Whew, that was a long title.
Either way, I'm asking this question, since I like to think about various things, and it occurred to me that there isn't really any simple, open source layers on top of GLSL, even if only to add simple things such as includes, or commonly used functions.
As research of sorts, I'm asking this question, since my own awareness of such languages is minuscule to say the least - I know of bgfx's shading language, and Unity's ShaderLab, but I don't really know what they accomplish - or why - being a relative newbie to computer graphics.
Alternatively, what would your wishlist for a shading language like this be? Mine so far is includes, some compatibility between versions, optional "hidden" inputs that allow easily accessing textures at pixel offsets, or image sizes, etc. and probably passes - for, say, two-pass gaussian blur.