Tracking issue for proc_macro conversion

[jebrosen]

Motivation

rocket_codegen currently uses Rust's plugin system, which is slated for future removal and is a leading cause of breakage on nightly Rust. Rocket should move to implementing its code generation through proc_macro and related features for better stability guarantees.

Related issues

• Stop using the quote_ macros #678: Stop using the quote_ macros. This conversion process supersedes that issue.
• Tracking issue for RFC 1566: Procedural macros rust-lang/rust#38356: proc_macro tracking issue
• Merge proc_macro_ expansion feature gates as proc_macro_hygiene rust-lang/rust#52121: merge several proc_macro features as proc_macro_hygiene

Questions/Concerns

• Forward compatibility, including everything to do with possible changes to feature gates, hygiene, and editions.
• Hygiene

Progress

Attribute macros:

• route and its variants (get, put, post, delete, head, patch, options)
• catch

Custom derives:

• derive(FromForm)
• derive(FromFormValue)
• derive(Responder)
• derive(UriDisplay)

Bang macros:

• routes!: Reimplement routes! and catchers! as proc_macros #680
• catchers!: PR: Reimplement routes! and catchers! as proc_macros #680
• uri!
• rocket_internal_uri! (some remaining work on the diagnostics)

Upstream:

• Compiler loses location information before calling macros (sometimes) rust-lang/rust#43081

Required rustc Features

These unstable features will need to be used by Rocket's proc_macro crate.

• proc_macro_diagnostic, proc_macro_span. Allows Rocket to provide useful error messages from macros.

These unstable features will need to be used by application crates that use Rocket's macros.

• proc_macro_hygiene. For macros that expand to non-items or macro definitions, including routes!, catchers!, and uri!, and #[route].
• decl_macro, for #[route].

Testing

• All existing tests for attribute syntax, etc.
• Verify that macros work via all applicable import mechanisms: macro_use (#[macro_use] extern crate rocket;), use_extern_macros (use rocket::macro), and proc_macro_path_invoc (#[derive(rocket::macro)], #[rocket::macro], or rocket::macro!()), and any others missing from this list. Ensure these cases are enough to cover compatibility with consumer crates that target the 2018 edition as well.

[SergioBenitez]

Note: rust-lang/rust#43081 (comment) is still an issue, and will surely lead to confusion. Also, Rocket is still using private APIs to work around not having an implementation for rust-lang/rust#54373 (comment).