ResidualAst mutates input *Ast

[tmthrgd]

Describe the bug
ResidualAst mutates the input *Ast which causes subsequent evaluations to fail or return incorrect results.

To Reproduce
Check which components this affects:

• parser
• checker
• interpreter

Sample expression and input that reproduces the issue:

Test setup:

func Test_ResidualAst_Modified(t *testing.T) {
	e, _ := cel.NewEnv(
		cel.Declarations(
			decls.NewVar("x", decls.NewMapType(decls.String, decls.Int)),
			decls.NewVar("y", decls.Int),
		),
	)
	ast, _ := e.Parse("x == y")
	prg, _ := e.Program(ast,
		cel.EvalOptions(cel.OptTrackState, cel.OptPartialEval),
	)
	for _, x := range []int{123, 456} {
		vars, _ := cel.PartialVars(map[string]interface{}{
			"x": x,
		}, cel.AttributePattern("y"))
		out, det, err := prg.Eval(vars)
		if !types.IsUnknown(out) {
			t.Fatalf("got %v, expected unknown", out)
		}
		if err != nil {
			t.Fatal(err)
		}
		residual, err := e.ResidualAst(ast, det)
		if err != nil {
			t.Fatal(err)
		}
		orig, err := cel.AstToString(ast)
		if err != nil {
			t.Fatal(err)
		}
		if orig != "x == y" {
			t.Errorf("parsed ast: got expr: %s, wanted x == y", orig)
		}
		expr, err := cel.AstToString(residual)
		if err != nil {
			t.Fatal(err)
		}
		want := fmt.Sprintf("%d == y", x)
		if expr != want {
			t.Errorf("residual ast: got expr: %s, wanted %s", expr, want)
		}
	}
}

=== RUN   Test_ResidualAst_Modified
    filter_test.go:83: parsed ast: got expr: 123 == y, wanted x == y
    filter_test.go:83: parsed ast: got expr: 123 == y, wanted x == y
    filter_test.go:91: residual ast: got expr: 123 == y, wanted 456 == y
--- FAIL: Test_ResidualAst_Modified (0.00s)

Expected behavior
ResidualAst should not be mutating it's input *Ast and should instead return a copy.

Additional context
Either ResidualAst or interpreter.PruneAst should be making a copy of the expr that can then be safely mutated.

[tmthrgd]

Actually the problem is in prune. All the lines like:

newExpr.GetExprKind().(*exprpb.Expr_SelectExpr).SelectExpr = ...

are mutating a pointer rather than allocating a new struct. These need to be changed to something like:

newExpr.ExprKind = &exprpb.Expr_SelectExpr{...}