust<S, T>(name: S, var: &mut T)
+where
+    S: Into<String>,
+    T: Clone + 'static,
+{
+    let mut registry = TESTVALUE_REGISTRY.write().unwrap();
+    // Clone the var here, since the argument is required to be 'static.
+    let mut clone = var.clone();
+    if let Some(entry) = registry.get_mut(&name.into()) {
+        if (*entry).type_id != TypeId::of::<T>() {
+            panic!("Type mismatch");
+        }
+        (*entry).cb.run(&mut clone);
+    }
+    *var = clone;
+}
+
+/// Define a test value adjustment (requires `failpoints` feature).
+#[macro_export]
+#[cfg(feature = "failpoints")]
+macro_rules! adjust {
+    ($name:expr, $var:expr) => {{
+        $crate::internal_adjust($name, $var);
+    }};
+}
+
+/// Define a test value adjustment (disabled, see `failpoints` feature).
+#[macro_export]
+#[cfg(not(feature = "failpoints"))]
+macro_rules! adjust {
+    ($name:expr, $var:expr) => {{}};
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/tests/tests.rs b/tests/tests.rs
index fecb53c..706541e 100644
--- a/tests/tests.rs
+++ b/tests/tests.rs
@@ -211,3 +211,36 @@ fn test_list() {
     fail::cfg("list", "return").unwrap();
     assert!(fail::list().contains(&("list".to_string(), "return".to_string())));
 }
+
+#[test]
+fn test_value_adjust() {
+    let f = || -> i32 {
+        let mut var = 1;
+        fail::adjust!("adjust_var", &mut var);
+        var
+    };
+    assert_eq!(f(), 1);
+
+    fail::set_callback("adjust_var", |vari| {
+        *vari = 2;
+    })
+    .unwrap();
+    assert_eq!(f(), 2);
+}
+
+#[test]
+fn test_value_adjust_raii() {
+    let f = || -> i32 {
+        let mut var = 1;
+        fail::adjust!("adjust_var1", &mut var);
+        var
+    };
+    {
+        let _raii = fail::ScopedCallback::new("adjust_var1", |var| {
+            *var = 2;
+        });
+        assert_eq!(f(), 2);
+    }
+
+    assert_eq!(f(), 1);
+}