Sharing a struct with trait objects as properties across threads

Question

I have the code below. With the commented out parts, it's working. When I uncomment the parts it does not compile anymore.

How can I adjust the commented parts to make them work, i.e., I want to make threads access the expression tree simultaneously.

When I try it, the compiler starts with errors about thread safeness.

I read the Rust book and know C/C++, but didn't understood everything about Rust type system and semantics yet.

use std::thread;
use std::sync::Arc;

pub trait Expr {
    fn run(&self) -> i32;
}

pub struct ConstantExpr {
    n: i32,
}

impl ConstantExpr {
    pub fn new(n: i32) -> Self {
        Self { n }
    }
}

impl Expr for ConstantExpr {
    fn run(&self) -> i32 {
        self.n
    }
}

pub struct AddExpr {
    expr1: Box<Expr>,
    expr2: Box<Expr>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr>, expr2: Box<Expr>) -> Self {
        Self { expr1, expr2 }
    }
}

impl Expr for AddExpr {
    fn run(&self) -> i32 {
        self.expr1.run() + self.expr2.run()
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr>,
}

impl Container {
    fn new() -> Self {
        Self {
            x: 0,
            cached_expr: Arc::new(AddExpr::new(
                Box::new(ConstantExpr::new(10)),
                Box::new(ConstantExpr::new(1)),
            )),
        }
    }
}

fn main() {
    let container = Arc::new(Container::new());

    let container1 = Arc::clone(&container);

    /*
    let thread1 = thread::spawn(move || {
        println!("thread1: {}", container1.x);
        println!("thread1: {}", container1.cached_expr.run());
    });
    */

    println!("main: {}", container.x);
    println!("main: {}", container.cached_expr.run());

    //thread1.join().unwrap();
}

The error:

error[E0277]: the trait bound `Expr + 'static: std::marker::Send` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

error[E0277]: the trait bound `Expr + 'static: std::marker::Sync` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

Shepmaster · Accepted Answer · 2018-01-15 02:24:52Z

I find the error message pretty straightforward:

the trait std::marker::Send is not implemented for Expr + 'static
required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Expr + 'static>
required because it appears within the type Container
required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Container>
required because it appears within the type [closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]
required by std::thread::spawn

You are trying to move your Arc<Container> to another thread, but it contains an Arc<Expr + 'static>, which cannot be guaranteed to be safely sent (Send) or shared (Sync) across threads.

Either add Send and Sync as supertraits to Expr:

pub trait Expr: Send + Sync { /* ... */ }

Or add them as trait bounds to your trait objects:

pub struct AddExpr {
    expr1: Box<Expr + Send + Sync>,
    expr2: Box<Expr + Send + Sync>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr + Send + Sync>, expr2: Box<Expr + Send + Sync>) -> Self {
        Self { expr1, expr2 }
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr + Send + Sync>,
}

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