"WGPUColorWriteMask_Alpha|WGPUColorWriteMask_Blue" creates a number with both the alpha and blue bits set, then "mask &" (that number) ands the mask with that number in which both bits are set, so it'll only return true if both alpha and blue are set. The parenthetical grouping of the operators is key here.

edit: oh, i think i'm wrong, nevermind.