How to store an Expression in a struct?

[tnychn]

I want to cache the compiled Expression by storing it in the Variable struct.
The expression is compiled using a single global static empty environment.

use std::cell::OnceCell;
use std::sync::OnceLock;

use minijinja::{Environment, Expression};

static ENVIRONMENT: OnceLock<Environment> = OnceLock::new();

pub struct Variable {
	_condition: String,
	condition: OnceCell<Expression<'static, 'static>>,
}

impl Variable {
	fn compile(&self) {
		let environment = ENVIRONMENT.get_or_init(|| Environment::empty());
		let expression = environment
			.compile_expression(&self._condition)
			.expect("cannot compile expression");
		let _ = self.condition.set(expression);
	}
}

But it failed to compile with the following error.

error[E0521]: borrowed data escapes outside of method
   |
33 |       fn compile(&self) {
   |                  -----
   |                  |
   |                  `self` is a reference that is only valid in the method body
   |                  let's call the lifetime of this reference `'1`
34 |           let environment = ENVIRONMENT.get_or_init(|| Environment::empty());
35 |           let expression = environment
   |  __________________________^
36 | |             .compile_expression(&self._condition)
   | |                                                 ^
   | |                                                 |
   | |_________________________________________________`self` escapes the method body here
   |                                                   argument requires that `'1` must outlive `'static`

How can I deal with the lifetimes of Environment and Expression, and the relationship between them and my struct in this case?

[mitsuhiko]

Today that's not possible, but in the next version of minijinja it will be possible with Environment::compile_expression_owned. See #383