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