Rust Piston can't find correct type hint

May 22, 2022

I am New to Rust and come from another programming language.

here are my dependencies

[dependencies]
piston_window = "0.123.0"
piston = "0.53.1"
gfx = "0.18.2"

I am having a problem with the manual type hinting

when I load a Texture without adding a type hint the texture is loaded and can be displayed in the window.

let dirt = piston_window::Texture::from_path(&mut window.create_texture_context(),
Path::new("assets/sprites/dirt.png"), Flip::None, &TextureSettings::new()).unwrap();

when I try to manually add the type hint which vscode recommends me, I get an Error.
e.g

let dirt: Texture<Resources>= piston_window::Texture::from_path(&mut window....

The Error can be reproduced by running the following main file in an environment with a dummy.png file

extern crate piston_window;
extern crate piston;
extern crate gfx;
use piston_window::{
    PistonWindow,
    WindowSettings,
    Flip,
    TextureSettings,
    Texture,
};

use std::path::Path; 

fn main(){
    let opengl_version = piston_window::OpenGL::V3_2;
    let mut window: PistonWindow = WindowSettings::new("test", [1280., 720.]).exit_on_esc(true).graphics_api(opengl_version).build().unwrap();
    let working = piston_window::Texture::from_path(&mut window.create_texture_context(), Path::new("dummy.png"), Flip::None, &TextureSettings::new()).unwrap();
    let failing:Texture<gfx::Resources> = piston_window::Texture::from_path(&mut window.create_texture_context(), Path::new("dummy.png"), Flip::None, &TextureSettings::new()).unwrap();
}

>Solution :

The actual type is Texture<gfx_device_gl::Resources>, not Texture<gfx::Resources>.

Note that gfx::Resources is a trait, not a type. In Texture<R> the R generic type must not be covariant, and even if it were the type would likely need to be Texture<dyn gfx::Resources>. While it’s true that gfx_device_gl::Resources implements gfx::Resources, that doesn’t on its own imply that Texture<gfx_device_gl::Resources> and Texture<dyn gfx::Resources> are compatible types. (The R type parameter would also need to be covariant.)

If you want to ensure that failing is a Texture without giving a particular resource type, you could use the annotation Texture<_> instead, which permits the compiler to infer Texture‘s generic type argument.