Donald Trump’s big idea to slap a 100% tariff on films made outside the U.S. might sound like a patriot’s dream — but according to one Hollywood heavyweight, it’s actually a 100% career-ender for the movie biz.

TMZ caught up with “Green Book” producer Charles Wessler, and let’s just say he didn’t sugarcoat his thoughts on Trump’s proposal to force filming back onto American soil … telling us it’s the stupidest idea he’s ever heard … but said it tracks, coming from Trump.

Charles breaks down how the movie biz can’t survive the move — budgets are part of it, but it’s also about creative freedom and authenticity … and also the practicality of certain locations that can’t be recreated in the U.S.

He points out a slew of major hits that couldn’t have happened if productions were locked into pricey American cities … and even points out Mel Gibson — one of Trump’s key celeb supporters — has himself headed overseas to shoot some of his films.

As for the international money flow, Charles tells us countries would just turn the tables, slapping tariffs right back on the U.S. and strangling global film trade.