Are warlocks able to abuse short rests to cast long-duration spells without consuming spell slots?

Question

From the description of the Warlock class:

Spell Slots

The Warlock table shows how many Spell Slots you have. The table also shows what the level of those slots is; all of your Spell Slots are the same level. To cast one of your warlock Spells of 1st level or higher, you must expend a spell slot. You regain all expended Spell Slots when you finish a Short or Long Rest.

...and the section on adventuring:

Short Rest

A short rest is a period of downtime, at least 1 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds.

Say a Warlock uses a spell slot to cast Hallucinatory Terrain - a spell that lasts 24 hours and requires no concentration (read: doesn't require strenuous activity). They then take a short rest to recover the used spell slot. Though this doesn't seem correct to me, I see no rules dictating that the spell ends. Are warlocks able to abuse short rests to cast long-duration spells without consuming spell slots?

Answer 1

You're correct.

Your analysis is correct--a warlock can cast some long-duration spells by effectively spending hours instead of spell slots.

This is not unbalanced (for a single-classed warlock).

First, the spell slots that the warlock recovers on a short rest can only go up to level 5. Any long-duration spell above level 5 is subject to Mystic Arcanum, which only allows the warlock to cast it once per long rest (as explained here).

So which spells on the list are both level 5 or less and last longer than an hour? My quick perusal through the list shows only Hallucinatory Terrain, Hex, and Dream. Dream is arguably a special case, and Hex only works on one target at a time because of concentration, so your example of Hallucinatory Terrain is essentially the only spell that can be "exploited" this way. Opinions may vary about whether this "exploit" is especially powerful or not, but given that it the spell is quite limited in scope and it requires uninterrupted rests, I'd hardly consider it to be abuse.

Multiclassed warlocks can benefit this way

As Bloodcinder points out, multiclass warlocks can use their warlock spell slots to cast spells from their other classes (PHB 164):

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature...

Thus, a warlock that has levels in wizard could cast a spell like Mage Armor, take a short rest to regain that spell slot, and then enjoy the benefits of Mage Armor for another 7 hours afterward.

As for whether this is balanced, I'd have to assume that the designers considered this possibility because the rule was written out so explicitly. After all, multiclassed spellcasters are delaying (or sacrificing) their ability to cast higher level spells, so there is some tradeoff to be considered.

Answer 2

No, there are no rules I'm aware of that dictate spells end during a rest. Keep in mind, though, that that's not "abusing" short rests; it's just how Warlocks work. Short rests aren't free - they require an hour of uninterrupted rest. If the party can secure that, there's no reason to deny the spell slot to a class that only gets 2 for most of their career.

Also note that the warlock has to have a spell slot free before the rest in order to cast a spell this way - and, as Icyfire pointed out, there are few Warlock spells that this could even affect. It's unlikely to unbalance your game.

Answer 3

The spell costs a slot when cast, as normal, and the warlock recovers all expended slots upon a short rest, regardless of whether the spell is still active.

To break that down, a spell slot is expended when the spell is cast, regardless of its duration.

When a character casts a spell, he or she expends a slot of that spell’s level or higher, effectively “filling” a slot with the spell.

System Reference Document PDF, p. 100
"Spell Slots" section of the "Spellcasting" chapter

And the warlock regains all expended slots upon completing a short rest.

You regain all expended spell slots when you finish a short or long rest.

System Reference Document PDF, p. 47
"Spell Slots" section of the "Warlock" class description

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Spells that might be overpowered to grant on such a short rest system are not on the warlock spell list and are either:

• available as an at-will Eldritch Invocation with a restriction.
  (e.g. mage armor via Armor of Shadows)
• available as a once-per-day Eldritch Invocation.
  (e.g. polymorph via Sculptor of Flesh)
• not available to warlocks at all.
  (e.g. animate dead)

Any spell of 1st-level through 5th-level on the warlock spell list is fit for the purpose of being cast two or more times per short rest, which should usually translate to six or more times per day.