`cfg(wasm)` unifying `target_arch="wasm32"` and `target_arch="wasm64"`

[nagisa]

In #80525 we added to nightly a cfg(wasm) which is set when either wasm32 or wasm64 targets are used. We don't have top-level cfg values for other architectures in a similar situation, only for windows vs unix which is set based on the OS.

This ended up being an insta-stable addition due to oversight of mine.

This issue is in order to gather consensus from T-lang members that we do indeed want this cfg. I will make myself a note to either remove this cfg or put it behind a feature gate before the next nightly->beta rollover that happens on May 6th if such consensus is not gathered.

Zulip discussion
PR adding the cfg

[nagisa]

@rfcbot fcp merge

[Amanieu]

Related: rust-lang/stdarch#974

[joshtriplett]

Based on discussions on Zulip, it sounds like we don't want to have this be insta-stable. However, I do think we want to have cfg(wasm), it just needs a feature-gate for a little while. (And we may want to stabilize it sooner rather than later, if there are no issues, so that people can start using it rather than writing wasm32-specific code.)

Gathering consensus on adding a feature-gated cfg(wasm) that's true on WebAssembly targets:

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[ehuss]

I'd like to note that having bare aliases like this can be confusing for some users. I occasionally run into users who get tripped up by unix/windows, and don't understand why other things like macos don't work, or just in general don't understand what "families" are. Having these bare aliases masks what the underlying fields are (target_family, target_os, target_arch), which I think contributes to that confusion. I'm concerned that adding more aliases like this will contribute to more confusion, as this would add a special case that is different from the family aliases. Seeing this, users may try other arch names without 32/64 and get confused, or not understand the difference between a family and an arch.

[joshtriplett]

@ehuss I would be in favor of adding aliases for other common cases, to make them simpler to write and simpler to read.

[ehuss]

I agree that they look easier to read (and are more aesthetically pleasing), but I am concerned that they are harder to understand, especially when they are "common" exceptions. Users sometimes learn by looking at existing code to build their mental model of how things work (instead of reading or remembering documentation), and when they try to apply their mental model to new situations (like writing cfg(arm)) it can be frustrating and confusing when it doesn't work. This is particularly insidious with cfg expressions because they don't necessarily fail at compile time, they are just ignored (baring RFC 3013, which I don't think will necessarily help here, and may actually be made worse by this change). Cargo in particular is unable to verify these things.

I don't want to oversell my objection here. It is very difficult to predict how well users will understand a concept like this. I can just say that there are some users who struggle with the current system caused by "unix" and "windows", and I am concerned this may contribute to more confusion. Adding more aliases may make it even harder to understand when the thing you want to match on isn't part of that blessed set of aliases.

[devsnek]

One thing to note specifically in the case of wasm is that wasm32 and wasm64 are not actually separate architectures, it is just a weird abstraction in llvm to represent wasm memory64 types. I would like to provide a very ergonomic way for the rust ecosystem to quickly coalesce on wasm-supporting crates targeting the wasm architecture without having to write out cfg(any(target_arch = "wasm32", target_arch = "wasm64")). I don't think it's reasonable for everyone who wants to target wasm architecture to know the specifics of how memory64 works and the choices that llvm made and that they should be making sure they're including all the wasm targets.

All that being said, I'm fine with other solutions besides the specific cfg(wasm) incantation, as long as we figure out something. For example, cfg(target_arch ~= "wasm") could also work (though I don't like that as much as cfg(wasm)).