No you can't.  Your "turn" doesn't end until your next turn begins. 

No, there's a distinction between "turn" and "round". Over the course of a round, each participant in initiative gets one turn. Sneak attack specifies turn, so you can use it once for every participant per round - if you can. Most abilities specify the start/end of a specific creature's turn, or have a time-based duration which is measured in rounds.

I used this spell a lot while playing Baldur's Gate, but it might have been effective because of how the game was coded, I want to use this this way -

Cast Melfs minute meteors, use bonus action for cantrip

next turn

Cast Haste on my Wizard - use the extra attack from haste to throw 2 meteors, use bonus action to cast a cantrip

next turn

Cast damaging spell - throw 2 meteors using my second action, use cantrip for bonus action

The Idea being that each turn you could cast a damaging spell, a cantrip and throw meteors so that your damage keeps stacking up, the higher level you are the more meteors you will have and the longer you can continue doing this,

Has anyone tried this? 

Horizon walker ranger also gets haste

if I’m concentrating it on another pc, and said pc dies, can I move it to a new target, like hex? Or does the spell just end?

Offhand attacks are bonus actions. What it means is that you cannot benefit from multiattack or extra attack features

I really wanted to cast two spells in one turn. monks are making spellcasters jealous. 

I don't really understand the point of this spell if you miss your next turn. Having some sort of drawback maybe, but that seems really excessive.

Quote from zeranguen >>

if I’m concentrating it on another pc, and said pc dies, can I move it to a new target, like hex? Or does the spell just end?

 No...sadly the spell would just end.

In most cases you can't transfer a spell... its the exceptions like Hunter's Mark or Hex that specifically allow you to transfer it to another creature.

Quote from Ferdomnach >>

I used this spell a lot while playing Baldur's Gate, but it might have been effective because of how the game was coded, I want to use this this way -

Cast Melfs minute meteors, use bonus action for cantrip

next turn

Cast Haste on my Wizard - use the extra attack from haste to throw 2 meteors, use bonus action to cast a cantrip

next turn

Cast damaging spell - throw 2 meteors using my second action, use cantrip for bonus action

The Idea being that each turn you could cast a damaging spell, a cantrip and throw meteors so that your damage keeps stacking up, the higher level you are the more meteors you will have and the longer you can continue doing this,

 

Has anyone tried this? 

 Unfortunately, both Melf's Minute Meteors and Haste are concentration spells. On top of that, Melf's requires a bonus action, so I think you might have confused the actions for meteors and your cantrip. And a hasted action only includes a weapon attack action, not a full action. Magic Items and spells can't be used as your hasted action.

Now, while Melf's is concentration, it's a bonus action to throw meteors. So there's nothing stopping you from tossing out a fireball and two meteors on the same turn. 

If you're, say, a melee-competent spellcaster with a friend, you could have them cast Haste on you, then cast the meteors and sling those around while you beat a monster senseless.

Ready action as main action, attack As haste action. You attack twice still, just 2 separate turns.

No actually, it’s anytime you take the attack action on your first turn of combat. For example, if you take 5 levels in ranger and 2 levels in fighter, your fighter action surge lets you take the “attack action” a second time, and with bonus action hunters mark that’s 6 attacks on your first turn of combat only. If you use a longbow/rapier/longsword that’s 8d8+6d6+dex/str, or 6d10+2d8+6d6+str if you two hand the longsword. That’s 6attacks on the first turn of combat, 2 of which do an additional 1d8, and every attack does +1d6. If you combine it with a rune knight fighter you can get an additional 2d6 on one attack on the first turn of combat. I have this exact build right now and I put damage out paladins clean.

No idea how to delete a comment so just gonna edit it dont mind me no one sees this

Quote from IamBrakX1 >>

"Posted Aug 28, 2020

Forgive me if I am wrong but Echo Knight + Action Surge + Haste lets you make twelve attacks for upwards of 24d6+60 (plus 36 if you have + 3 weapons) damage in around no? 140 damage in a round for up to two rounds for an average of about 300 damage in two rounds?"

No...I think the math is off. When you take the attack action, you get x weapon attacks with Extra Attack.  At lvl 20, you would get 4. Action Surge for 4 more, but haste would only give you 1 more weapon attack. Echo says "Whenever you take the Attack action, you can make one additional melee attack from the echo’s position."
At this point it is Attack action (4) + Echo (1) + Action Surge Attack (4) + Echo (1) + Haste (1) (because its still the same turn)... Unless you take Pole arm master to add in another 2 bonus action attacks with the Action Surge for a total of 13.

So (1d10+4 (granted Str is 18) and +3 (magic weapon))x11 + (1d4+7)x2 = 11d10+2d4+99 or average of 159 in 1 round

"That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action."

The haste attack is still considered an Attack Action even though it does only 1 attack per the clause so this should be allowed to trigger for the Unleashed Incarnation. So that's an extra 1d10+4(granted Str is 18). This also naturally assumes you have minimum of 16 con as the amount of times you can trigger Unleashed Incarnation per long rest is equal to your Con modifier.

Okay so if I playing sorcerer and cast haste on my self. I can twin fire bolt for my 1st action but I can’t twin and cast fire bolt with my second action? But a fighter can attack with his sword both actions?

haste specifically says "That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action."

sooo, yeah. a fighter would get an extra weapon attack (only 1, no extra attack) but a caster would not get another cantrip. although this could always be houseruled ofc. This is probably because of the variety of cantrips available compared to the attack action only ever being an attack.

Wait, can't you cast this on an enemy then end concentration immediately to stun it for a round? Not my idea btw

Hear me out... What happens if you haste a Quickling? For example, summoning two quicklings with the Conjure Woodland beings, and asking your wizard friend to cast haste on one of them to make them scout a certain place we need to enter, and if you have a rogue who can create a telephatic link in between you, the quicklings and himself... is this a viable strategy? or too much for a scout?

This spell says it can only target a willing creature, so you'd have to trick and/or charm spell it into thinking you're friendly to make it lose a turn by casting and dropping it. Not very efficient for something the command spell could replicate. As for this not being great to use on casters because you cant cast spells with it, this isn't a damage amping spell, it's utility. A squishy wizard should love being able to cast a spell, and dash 120 feet, or move 60 with a disengage, hide, or pop a potion, all with bonus to armor and dex saves, also doubling any + to movement like wood elf, longstride, or mobile. Looking at this as a "more attacking" spell is very reductive from what it's actually good for. Even allowing it to cast a cantrip could be iffy, a 3d10 or 4d10 firebolt is one thing, but that'd open the door for twincasting or eldritch blasting. 

You can Hide but not Dodge... with Haste...  The heck?