Tafsir of At-Tur 52:19

(Eat and drink in satisfaction): That is, it is said to them, "Eat and drink," a consumption and a drinking that is satisfying, or eat and drink food and drink that is satisfying. The statement implies the omission of a verb [of saying]. "In satisfaction" (hani'an) is in the accusative case as a verbal noun (masdar), because it is an adjective for a verbal noun, or it is in the accusative because it is a direct object. Regardless of the case, both verbs contend for governance over it. That which is hani' (satisfying) is everything that entails no hardship and is not followed by any ill effect.

(For what you used to do): That is, because of it or in exchange for it. The particle ba' in both interpretations relates to "Eat" and "drink" based on their contention for governance.

Al-Zamakhshari permitted the possibility that the ba' is extra/redundant and that what follows it is the subject (fa'il) of "satisfying" (hani'an), as in the saying of Kuthayyir: Satisfying and wholesome, without any lurking ailment, for 'Azza is that which she has made lawful of our honor.

In that verse, there is a subject for "satisfying" based on it being an adjective, which originally functions as a verbal noun whose verb is omitted obligatorily due to frequent usage; as if one said: "May that which she has made lawful of our honor be satisfying to 'Azza."

In this case, just as it is permissible for "what you used to do" here to be the subject due to the extra ba', following the meaning "May what you used to do be satisfying to you," it is also possible to consider the subject as an implicit pronoun referring to the "eating" or "drinking" indicated by their verbs.

However, the objection to this is that the [grammatical] addition in the subject position is not proven to be heard [in standard speech] in the scope of common usage, except in the case of "subject" by way of controversy, nor is it analogical in such an instance. Furthermore, the statement would still require the estimation of an added term—i.e., "the reward of what you used to do"—and in that, there is a type of affectation.