Rollup of 8 pull requests#70024

Merged

bors merged 18 commits intorust-lang:masterfrom

Centril:rollup-cppmaxr

Mar 15, 2020

Commits on Mar 3, 2020

Implement From<&mut str> for String
lopopolo
committed

Commits on Mar 4, 2020

Add unborrow to reset RefCell borrow state
197g
committed

Commits on Mar 11, 2020

Commits on Mar 12, 2020

ast: `Mac`/`Macro` -> `MacCall`
petrochenkov
committed

Commits on Mar 13, 2020

rustc_metadata: Remove `rmeta::MacroDef`
petrochenkov
committed

Commits on Mar 14, 2020

Return feature gate as a `Symbol`
ecstatic-morse
committed

Commits on Mar 15, 2020