Sharing a struct with trait objects as properties across threads - rust

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`

Answer 1

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>,
}
See also:
How can I share references across threads?
Multithreaded application fails to compile with error-chain
Is there any way to implement the Send trait for ZipFile?
How do I share a generic struct between threads using Arc<Mutex<MyStruct<T>>>?