Macro def_programs 

macro_rules! def_programs {
    (
    SHADERS [
      $($shader:ident, $sourcepath:expr),*
    ]
    PROGRAMS [
      $(
      program $program:ident {
        vertex_shader:   $vert_shader:ident
        $(tessellation_control_shader: $tess_ctrl_shader:ident
        tessellation_evaluation_shader: $tess_eval_shader:ident)*
        $(geometry_shader: $geom_shader:ident)*
        fragment_shader: $frag_shader:ident
      }
      )*
    ]

  ) => { ... };
    ( @def_programs
    SHADERS [ $($shader:ident),* ]
    PROGRAMS [
      $(
      program $program:ident {
        vertex_shader:   $vert_shader:ident
        $(tessellation_control_shader: $tess_ctrl_shader:ident
        tessellation_evaluation_shader: $tess_eval_shader:ident)*
        $(geometry_shader: $geom_shader:ident)*
        fragment_shader: $frag_shader:ident
      }
      )*
    ]
  ) => { ... };
    (
    @def_shaders $($shader:ident, $sourcepath:expr),*
  ) => { ... };
    (@option_shader $shaders:ident [$shader_id:ident]) => { ... };
    (@option_shader $shaders:ident []) => { ... };
}

Define a list of shaders with source paths and a list of shader programs using those shaders.

This will create two enums in local scope named ShaderId and ProgramId with variants from the given identifiers.