What are the specific happens-before guarantees for channel operations in Go's memory model?

Go's memory model specifies precise happens-before rules for channels. Knowing these is necessary for writing correct concurrent code that works across CPU architectures without data races.

// Rule 1: send completes before receive returns
var data string
ch := make(chan struct{})
go func() {
    data = "shared"  // write
    ch <- struct{}{} // send: completes before <-ch returns
}()
<-ch               // receive: data write is guaranteed visible
fmt.Println(data)  // "shared" — safe

// Rule 2: close happens-before zero-value receive
var ready bool
done := make(chan struct{})
go func() { ready = true; close(done) }()
<-done
fmt.Println(ready) // guaranteed: true

// Rule 3: unbuffered — receive happens-before send completes
// (sender cannot proceed until receiver has the value)

// Rule 4: buffered channel as semaphore
// cap=1 channel: 1st receive happens-before 2nd send completes
limit := make(chan struct{}, 1)
var shared int
go func() {
    limit <- struct{}{} // 1st send
    shared = 42
    <-limit            // 1st receive: happens-before 2nd send
}()
go func() {
    limit <- struct{}{} // 2nd send — cannot complete until 1st recv done
    fmt.Println(shared) // guaranteed to see 42
    <-limit
}()

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