cmd/compile: skip slicebytetostring when argument doesn't escape #6714
Description
In this function,
func hi() string {
b := make([]byte, 0, 100) // 1st alloc
b = append(b, "Hello, "...)
b = append(b, "world.\n"...)
return string(b) // 2nd alloc, but could be removed.
}
The final line currently generates a call to runtime.slicebytetostring, causing a copy
of b to be allocated.
But the compiler already knows that b doesn't escape. It could instead return a string
header re-using the []byte memory.