Where is the 'static lifetime requirement coming from when using a trait type in std::rc::Rc and how can I prevent it? E.g. when trying to compile this code
trait Handler{}
fn add(a: std::rc::Rc<Handler>) {
}
fn main() {
add(0);
}
rust reports
error[E0308]: mismatched types
--> test.rs:7:9
...
= note: expected type `std::rc::Rc<Handler + 'static>`
found type `{integer}`
NOTE: the error itself is expected; I am just interested in the Handler + 'static diagnostic output. The Real program creates instances of a trait type, stores them into a HashMap and runs a type specific function on it. It fails to compile with
| - borrowed value only lives until here
|
= note: borrowed value must be valid for the static lifetime...
at this place.
Second example
The following code is more real-world and demonstrates the issue perhaps better:
trait Handler {
}
struct SomeHandler<'a>(&'a u32);
impl <'a> SomeHandler<'a> {
fn new(a: &'a u32) -> std::rc::Rc<Handler> {
std::rc::Rc::new(SomeHandler(a))
}
}
impl <'a> Handler for SomeHandler<'a> {
}
fn main() {
let a: u32;
SomeHandler::new(&a);
}
It fails with
8 | std::rc::Rc::new(SomeHandler(a))
| ^
= note: but, the lifetime must be valid for the static lifetime...
= note: ...so that the expression is assignable:
expected std::rc::Rc<Handler + 'static>
found std::rc::Rc<Handler>
Why explicit lifetimes do not work
The simple demo might be fixed by adding an explicit lifetime (e.g. Rc<Handler + 'a>). Unfortunately, this is not an option (nor trying to make anything 'static) because real code is intended to look like
struct PollRegistry {
...
handlers: std::collections::HashMap<mio::Token, std::rc::Weak<PollHandler>>,
}
impl PollRegistry {
fn register<'a>(&mut self, handler: &std::rc::Rc<PollHandler>,
interest: mio::Ready, opts: mio::PollOpt)
-> std::io::Result<()> {
....
self.handlers.insert(token, std::rc::Rc::downgrade(handler));
}
}
and methods in PollHandler specializations create and own other PollHandler specializations which are registered in the registry by these methods.
rustc 1.27.1
