How is suffering failed death saves from damage at 0 hit points affected by save reroll features like Indomitable or Diamond Soul?

Question

Relevant snippets:

Damage at 0 Hit Points.

If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead.

Indomitable

Beginning at 9th level, you can reroll a saving throw that you fail. If you do so, you must use the new roll, and you can't use this feature again until you finish a long rest.

Diamond Soul

Beginning at 14th level, your mastery of ki grants you proficiency in all saving throws.

Additionally, whenever you make a saving throw and fail, you can spend 1 ki point to reroll it and take the second result.

Can you reroll the automatic fail death saves with these (and/or others)? I kinda think Indomitable works, but I'm less sure that "suffering a death save failure" counts as "making" the save for Diamond Soul.

Answer 1

No dice were rolled, so these features do not do anything

Indomitable explicitly says you can reroll a saving throw, thus, you must have rolled a saving throw, which does not happen when taking damage while at 0 HP.

Meanwhile Diamond Soul requires you to make a saving throw and the rules on "Saving Throws" state:

[...] To make a saving throw, roll a d20 and add the appropriate ability modifier. [...]

You did not roll a d20, so you did not make a saving throw and Diamond Soul cannot be applied.

Answer 2

Suffering a death saving throw failure is not the same as failing a death saving throw

I believe that, as the rules are worded, there are Death Saving Throws and the failures (or successes) that arise from those throws.

The rule for damage at 0 hit points says that the character in that situation suffers a failure. It does not say, for example, that the character immediately makes a throw with disadvantage or some other wording that indicates a throw is to be performed.

Rather, they accrue the failures without rolling a saving throw. Because no roll is made, abilities that manipulate the roll of a saving throw are not applicable.