Lifetime Issue with Associated Types - rust

Question

I've been pulling my hair out over the last week due to this incredibly annoying issue with lifetimes.
The problem occurs when I try to put a reference to a Buffer inside a DataSource, which is then referenced to a DrawCommand. I'm getting the error: vertex_data_source does not live long enough.
src/main.rs:65:23: 65:41 error:
src/main.rs:65 data_source: &vertex_data_source
^~~~~~~~~~~~~~~~~~
src/main.rs:60:51: 67:2 note: reference must be valid for the block suffix following statement 3 at 60:50...
src/main.rs:60 let vertices = VertexAttributes::new(&buffer);
src/main.rs:61
src/main.rs:62 let vertex_data_source = factory.create_data_source(vertices);
src/main.rs:63
src/main.rs:64 let command: DrawCommand<ResourcesImpl> = DrawCommand {
src/main.rs:65 data_source: &vertex_data_source
...
src/main.rs:62:67: 67:2 note: ...but borrowed value is only valid for the block suffix following statement 4 at 62:66
src/main.rs:62 let vertex_data_source = factory.create_data_source(vertices);
src/main.rs:63
src/main.rs:64 let command: DrawCommand<ResourcesImpl> = DrawCommand {
src/main.rs:65 data_source: &vertex_data_source
src/main.rs:66 };
src/main.rs:67 }
It says vertex_data_source has to be valid for the block suffix following statement 3 at line 60. My interpretation of that error is that vertex_data_source should be defined before line 60. But to create the vertex_data_source in the first place I need access to those VertexAttributes on line 60, so I can't just swap the order around.
I feel like all the 'a lifetimes sprinkled over my code need to be split into 2 or maybe just removed, however I've tried every combination that seemed sensible and I'm not out of ideas.
Below is a greatly simplified example of my code that demonstrates the problem. I would really appreciate a sanity check and hopefully a fresh mind might be able to spot the issue. (every time before a few days of fiddling has produced a fix but this time I'm stumped).
use std::cell::RefCell;
use std::marker::PhantomData;
pub struct DrawCommand<'a, R: Resources<'a>> {
pub data_source: &'a R::DataSource
}
pub trait Resources<'a> {
type DataSource: 'a;
type Buffer: 'a;
}
pub struct DataSource<'a> {
id: u32,
attributes: Vec<VertexAttributes<'a, ResourcesImpl<'a>>>,
current_element_array_buffer_binding: RefCell<Option<Buffer<'a>>>
}
pub struct Buffer<'a> {
context: &'a GraphicsContextImpl
}
pub struct GraphicsContextImpl;
pub struct ResourcesImpl<'a> {
phantom: PhantomData<&'a u32> // 'a is the lifetime of the context reference
}
impl<'a> Resources<'a> for ResourcesImpl<'a> {
type Buffer = Buffer<'a>;
type DataSource = DataSource<'a>;
}
struct Factory<'a> {
context: &'a GraphicsContextImpl
}
impl<'a> Factory<'a> {
/// Creates a buffer
fn create_buffer<T>(&self) -> Buffer<'a> {
Buffer {
context: self.context
}
}
fn create_data_source(&self, attributes: Vec<VertexAttributes<'a, ResourcesImpl<'a>>>) -> DataSource<'a> {
DataSource {
id: 0,
attributes: attributes,
current_element_array_buffer_binding: RefCell::new(None)
}
}
}
fn main() {
let context = GraphicsContextImpl;
let factory = Factory {
context: &context
};
let buffer = factory.create_buffer::<u32>();
let vertices = VertexAttributes::new(&buffer);
let vertex_data_source = factory.create_data_source(vec!(vertices));
let command: DrawCommand<ResourcesImpl> = DrawCommand {
data_source: &vertex_data_source
};
}
pub struct VertexAttributes<'a, R: Resources<'a>> {
pub buffer: &'a R::Buffer,
}
impl<'a, R: Resources<'a>> VertexAttributes<'a, R> {
pub fn new(buffer: &'a R::Buffer) -> VertexAttributes<'a, R> {
VertexAttributes {
buffer: buffer
}
}
}
Thanks very much in advance.
EDIT:
I've updated the code to better reflect my actual implementation.
By the way - replacing this:
let vertex_data_source = factory.create_data_source(vec!(vertices));
With this:
let vertex_data_source = DataSource {
id: 0,
attributes: vec!(vertices),
current_element_array_buffer_binding: RefCell::new(None)
};
Doesn't solve the issue.

Answer 1

This allows your example to compile:
pub struct DrawCommand<'a : 'b, 'b, R: Resources<'a>> {
pub data_source: &'b R::DataSource
}
However, I'm finding it extremely difficult to create a more minimal example. As best I can determine, you have an issue because you are declaring that you will hold a reference to an item that itself has a reference, and that those two references need to have a common lifetime ('a). Through some combination of the other lifetimes, this is actually impossible.
Adding a second lifetime allows the reference to the DataSource to differ from the reference of the DataSource itself.
I'm still going to try to create a more minified example.