update nightly book

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mark 2018-06-26 23:59:01 -05:00
parent 8eb4941e30
commit 63c2d06a0d

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# `macro_at_most_once_rep`
The tracking issue for this feature is: TODO(mark-i-m)
NOTE: This feature is only available in the 2018 Edition.
The tracking issue for this feature is: #48075
With this feature gate enabled, one can use `?` as a Kleene operator meaning "0
or 1 repetitions" in a macro definition. Previously only `+` and `*` were allowed.
For example:
```rust
```rust,ignore
#![feature(macro_at_most_once_rep)]
macro_rules! foo {