Halsey has congratulated her close friend Machine Gun Kelly on his ''reinvention''.

The 'Without Me' hitmaker teamed up on the track 'Forget Me Too' on the rapper's punk rock LP, 'Tickets To My Downfall', which is out now, and she's posted a touching tribute to the musician - real name is Colson Baker - who she hailed ''punk as f***''.

Halsey also admitted it's ''crazy'' that they took so long to appear on a track together because they've known each other ''since the dawn of time''.

Alongside a series of snaps of the pair, Halsey wrote on Instagram: ''Tickets to My Downfall featuring the song ''Forget Me Too'' is out now! The day has finally arrived that @machinegunkelly and I have a song together. I know it may seem impossible that we haven't already done a song together, because we have been friends since the dawn of time. Believe me. It's crazy even to us that it took so long to do it. Just seems like it's a meant to be type of thing. And now it is. Here. And f****** SICK. I'm so proud of your reinvention Colson. You've always been a rockstar in everything you've done whether it's acting, modeling, being an amazing father, or exploring other genres. But now you have a whole entire album full of songs that reinforce what we already know. YOU'RE PUNK AS F***, KID!!!!!!! Congrats! (sic)''

The album is produced by and features the drumming of Blink-182's Travis Barker.

Meanwhile, MGK recently admitted he wants his music to be unpredictable.

The 'Rap Devil' star moved away from his hip hop roots for his fifth album, but he insisted rather than changing style he's simply broadening his horizons.

He said: ''I just want to go against what everyone else thinks.

''I make a pop-punk record ... and everyone's like, 'You switched genres!'

''To me, I just added on to the multiple things I've already done anyway, but, sure, what a narrow-minded way to look at things.''