Cargo Features for *WASM* environments are sub-optimal.

[stephenmartindale]

Having exploited the instant crate in my toy WebAssembly game that I've been playing around with, I have come to the conclusion that the way that Cargo features are defined for this crate is a little sub-optimal for two reasons:

1. They're not self documenting. That is to say, if one intends to target wasm32 using wasm-bindgen and expects that instant will, therefore, use the JavaScript performance.now(), one MUST express the wasm-bindgen feature flag for instant (similarly, for stdweb) but the requirement to do so is easy to miss and missing it results in a rather obtuse error related to JavaScript module imports, at runtime.

2. Cargo feature flags are supposed to be strictly additive. The Cargo Book says: "...enabling a feature should not disable functionality..." The existence of the inaccurate feature directly contravenes this recommendation and is, for me, an issue because my use-case requires accurate, high-precision performance counters and thus requires me to be sure that inaccurate will not be set.

   • Tangent: I didn't go to all that trouble setting COOP / COEP headers just for another Cargo dep. to sink my timer precision!

I'm happy to help fix this and submit a merge request, improving the use of feature flags, but I am not entirely sure how this should or even could be done, without introducing breaking changes to the crate. My first thought was to handle the first problem with a compilation warning that could be dismissed by setting a feature -- perhaps named js-extern-now -- indicating that the user did, indeed, expect instant to find now() as a JS extern. To handle case 2, perhaps introducing a precise feature which overrides (or conflicts) with inaccurate would help but that seems somewhat messy.

[stephenmartindale]

Further on this topic, separated into a comment to preserve the purity of the issue OP, here is a link to an external, down-stream issue (rustwasm/wasm-bindgen#2215) which demonstrate that problem number 1 means that users that consume other crates, like parking_lot, in the WASM environment are expected to know about instant's need for either the wasm-bindgen or stdweb features.

I, myself, stumbled upon this while using wgpu which, in turn, uses parking_lot. I later learned that instant is a very useful crate and interesting to me, too (hence item 2 in the OP) but, at the time, I had never heard of it and certainly did not know its usage requirements.