Halsey and G-Eazy don't seem to care much about keeping things professional ... they'll make out in the middle of a show if they damn well please.
Halsey was joined by her rapper boyfriend mid-set of her concert Friday night at the Forum in L.A. ... and the 2 busted out a rendition of G-Eazy's new track "No Limit" -- she covered Cardi B's part.
When it was over the crowd gave it up for the special guest appearance ... and went wild for the stage smooch.
The couple reportedly began dating a few months ago, and G-Eazy recently teased an upcoming album by posting a pic with Halsey ... so there might be more collabs to come.