PromQL: Evaluation changes structure of the AST

[slrtbtfs]

As discussed here #6612 (comment) the PromQL engine currently modifies the structure of the AST being evaluated.

These AST mutations could be prevented with reasonable effort.

Is this something that should be fixed?

@brian-brazil @juliusv

[brian-brazil]

The structure of the AST shouldn't be changing. I know we also mutate some stuff in passing that is not the structure.

[juliusv]

@brian-brazil Doesn't this mutate the structure?

prometheus/promql/engine.go

Lines 1062 to 1063 in 0c8e9ef

	// Replacing SubqueryExpr with MatrixSelector.
	e.Args[i] = ev.evalSubquery(subq)

[brian-brazil]

Yeah, that's changing the structure.

[bboreham]

Discussed at bug-scrub: general agreement that the AST should not be modified during the evaluation phase.
Perhaps an explicit "optimize" function would want to modify the AST, but that should happen prior to evaluation.