runtime, cmd/compile: implicit allocations allowed in the runtime

[randall77]

We have a test in the compiler that ensures that we don't have any implicit allocations in the runtime. We want all allocations that the runtime does to be explicit (to avoid things like self-deadlock when allocating inside malloc).
That test doesn't catch all the allocations. In particular, it doesn't catch the allocation required to assign non-pointer types to interfaces. There may be others, I haven't checked.

I hacked a simple test into the compiler, and found a bunch of situations where this implicit allocation happens. An example:

func goPanicIndex(x int, y int) {
	panicCheck1(getcallerpc(), "index out of range")
	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
}

This code needs to allocate a boundsError and put its address in the data field of an interface.
All current missed interface-conversion allocations are of this form (an argument to panic).

I ran into this issue in a CL I'm working on where implicit allocations were happening in a bad place, and it took me a while to track it down. It would have been much easier for the compiler to give me an error.

Update #42076

@josharian

[randall77]

p.s. The panicCheck1 call ensures we can malloc (and throws instead if we can't).