为什么此特性/实现不兼容-约束寿命与具体寿命

Question

I'm struggling with this error rustc gives me:

error: method `create_shader_explicit` has an incompatible type for trait: expected bound lifetime parameter 'a, found concrete lifetime

My trait declaration is pretty much this:

pub trait GraphicsContext<R: Resources> {

    /// Creates a shader object
    fn create_shader<'a>(&'a self, shader::Stage, source: &str) -> 
        Result<handle::Shader<R>, shader::CreateError>;

}

Here's my implementation,

pub struct OpenGLResources<'a> {
    phantom: PhantomData<&'a u32> 
}

impl<'a> Resources for OpenGLResources<'a> {
    type Shader = Shader<'a>;
}

impl<'z> GraphicsContext<OpenGLResources<'z>> for OpenGLGraphicsContext {

    /// Creates a shader object
    fn create_shader<'a>(&'a self, stage: shader::Stage, source: &str) -> 
        Result<handle::Shader<OpenGLResources>, shader::CreateError> {

        let shader = Shader::new(self, stage);
        try!(shader.compile_from_source(source));

        Ok(shader)
    }

}

In other questions on StackOverflow, they are missing things like <'a> between create_shader and () , however when I compare the fn definitions in mine they look identical.

EDIT:

Changing the definition inside impl to the following fixes that issue

fn create_shader<'a>(&'a self, stage: shader::Stage, source: &str) ->     
    Result<handle::Shader<OpenGLResources**<'z>**>, shader::CreateError>

But then the issue is that 'a and 'z need to be the same lifetime. If I change it to this:

fn create_shader(**&'z** self, stage: shader::Stage, source: &str) -> 
    Result<handle::Shader<OpenGLResources<'z>>, shader::CreateError>

The impl block works, but then I need a way of specifying the 'z lifetime in the trait definition. I tried the following:

pub trait<'z> GraphicsContext<R: Resources<'z>>

But it didn't work.

Answer 1

When comparing things like this, you need to remember to expand all the generics so that you can actually compare it all. In this case, you haven't expanded R . If you do, the answer becomes obvious: R is OpenGLResources<'z> , linking the OpenGLResources to the impl block, whereas your method definition has elided the lifetime on OpenGLResources , causing it to be inferred as self 's lifetime, which is 'a .

Answer 2

Thanks the hints of @Chris Morgan I managed to implement this functionality and its now working fine.

If we start with the base trait with the 'a lifetime included:

trait Resources<'a> {
    type Shader: Shader;
    type ShaderProgram: ShaderProgram;
}

Then implement it for OpenGL. (note the PhantomData struct)

struct OpenGLResources<'a> {
   phantom: PhantomData<&'a u32> // 'a is the lifetime of the context reference
}

impl<'a> ResourcesTrait<'a> for Resources<'a> {
    type Shader = Shader<'a>;
    type ShaderProgram = ShaderProgram<'a>;
    type CommandBuffer = CommandBuffer;
    type CommandBufferBuilder = CommandBufferBuilder;
} 

Its a bit verbose, but the GraphicsContext trait works fine too now. The 'a lifetime goes in the type parameters part.

trait GraphicsContext<'a, R: Resources<'a>> {

    fn create_shader(&'a self, ty: Type, source: &str) -> Result<R::Shader, ShaderCreationError>

}

Finally this is the required code in the graphics context implementation. It is extremely verbose with the 'a lifetimes sprinkled everywhere but at least it works!

impl<'a> GraphicsContext<'a, Resources<'a>> for OpenGLGraphicsContext