proc-macro-workshop | write Rust procedural macros [ Rust Latam conference | Wrapper library

Here is a Playground repro: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=6fb228ef5b3f3a100ba410d7c3e9714c

So, indeed what you mention is true: the expanded code could be copy-pasted and it would work. When this happens (having behavior from macro expansion and "manual copy-pasted expansion" differ), there are two possibilities:

• macro_rules! metavariables

  When emitting code using macro_rules! special captures, some of these captures are wrapped with special invisible parenthesis that already tell the parser how the thing inside should be parsed, which make it illegal to use in other places (for instance, one may capture a $Trait:ty, and then doing impl $Trait for ... will fail (it will parse $Trait as a type, thus leading to it being interpreted as a trait object (old syntax)); see also https://github.com/danielhenrymantilla/rust-defile for other examples.

  This is not your case, but it's good to keep in mind (e.g. my initial hunch was that when doing $tt::$rhs if $tt was a :path-like capture, then that could fail).

• macro hygiene/transparency and Spans

  Consider, for instance: