Tracking issue: declarative macros 2.0

[nrc]

Tracking issue for declarative macros 2.0 (aka macro aka decl_macro aka macros-by-example).

RFC: https://rust-lang.github.io/rfcs/1584-macros.html

RFC PR: rust-lang/rfcs#1584

cc @rust-lang/compiler

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Tasks

• Complete the draft hygiene prototype and RFC.
  • Hygiene for items, explicit imports, lexical scopes, and module scopes.
  • Hygiene for globs and trait methods (w.r.t. extension trait candidates in scope for a method call).
  • Hygiene for type directed name resolutions (i.e. methods, associated types, and fields).
  • Support macro in blocks as well as modules.
  • Implement inter-crate hygiene, except for nested macros.
  • Implement "hygiene bending" for when users want a name from macro def to "escape".
    • Examples: New declarative macros, functions and fields not being recognized #91249, Declaring modules inside decl macros 2.0 #46342
  • Implement inter-crate hygiene for nested macros (pending macro def encoding).
  • Make private_in_public hygienic?
  • Make unsafe and lints hygienic (if appropriate)?
• Add variant ast::ItemKind::MacroDef for macro_rules! items and macro items (PR syntax: add ast::ItemKind::MacroDef, simplify hygiene info #40220).
• Encode macros in the crate metadata using TokenStream, not String.
• Fix Inferred types should be checked for privacy violations #30476 (private_in_public details).
• Fix span issues If an error is not affected by a macro, don't print macro notes #30506 and Spans for Paths can be incorrect #39450 (PR macros: improve Span's expansion information #40597).
• Land macro behind a feature gate (PR Initial implementation of declarative macros 2.0 #40847).
• Future-proof matchers (e.g. $e:expr) by employing a simpler, more general grammar.
• Allow fragments (e.g. $e where $e:expr) to be parsed in more contexts (c.f. Macro matching is incorrect for macros invoked in macro expansions #26361).
• Decide whether we want macro invocations in identifier positions and/or eager expansion.
• Remove $:meta matcher (Remove $:meta matcher in decl macros #49629)

Potentially blocking issues:

• Declarative macros: no error about duplicate trait items #71614

[CryZe]

@nrc Typo in Issue Name: "issuse"

[jseyfried]

Tasks

(dtolnay edit: moved the checklist up to the OP)

[nrc]

We need to RFC a whole bunch of stuff here. In particular, I would like to propose some new syntax for declaring macros and we should RFC the changes to matchers.

[tikue]

Can the hygiene RFC mention pattern hygiene? This in particular scares me:

// Unsuspecting user's code
#[allow(non_camel_case_types)]
struct i(i64);

macro_rules! ignorant_macro {
    () => {
        let i = 0;
        println!("{}", i);
    };
}

fn main() {
    // oh no!
    ignorant_macro!();
}