Drawing vertices without vertex buffer not possible with OpenGL

[maxammann]

Description
I'm drawing several instances of vertices which are statically defined in a vertex shader. I'm drawing 7 instances of 6 vertices like in the following. The data for each instance is bound to slot 0.

​pass.set_vertex_buffer(0, self.instances.slice(..));
pass.draw(0..6, 0..7);

This works nicely on Vulkan. I get 7 squares with the vertex shader below. On OpenGL and WebGL nothing is drawn.

WGSL shader:

struct CameraUniform {
    view_proj: mat4x4<f32>;
    view_position: vec4<f32>;
};

struct GlobalsUniform {
    camera: CameraUniform;
};

[[group(0), binding(0)]] var<uniform> globals: GlobalsUniform;

struct VertexOutput {
    [[location(0)]] v_color: vec4<f32>;
    [[builtin(position)]] position: vec4<f32>;
};

var<private> VERTICES: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
    vec2<f32>(0.0, 0.0),
    vec2<f32>(0.0, EXTENT),
    vec2<f32>(EXTENT, 0.0),
    vec2<f32>(EXTENT, 0.0),
    vec2<f32>(0.0, EXTENT),
    vec2<f32>(EXTENT, EXTENT)
);

[[stage(vertex)]]
fn main(
    //[[location(0)]] a_position: vec2<f32>,
    [[location(4)]] mask_offset: vec2<f32>,
    [[location(5)]] target_width: f32,
    [[location(6)]] target_height: f32,
    [[location(7)]] debug_color: vec4<f32>,
    [[builtin(vertex_index)]] vertex_idx: u32,
    [[builtin(instance_index)]] instance_idx: u32 // instance_index is used when we have multiple instances of the same "object"
) -> VertexOutput {
    let a_position = VERTICES[vertex_idx];

    let scaling: mat3x3<f32> = mat3x3<f32>(
            vec3<f32>(target_width,   0.0,            0.0),
            vec3<f32>(0.0,            target_height,  0.0),
            vec3<f32>(0.0,            0.0,            1.0)
    );

    let z = 0.0;

    let world_pos_3d = vec3<f32>(a_position + mask_offset, z);
    let world_pos = scaling * world_pos_3d;

    let position = globals.camera.view_proj * vec4<f32>(world_pos, 1.0);

    return VertexOutput(debug_color, position);
}

Repro steps
None right now.

Expected vs observed behavior
Right now nothing is drawn with OpenGL. I would expect that drawing without a vertex buffer works with OpenGL.

Extra materials
None

Platform
wgpu: 0.12 (latest)
OpenGL: ES 3.2 Mesa 21.3.2

[maxammann]

Here is a renderdoc log:
renderdoc.rdc.zip

And a spectorjs log:
spectorjs.json.zip

[maxammann]

Alright, so I noticed that the issue is not the rendering code, but my shader. It works if I move the VERTICES array into the main():

[[stage(vertex)]]
fn main(
    //[[location(0)]] a_position: vec2<f32>,
    [[location(4)]] mask_offset: vec2<f32>,
    [[location(5)]] target_width: f32,
    [[location(6)]] target_height: f32,
    [[location(7)]] debug_color: vec4<f32>,
    [[builtin(vertex_index)]] vertex_idx: u32,
    [[builtin(instance_index)]] instance_idx: u32 // instance_index is used when we have multiple instances of the same "object"
) -> VertexOutput {
var VERTICES: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
    vec2<f32>(0.0, 0.0),
    vec2<f32>(0.0, EXTENT),
    vec2<f32>(EXTENT, 0.0),
    vec2<f32>(EXTENT, 0.0),
    vec2<f32>(0.0, EXTENT),
    vec2<f32>(EXTENT, EXTENT)
);

    var a_position = VERTICES[vertex_idx];

    // Encode a square within the shader

    let scaling: mat3x3<f32> = mat3x3<f32>(
            vec3<f32>(target_width,   0.0,            0.0),
            vec3<f32>(0.0,            target_height,  0.0),
            vec3<f32>(0.0,            0.0,            1.0)
    );

    let z = 0.0;

    let world_pos_3d = vec3<f32>(a_position + mask_offset, z);
    let world_pos = scaling * world_pos_3d;

    let position = globals.camera.view_proj * vec4<f32>(world_pos, 1.0);

    return VertexOutput(debug_color, position);
}

[maxammann]

Closed in favor of gfx-rs/naga#1638