I've made changes that allow (not force) the merging of shader stages into one invokation of glassful. Each stage's main is marked by an attribute of the stage. Everything besides a stage's main is copied into every stage, not including other stages' mains. Function parameters not mut become in values in glsl, and parameters marked mut become out values, and if there's a return value, then that becomes the very last out value. A couple uppercase types are converted into glsl types as well, including Pnt*, Vec*, Mat* into their (lowercase) glsl counterparts. The lowercase versions could still be used for function syntax if that is clearer. My changes to the README.md say more.

Be warned: shaders.rs has alot of things commented out, I've been playing with syntax there. But, what I've outlined should work.

Am I doing something wrong here:

let fragment_shader_src = glassful! {
    #![version="140"]

    #[varying] static v_coord: vec3 = UNINIT;

    fn main() {
        if (abs(v_coord.z) == 0.25) {
            gl_FragColor = vec4(0., 1., 0., 1.);
        } else if (abs(v_coord.x) == 0.25) {
            gl_FragColor = vec4(0., 0., 1., 1.);
        } else if (abs(v_coord.y) == 0.25) {
            gl_FragColor = vec4(1., 0., 0., 1.);
        }
    }
};

I get:

thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: CompilationError("0:5(8): error: syntax error, unexpected IF\n")', ../src/libcore/result.rs:731```

e.g.

const UNIFORMS: Uniforms;

fn vertex(v: Attributes) -> (vec4, Varyings) { ... }
fn fragment(v: Varyings) -> vec4 { ... }

where those struct definitions are lifted up from glium (c.f. #2)

We only support a tiny subset of Rust. It could be nice to have a 100 line grammar for shader programs instead of the grammar being the 6,000 line Rust parser + a pile of ad-hoc restrictions.

We could also make small changes to the syntax, e.g. removing the mandatory initializers on statics. As long as the tokens are compatible with Rust, the macro re-parses from source anyway.

Using the actual libsyntax parser was mostly a hack to get the proof of concept done in one day :P