How do I work with scoped threads in Rust?

Walkthrough

Scoped threads allow you to spawn threads that can borrow data from the parent scope, rather than requiring owned data with 'static lifetime. This is achieved through std::thread::scope, which guarantees all spawned threads complete before the scope exits.

Key benefits:

Borrow non-'static data — Threads can reference local variables
No Arc needed — Share stack data directly without reference counting
Guaranteed join — All threads automatically join at scope end
Clean syntax — More ergonomic than manual thread management

Before scoped threads (Rust 1.63+), sharing data with threads required Arc or moving ownership. Now you can borrow stack data safely within a scope.

The scope function takes a closure that receives a &Scope reference, which provides spawn() to create scoped threads.

Code Examples

Basic Scoped Thread Usage

use std::thread;
 
fn main() {
    let data = vec![1, 2, 3, 4, 5];
    
    // Spawn threads that borrow `data` from the parent scope
    thread::scope(|s| {
        for i in 0..5 {
            // Each thread borrows data - no Arc needed!
            s.spawn(move || {
                println!("Element {}: {}", i, data[i]);
            });
        }
    }); // All threads are joined here before scope exits
    
    println!("All threads completed, data is still valid: {:?}", data);
}

Parallel Sum with Scoped Threads

use std::thread;
 
fn parallel_sum(data: &[i32]) -> i32 {
    let mut sums = vec![0; 4]; // One sum per thread
    
    thread::scope(|s| {
        let chunk_size = (data.len() + 3) / 4;
        
        for (i, chunk) in data.chunks(chunk_size).enumerate() {
            let sum_ref = &mut sums[i];
            
            s.spawn(move || {
                *sum_ref = chunk.iter().sum();
            });
        }
    });
    
    sums.iter().sum()
}
 
fn main() {
    let data: Vec<i32> = (1..=100).collect();
    let result = parallel_sum(&data);
    println!("Sum of 1..100: {}", result); // 5050
}

Mutating Shared Data

use std::thread;
use std::sync::Mutex;
 
fn main() {
    let counter = Mutex::new(0);
    let data = vec![10, 20, 30, 40, 50];
    
    thread::scope(|s| {
        for value in &data {
            s.spawn(|| {
                let mut num = counter.lock().unwrap();
                *num += value;
            });
        }
    });
    
    println!("Total: {}", *counter.lock().unwrap()); // 150
}

Returning Values from Scoped Threads

use std::thread;
 
fn main() {
    let data = vec!["hello", "world", "rust", "scoped", "threads"];
    
    let results: Vec<String> = thread::scope(|s| {
        // Collect the JoinHandles
        let handles: Vec<_> = data.iter()
            .map(|&word| {
                s.spawn(move || {
                    word.to_uppercase()
                })
            })
            .collect();
        
        // Join all threads and collect results
        handles.into_iter()
            .map(|h| h.join().unwrap())
            .collect()
    });
    
    println!("Results: {:?}", results);
}

Parallel Map Pattern

use std::thread;
 
fn parallel_map<T, U, F>(items: &[T], f: F) -> Vec<U>
where
    T: Sync,
    U: Send,
    F: Fn(&T) -> U + Sync,
{
    thread::scope(|s| {
        items.iter()
            .map(|item| s.spawn(|| f(item)))
            .collect::<Vec<_>>()
            .into_iter()
            .map(|handle| handle.join().unwrap())
            .collect()
    })
}
 
fn main() {
    let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8];
    
    let squares = parallel_map(&numbers, |n| n * n);
    println!("Squares: {:?}", squares);
    
    let cubes = parallel_map(&numbers, |n| n * n * n);
    println!("Cubes: {:?}", cubes);
}

Comparing Scoped vs Regular Threads

use std::thread;
use std::sync::Arc;
 
fn main() {
    let data = vec![1, 2, 3, 4, 5];
    
    // Regular thread requires Arc for shared data
    println!("=== Regular Thread (requires Arc) ===");
    {
        let data = Arc::new(data.clone());
        let handles: Vec<_> = (0..5)
            .map(|i| {
                let data = Arc::clone(&data);
                thread::spawn(move || {
                    println!("Regular thread {}: {}", i, data[i]);
                })
            })
            .collect();
        
        for h in handles {
            h.join().unwrap();
        }
    }
    
    // Scoped thread can borrow directly
    println!("\n=== Scoped Thread (borrows directly) ===");
    {
        thread::scope(|s| {
            for i in 0..5 {
                s.spawn(move || {
                    println!("Scoped thread {}: {}", i, data[i]);
                });
            }
        });
    }
    
    // Data is still available after scope
    println!("\nData after scoped threads: {:?}", data);
}

Parallel File Processing

use std::thread;
use std::fs;
 
fn main() {
    let files = vec![
        "src/main.rs",
        "Cargo.toml",
    ];
    
    let results = thread::scope(|s| {
        files.iter()
            .map(|path| {
                s.spawn(move || {
                    fs::read_to_string(path)
                        .map(|content| (path.to_string(), content.lines().count()))
                })
            })
            .collect::<Vec<_>>()
    });
    
    for handle in results {
        match handle.join().unwrap() {
            Ok((path, lines)) => println!("{}: {} lines", path, lines),
            Err(e) => println!("Error: {}", e),
        }
    }
}

