[StackOverflow/rust] Using higher-ranked trait bounds with generics

### ROOT CAUSE The issue arises when a generic function or trait requires a closure or trait object with a longer lifetime than its own scope. Rust's trait system requires explicit lifetime annotations for trait objects (e.g., `&dyn Trait`), but closures like `Fn`/`FnMut` are inherently higher-ranked due to their design. Mixing these can lead to lifetime mismatches. ### CODE FIX Replace `&dyn Trait` with `impl Trait` or use `Box` for higher-ranked trait bounds. Here's a concrete solution: **Before (error-prone):** ```rust fn process_data(mut f: F) where F: FnMut(&str) -> bool, { // ... } // Usage let mut f = |x: &str| x.starts_with('a'); process_data(f); // Error: trait object `&mut dyn FnMut(&str) -> bool` lacks a static lifetime ``` **After (fixed):** ```rust fn process_data(f: Box bool>) { // ... } // Usage let f = Box::new(|x: &str| x.starts_with('a')); process_data(f); ``` **Explanation:** - Use `Box` instead of `&dyn Trait` to avoid lifetime mismatches. - `FnMut` traits are higher-ranked by default, allowing them to be stored and used across different lifetimes. - If the closure captures data, use `move ||` and wrap it in a `Box` to ensure proper ownership and lifetime extension. This solution leverages Rust's built-in higher-ranked traits (`Fn`, `FnMut`) to resolve the issue without manual lifetime annotations.