When can a Barbarian, who has just levelled up, use the additional Rage gained from that level?

Question

At certain levels, Barbarians gain additional Rages to use between Long Rests. My question is, when can these additional Rages be used? The PHB seems a little unclear on this.

PHB pg.48

Once you have raged the number of times shown for your barbarian level in the Rages column of the Barbarian table, you must finish a long rest before you can rage again.

This description doesn't really seem to account for levelling up between long rests. With that, there seem to be two options:

1. The Barbarian can use their additional Rage immediately after levelling up. In this option, the additional Rage comes in as an unused Rage. This option uses the idea that if a level 11 Barbarian has raged 4 times, he is out of rages. If after he finishes his 4th Rage, he levels up and becomes a level 12 Barbarian, he would then gain a 5th Rage that is unused. Being as he has only raged 4 times since his last Long Rest, it would stand that he would have 1 more Rage.

2. The Barbarian must wait until after his next Long Rest to use his additional Rage. In this option, when a Barbarian gains a level and an additional Rage, it is accounted as apart of the Barbarian's total Rages, not available Rages. This option is similar to a players Hit Points when they level, as in the player gains Hit Points to their Hit-Point Maximum, but not their Hit Point Total.

PHB pg.15

Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum.

Levelling mechanics at my table:

The Rogue gets their additional sneak dice right away. The same goes for the Monk and their additional Ki points. Casters, while they get their additional spell slots immediately on level up, they must wait until a rest (short or long depending on the class) and then prepare spells for any new level of spells they have gained (ie: they have 1 level 5 spell slot, but no prepared level 5 spells).

Answer 1

A Barbarian, who has just levelled up may immediately use the additional Rage that they gained from that level.

In general, you level up when you reach the total combined experience amount in the table for Character Advancement for that level (PHB 15). Once you gain a level, you immediately gain the benefits.

Beyond first level (PHB 15):

A character who reaches a specified experience point total advances in capability. This advancement is called gaining a level. When your character gains a level, his or her class often grants additional features, as detailed in the class description.

Normally experience points are awarded after an encounter is solved, many DMs handle house rule this differently, however.

Experience Points (DMG 260-261):

Experience points (XP) fuel level advancement for player characters and are most often the reward for completing combat encounters.

Many tables use the variant rule: Training to Gain Levels (DMG 131)

you can require characters to spend downtime training or studying before they gain the benefits of a new level. If you choose this option, once a character has earned enough experience points to attain a new level, he or she must train for a number of days before gaining any class features associated with the new level. The training time required depends on the level to be gained, as shown on the Training to Gain Levels table. The training cost is for the total training time.

So this depends on how the table handles the moment when the level-up happens.

At your table none-caster characters immediately gain the benefits (and casters who aren't clerics, druids, paladins or wizards are nerfed, but you are a Barbarian - not a caster, so this doesn't matter for you, but you maybe want to talk to your DM about how spells are prepared and which classes have to prepare their spells).

Rage (PHB 48):

Once you have raged the number of times shown for your barbarian level in the Rages column of the Barbarian table, you must finish a long rest before you can rage again.

So when you gain a level, this text immediately benefits you. You gain a new maximum amount of rages that is active until you take a long rest.

So you gain one more rage for your current day (or other time until the next rest depending on optional rules such as Gritty Realism) unless your next gained level is 20, then you get an unlimited amount of maximum rage uses.

The deciding factor is:

Once you have raged the number of times shown

The number of times that you can rage depends on the maximum number and increases if the maximum number increases.