Nested Scopes

use std::thread;
 
fn main() {
    let outer_data = vec![10, 20, 30];
    
    thread::scope(|outer_scope| {
        for (i, val) in outer_data.iter().enumerate() {
            outer_scope.spawn(move || {
                println!("Outer thread {}: processing {}", i, val);
                
                // Nested scope
                let inner_data = vec![*val; 3];
                
                thread::scope(|inner_scope| {
                    for (j, inner_val) in inner_data.iter().enumerate() {
                        inner_scope.spawn(move || {
                            println!("  Inner thread {}.{}: {}", i, j, inner_val);
                        });
                    }
                });
            });
        }
    });
    
    println!("All nested scopes completed");
}

Matrix Multiplication with Scoped Threads

use std::thread;
 
type Matrix = Vec<Vec<i32>>;
 
fn matrix_multiply(a: &Matrix, b: &Matrix) -> Matrix {
    let rows_a = a.len();
    let cols_b = b[0].len();
    let cols_a = a[0].len();
    
    // Initialize result matrix
    let result = vec![vec![0; cols_b]; rows_a];
    let result = thread::scope(|s| {
        // Create mutable references for each row
        let mut handles = Vec::new();
        
        for i in 0..rows_a {
            let result_ref = &result[i];
            
            // We need interior mutability for parallel writes to different rows
            // Here we use a different approach: compute each row separately
            handles.push(s.spawn(move || {
                (0..cols_b).map(|j| {
                    (0..cols_a).map(|k| a[i][k] * b[k][j]).sum()
                }).collect::<Vec<i32>>()
            }));
        }
        
        handles.into_iter()
            .map(|h| h.join().unwrap())
            .collect()
    });
    
    result
}
 
fn main() {
    let a = vec![
        vec![1, 2, 3],
        vec![4, 5, 6],
    ];
    
    let b = vec![
        vec![7, 8],
        vec![9, 10],
        vec![11, 12],
    ];
    
    let result = matrix_multiply(&a, &b);
    
    for row in &result {
        println!("{:?}", row);
    }
}

Error Handling in Scoped Threads

use std::thread;
 
fn main() {
    let data = vec![1, 2, 3, 4, 5];
    
    let results: Vec<Result<i32, String>> = thread::scope(|s| {
        data.iter()
            .map(|&n| {
                s.spawn(move || {
                    if n == 3 {
                        Err("Three is not allowed!".to_string())
                    } else {
                        Ok(n * 2)
                    }
                })
            })
            .collect::<Vec<_>>()
            .into_iter()
            .map(|h| h.join().unwrap())
            .collect()
    });
    
    for result in results {
        match result {
            Ok(value) => println!("Success: {}", value),
            Err(e) => println!("Error: {}", e),
        }
    }
}

Thread Pool Simulation with Scoped Threads

use std::thread;
use std::sync::Mutex;
 
fn process_in_parallel<T, R, F>(items: &[T], num_threads: usize, f: F) -> Vec<R>
where
    T: Sync,
    R: Send,
    F: Fn(&T) -> R + Sync,
{
    let results = Mutex::new(Vec::with_capacity(items.len()));
    
    thread::scope(|s| {
        let chunk_size = (items.len() + num_threads - 1) / num_threads;
        
        for chunk in items.chunks(chunk_size) {
            s.spawn(|| {
                let chunk_results: Vec<R> = chunk.iter().map(&f).collect();
                results.lock().unwrap().extend(chunk_results);
            });
        }
    });
    
    results.into_inner().unwrap()
}
 
fn main() {
    let urls = (0..20).map(|i| format!("https://example.com/{}", i)).collect::<Vec<_>>();
    
    let results = process_in_parallel(&urls, 4, |url| {
        // Simulate work
        format!("Fetched: {}", url)
    });
    
    println!("Processed {} items", results.len());
}

Scoped Threads with Channels

use std::thread;
use std::sync::mpsc;
 
fn main() {
    let data = vec![1, 2, 3, 4, 5];
    let (tx, rx) = mpsc::channel();
    
    thread::scope(|s| {
        // Producer threads
        for &value in &data {
            let tx = tx.clone();
            s.spawn(move || {
                tx.send(value * 2).unwrap();
            });
        }
        
        // Drop the original sender so rx.iter() terminates
        drop(tx);
    });
    
    // Collect all results
    let results: Vec<_> = rx.iter().collect();
    println!("Results: {:?}", results);
}

Summary

| Feature | Scoped Threads | Regular Threads | |---------|---------------|-----------------| | Borrow stack data | ✅ Yes | ❌ No (requires 'static) | | Requires Arc | ❌ No | ✅ Often needed | | Automatic join | ✅ Yes | ❌ Manual .join() | | Lifetime flexibility | ✅ Can use local data | ❌ Owned only |

Key Methods:

| Method | Description | |--------|-------------| | thread::scope(|s| {...}) | Create a scope for spawning threads | | s.spawn(|| {...}) | Spawn a thread within the scope | | handle.join() | Wait for thread to complete, get result |

When to Use Scoped Threads:

Processing local/stack data in parallel
Avoiding Arc overhead for simple sharing
Parallel map/filter operations
Batch processing with borrowed data

Key Points:

All spawned threads complete before scope exits
Threads can borrow non-'static data
No need for Arc when sharing stack data
spawn() returns a JoinHandle like regular threads
Exceptions in threads propagate through join() as Result
Requires Rust 1.63+ (or use crossbeam crate for older versions)

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