Rollup merge of #25710 - Manishearth:no_decorator_clone, r=sfackler

fixes #25683

I have a very nonscientific measurement of the data via valgrind/massif [here](https://gist.github.com/Manishearth/4c47f15f6835cb3957c4)

I measured the memory usage for both --pretty=expanded and -Z no-trans

It *seems* like there's a 20-25MB decrease during expansion on stage2 librustc; but I'm not quite sure.

r? @eddyb

(have not yet run tests, but it compiles fine, might want to wait before giving r+)

cc @nrc @huon