Snippet

HRTB (Higher-Rank Trait Bounds) for Abstracting Lifetimes in Closure Arguments

This example shows how to use Higher-Rank Trait Bounds (HRTB) with the `for<'a>` syntax. By using HRTB, we specify that the closure must accept a reference with any lifetime `'a` (specifically, lifetimes internal to `execute_with_borrow`), rather than being locked into a single specific lifetime chosen by the caller.

snippet.rs

rust

pub fn execute_with_borrow<F>(f: F)
where
    for<'a> F: Fn(&'a str) -> &'a str,
{
    let local_data = String::from("temporary string");
    let result = f(&local_data);
    println!("Processed result: {}", result);
}
 
fn main() {
    execute_with_borrow(|s| {
        if s.len() > 5 {
            &s[..5]
        } else {
            s
        }
    });
}

Breakdown

for<'a> F: Fn(&'a str) -> &'a str

Declares that the closure type `F` must be callable with a reference of *any* lifetime `'a`.

let local_data = String::from("...")

A local string allocated inside the function scope.

let result = f(&local_data)

Passes a reference to `local_data` to the closure. The lifetime of this reference is local to this function call.

|s| if s.len() > 5 { &s[..5] } else { s }

The closure borrows the input reference and returns a slice tied to that same arbitrary lifetime.

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HRTB (Higher-Rank Trait Bounds) for Abstracting Lifetimes in Closure Arguments

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