Unstable book uses box_syntax

[Noratrieb]

rust/src/doc/unstable-book/src/language-features/lang-items.md

Line 19 in 4cb92cc

#![feature(lang_items, box_syntax, start, libc, core_intrinsics, rustc_private)]

There is no such thing as box_syntax anymore :D

[Noratrieb]

plugin.md does the same.

[fee1-dead]

It probably makes sense to search across the repository (including submodules) any references to this removed feature and remove them

@rustbot claim