refactor reflect.SliceHeader uses to allow tinygo cross-compilation

[orsinium]

The Len and Cap fields of reflect.SliceHeader are int in Go and uintptr in TinyGo. The PR adjusts all places that set these fields to be compatible with both Go and TinyGo.

Related issue: #1854

[ncruces]

Do we need this now that we floor on 1.20?
I've been meaning to do a pass on APIs like unsafe.SliceData.

[orsinium]

Do we need this now that we floor on 1.20?

What does it mean? Do you ask me? Sorry, I'm out of the loop. I build my changes on top of the latest main branch.

I've been meaning to do a pass on APIs like unsafe.SliceData.

It would be great to get rid of it eventually since it is deprecated but I'd be glad if we can get Wazero compiling to TinyGo without blocking it with refactoring. The fork we have for TinyGo is already far behind the upstream, and keeping it up-to-date is hard.

[ncruces]

What does it mean? Do you ask me? Sorry, I'm out of the loop. I build my changes on top of the latest main branch.

wazero has a policy of supporting current Go version minus 2. Today, that means Go 1.20.

unsafe.SliceData was introduced in 1.20. It couldn't be used when the reflect.SliceHeader code was originally written, but it can now.

Does TinyGo support unsafe.SliceData? If so, that's the correct change. I don't mind doing that change myself if it helps you. But maybe this doesn't apply to all cases where reflect.SliceHeader is being used.

[ncruces]

OK… so, I'm sorry.

I see that in most places were using the deprecated reflect.SliceHeader to skirt around a go vet warning: possible misuse of unsafe.Pointer.

So we need to pick our poison. If we do stick with reflect.SliceHeader, we should remove the TODOs for updating to unsafe.Slice.

I'm still pretty sure this is skirting around the rules, not fixing anything, otherwise this would be a fine way to avoid the "you're reviving a uintptr which might break the GC and you really shouldn't" problems we were having.

Anyway, this is a longer, hairier, discussion which shouldn't block this PR.

[orsinium]

@ncruces applied code suggestions ✅

[evacchi]

since reflect_{go,tinygo}.go are new I think we can rename reflect_go.go to reflect.go so that it is clearer that's the default

[orsinium]

Applied suggestion ✅

[evacchi]

merging this, but I am not sure why tinygo is reaching this sub-package at all. It would be worth investigating if it's possible to drop the entire surface of the compiler.