Related to This Question, and meant to provide a basis upon which that question can be answered.
Keep in mind that this is a Rules as Written question:
Questions about the rules as written are those which:
- Are investigating, as a priority, literal interpretations of the rules, even if they lead to absurd situations...
So, although this situation is absurd, it is a valid question by the site's parameters.
Trolls have the Regeneration trait:
Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
This means that, if reduced to 0 HP, the troll automatically regenerates 10 HP at the start of its turn, only dying if Regeneration is overridden, via fire or acid damage.
The disintegrate spell description says:
A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6 + 40 force damage. If this damage reduces the target to 0 hit points, it is disintegrated.
A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell.
So, if we take both instances Rules as Written, this would mean that a Troll dropped to 0 HP would be disintegrated but not dead, because it
dies only if it starts its turn with 0 HP and doesn't regenerate
the target takes 10d6 + 40 force damage
The creature can be restored to life only by means of a true resurrection or a wish spell.
and because disintegrate deals force damage, not fire or acid damage, and thus the Troll is technically still able to regenerate, and thus doesn't die.
So what happens?
Of course, this breaks down into certain key points which must be considered in making a solid answer:
- Does the ash regenerate?
- Can it take actions?
- Does it get a turn at all?
- If any or all of the prior answers are no, isn't the troll basically dead?
- What does this even look like in-game?
- And how does this interact with the text, "...can be restored to life only by means of a true resurrection or wish spell", since the troll never technically died?