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| -rw-r--r-- | doc/tutorial-macros.md | 21 |
1 files changed, 13 insertions, 8 deletions
diff --git a/doc/tutorial-macros.md b/doc/tutorial-macros.md index 995944b2fbd..c7e1ada648e 100644 --- a/doc/tutorial-macros.md +++ b/doc/tutorial-macros.md @@ -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. -2. The parser must have eliminated all ambiguity by the time it reaches a +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 |