Are damage dice from features such as Sneak Attack considered part of "the attack's damage" for the purposes of features such as the Piercer feat?

Question

This question stems from looking at the rules on Critical Hits:

When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together [...]

[...] If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.

Are Sneak Attack's dice part of "the attack's damage"?. An example where this matters is the Piercer feat, which states (emphasis mine):

[...] Once per turn, when you hit a creature with an attack that deals piercing damage, you can reroll one of the attack's damage dice, and you must use the new roll. [...]

So a specific example of my question could be this: Does the Piercer feat allow you to reroll a Sneak Attack damage die?

Answer 1

The feature is called Sneak Attack.

I think that asking for a clear answer with precise argumentation using the rules of the game is bringing to bear an unrealistic expectation on both the reader of the rules, and the rules themselves.

How do we know the Sneak Attack dice are part of the attack's damage dice or something else? Obviously, the rules aren't terribly concerned with spelling this out for us, since we don't have an applicable general rule, and features generally make no habit of saying "this is part of the attack's damage" or "this is not a part of the attack's damage". So we use other means to figure it out.

In this case, let's just read the name of the feature:

Sneak Attack

It's got the word "attack" right there in the feature name. My attack did more damage because I exploited my target's vulnerability:

you know how to strike subtly and exploit a foe’s distraction.

It is not as though my dagger did its usual damage, but my sneakiness brought about some additional trauma that is separate from my dagger's stabbiness. No. My attack did more damage because I stabbed the dude in the neck when he wasn't paying attention, rather than having to fight fair against an alert opponent. One attack, one damage roll, it's all damage dice from the attack.

And if I get a feat that makes me even better at stabbing, you bet it is going to make me better at stabbing people while I'm being sneaky.

Answer 2

All damage that flows directly from a successful attack is "the attack's damage"

This includes weapon damage, magical damage (like the cold damage of a frost brand), class abilities like sneak attack or divine smite, spells like Hex or Hunter's Mark and so on. Basically, if the damage can be done simply by succeeding on the attack roll, it's "the attack's damage". Even the poison damage from a Giant Spider's bite is part of "the attack's damage" because even though a successful saving throw reduces it, the damage comes directly from the attack.