Macros-by-example and their limitation to consuming only one token tree

[kpreid]

@zackw

Assuming use bitflags::bitflags has appeared earlier in the file, the parsing of these examples would be almost entirely up to the definition of bitflags::bitflags, just as it is now for procedural macros.

This is not correct. Procedural macros can be invoked in three forms, as per the Rust Reference:

Procedural macros allow creating syntax extensions as execution of a function. Procedural macros come in one of three flavors:

• Function-like macros - custom!(...)
• Derive macros - #[derive(CustomDerive)]
• Attribute macros - #[CustomAttribute]

None of these three forms allow arbitrary multi-token-tree parsing.

• Function-like macros receive the contents of one set of brackets — just like macro_rules! macros.
• Derive and attribute macros receive one item definition, which must still match the Rust grammar for items.

It would certainly be useful for macro_rules! to be usable in derive and attribute position (see Declarative `macro_rules!` attribute macros by joshtriplett · Pull Request #3697 · rust-lang/rfcs · GitHub, Declarative `macro_rules!` derive macros by joshtriplett · Pull Request #3698 · rust-lang/rfcs · GitHub, macro_rules_attribute) but that’s not the same as parsing arbitrary syntax outside of brackets.