You know how you can do //go:build windows but then also if runtime.GOOS == "windows"? What about the runtime.GOOS equivalent for //go:build wasm.signext?

if runtime.GOARCH == "wasm" && ??? {
  fmt.Println("You're using WASM with signext")
} else if runtime.GOARCH == "wasm" {
  fmt.Println("You're using WASM but without signext")
} else {
  fmt.Println("You're not using WASM at all")
}

Or any other GO${GOARCH} arch-specific Go settings like GOARM=7 or GO386=sse2 or GOAMD64=v1 or whatever.

You'd think that you can't and that the only way to get that information would be to stash some //go:build information into a const like this:

//go:build wasm.signext
package main
const TheMessage = "You're using WASM with signext"

//go:build wasm && !wasm.signext
package main
const TheMessage = "You're using WASM without signext"

//go:build !wasm
packag main
const TheMessage = "You're not using WASM at all"

...but that's tedious. There's a more runtime-y way to get that information: runtime/debug.ReadBuildInfo()

bi, _ := debug.ReadBuildInfo()
var gowasm string
for _, bs := range bi.Settings {
  if bs.Key == "GOWASM" {
    gowasm = bs.Value
    break
  }
}

if runtime.GOARCH == "GOWASM" && slices.Contains(strings.Split(gowasm, ","), "signext") {
  fmt.Println("You're using WASM with signext")
} else if runtime.GOARCH == "wasm" {
  fmt.Println("You're using WASM but without signext")
} else {
  fmt.Println("You're not using WASM at all")
}

The key is the BuildSetting structs that are attached to the BuildInfo module info struct that we get from ReadBuildInfo(). https://pkg.go.dev/runtime/debug#BuildSetting