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Add examples to the parsing limitations section of the macro tutorial.

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(Thanks to bstrie for pointing them out!)
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Paul Stansifer 2012-10-17 21:40:39 -04:00
parent fdd7b4d8f4
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doc/tutorial-macros.md
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@ -155,17 +155,22 @@ If it is under more, it'll be repeated, as appropriate.
## Parsing limitations
The macro parser will parse Rust syntax with two limitations:
1. The parser will always parse as much as possible. For example, if the comma
were omitted from the syntax of `early_return!` above, `input_1 [` would've
been interpreted as the beginning of an array index. In fact, invoking the
macro would have been impossible.
For technical reasons, there are two limitations to the treatment of syntax
fragments by the macro parser:
1. The parser will always parse as much as possible of a Rust syntactic
fragment. For example, if the comma were omitted from the syntax of
`early_return!` above, `input_1 [` would've been interpreted as the beginning
of an array index. In fact, invoking the macro would have been impossible.
2. The parser must have eliminated all ambiguity by the time it reaches a
`$name:fragment_specifier` declaration. This limitation can result in parse
errors when declarations occur at the beginning of, or immediately after,
a `$(...)*`. Changing the invocation syntax to require a distinctive
token in front can solve the problem.
a `$(...)*`. For example, the grammar `$($t:ty)* $e:expr` will always fail to
parse because the parser would be forced to choose between parsing `t` and
parsing `e`. Changing the invocation syntax to require a distinctive token in
front can solve the problem. In the above example, `$(T $t:ty)* E $e:exp`
solves the problem.
## A final note