CODEWITHSUNDEEP
rust
quux00
quux00

Reputation: 14604

In Rust, is there any way to change a struct/type to be "Send"?

I've hit the issue a couple of times of trying to pass things into the spawn function (to create a new thread/task) and have the compiler tell me error: cannot capture variable of type "blah blah", which does not fulfill "Send", in a bounded closure.

Is there a way to transform a type be able to fulfill "Send" or is that fixed based on some set of rules?

For example, I can easily implement the ToStr trait by using a directive like this:

#[deriving(ToStr, Rand)]
struct Point {
    x: int,
    y: int,
}

Can I do something similar for the Send trait? Or are "kind" Traits treated differently?

Here's a concrete example of this issue - is there some way to overcome it?

fn create_process_options(cmdinfo: &CmdInfo) -> (ProcessOptions, Option<FileDesc>) {
// ... omitted
}

// "po" is of type std::run::ProcessOptions
let (po, filedesc_opt) = create_process_options(&cmdinfo);
spawn(proc() {
    let mut ps = Process::new(cmdinfo.program, cmdinfo.args, po).expect("darn");
    ps.finish();
});

Compiler error:

error: cannot capture variable of type `std::run::ProcessOptions<>`, which does not fulfill `Send`, in a bounded closure
let mut process = Process::new(cmdinfo.program, cmdinfo.args, po).expect("darn");
                                                              ^~
note: this closure's environment must satisfy `Send`
let mut process = Process::new(cmdinfo.program, cmdinfo.args, po).expect("darn");

Upvotes: 4

Views: 5384

Answers (1)

reem
reem

Reputation: 7236

Send is a rust Kind, the other things you mentioned are Traits. While both can be used to bound generics, they are in fact quite different. You have to opt-in to a Trait, but what Kinds a type has are inferred based on their contents - besides changing the contents you can't change the Kind of a type.

For most Kinds, the rule is "a type X is of Kind Y if all the members of X are of Kind Y."

In this case, since being Send requires that you fulfill 'static, which means that they don't contain any non-'static references. Since ProcessOptions contains a non-static lifetime Option<&'a Path>, like Chris Morgan detailed in his comment, ProcessOptions does not fulfill Send.

Upvotes: 8

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