How to wrap a struct with lifetime parameter?

[lochbrunner]

🐛 Bug Reports

This is different to issue 246.

🌍 Environment

• Your operating system and version: 4.15.0-50-generic Add PyDict::iter #54-Ubuntu SMP
• Your python version: Python 3.6.8
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: Anaconda 64bit
• Your rust version (rustc --version): rustc 1.34.0-nightly (e1c6d0057 2019-02-22)
• Are you using the latest pyo3 version? Have you tried using latest master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")? version: 0.7.0

💥 Reproducing

When using a struct with lifetime as follow:

use pyo3::prelude::*;

#[pyclass]
struct MyClass<'a> {
    r: &'a u32,
}

results in the compiler error:

error[E0106]: missing lifetime specifier
--> pycore/src/trace.rs:114:8
|
114 | struct MyClass<'a> {
| ^^^^^^^ expected lifetime parameter
error: aborting due to previous error

Using dependency:

[dependencies]
pyo3 = {version = "0.7.0", features = ["extension-module"]}

[kngwyu]

It's not allowed now since it's really difficult to handle the lifetime from Python side.

[birkenfeld]

For illustration, the issue is that as a pyclass, the object has to be able to be passed into Python code. For that, it has to be put onto the Python heap, at which point the lifetime is out of control of Rust and the borrow checker. It can be kept alive potentially indefinitely by Python references.

[birkenfeld]

That said, the error could be a lot clearer :)

[kngwyu]

That said, the error could be a lot clearer :)

Agreed 👍

[lochbrunner]

@birkenfeld I thought so already...

[konstin]

PR with better error message has been merged

[chrisrossi]

Is there a workaround for the fundamental issue here? If I need to wrap a struct from a pure-Rust library that uses lifetimes, how do I do that, short of rewriting the pure-Rust library to work within PyO3 limitations? Is there some clever hack/indirection to get around this limitation?

[adamreichold]

You could only unsafely transmute MyStruct<'a> into MyStruct<'static> and store that inside a #[pyclass]. Of course, it will then be your responsibility to ensure that the correct lifetimes will be applied during usage. (Generally speaking, this seems unlikely to work as the lifetime 'a in MyStruct<'a> suggests that it references data stored elsewhere. This relationship can just not be managed automatically using the Python heap.)