Rollup of 6 pull requests#67440

Merged

bors merged 13 commits intorust-lang:masterfrom

Mark-Simulacrum:rollup-z59a7ky

Dec 20, 2019

Commits on Dec 12, 2019

Change fmt docs for more delegations
elichai
committed

Commits on Dec 15, 2019

Replace prints in fmt docs with asserts
elichai
committed

Commits on Dec 18, 2019

Add benchmarks for string::insert(_str)
llogiq
committed

Commits on Dec 19, 2019