Legendary Songwriter Diane Warren -- Snoop Dogg Got Me SOOOO HIGHHHH

Legendary songwriter Diane Warren -- who wrote pretty much every hit song you've ever heard -- tells TMZ, it took her 56 years to reach the highest point of her career ... when Snoop Dogg got her blitzed out of her mind on some serious sticky icky.

Diane -- whose hits include Toni Braxton's "Unbreak My Heart" and Aerosmith's "I Don't Want to Miss a Thing" -- recently wrote Snoop's new song "The Good Good" ... and when they recorded it at the studio, Diane says they got high as kites together.

The best part of the video is when Diane's friends tell her not to talk about the weed session on camera ... and she blows 'em off saying -- "I don't care, I'm not running for office. I don't give a f**k."

It's pretty hilarious ... Diane's got a great sense of humor ... watch the clip.