Tracking issue for internal syntax #110680

This is a tracking issue for the internal feature internal syntax.

It's gated behind #![feature(internal_syntax)] and hosts an assortment of syntaxes that conventionally follow the scheme k#IDENT($( $tt )*) where IDENT is the name of the construct and k# (from MCP 945) forces IDENT to be interpreted as a keyword.

This feature once used to be called builtin_syntax and the individual constructs used to be prefixed with builtin # instead of k#. It was originally proposed in MCP 580.

Usually, these internal syntactic constructs are then wrapped by & exposed via a non-internal macro in the library core. The macro is usually a decl macro 2.0 that's annotated with #[diagnostic::opaque] & bears the same name as the internal construct.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the scaffolding for the builtin # syntax
• Replace builtin # with k#
• Migrate all built-in macros that qualify to wrapper macros containing internal syntax
  • asm (ast::ExprKind::InlineAsm)
  • format_args (ast::ExprKind::FormatArgs)
  • gca (ast::{Expr,Type}Kind::GcaMacro (needs renaming))
  • global_asm (ast::ItemKind::GlobalAsm)
  • include_bytes (ast::ExprKind::IncludedBytes)
  • naked_asm (ast::ExprKind::InlineAsm)
  • offset_of (ast::ExprKind::OffsetOf)
  • pattern_type (ast::TyKind::Pat)
  • type_ascribe (ast::ExprKind::Type)
  • view_type (ast::TypeKind::View)
• Maybe migrate certain (unstable) temporary syntactic constructs to internal syntax unless it's considered too churn-y for users of the language
  • do yeet (ast::ExprKind::Yeet)

Reference

For reference, here's a list of all current internal syntactic constructs:

• deref (ast::PatKind::Deref)
• field_of (ast::TyKind::FieldOf)
• offset_of (ast::ExprKind::OffsetOf)
• type_ascribe (ast::ExprKind::Type)
• unwrap_binder (ast::ExprKind::UnsafeBinderCast(Unwrap))
• wrap_binder (ast::ExprKind::UnsafeBinderCast(Wrap))

Unresolved questions

None so far.

Implementation history

• Implement builtin # syntax and use it for offset_of!(...) #110694
• Implement macro-based deref!() syntax for deref patterns #122793
• Make type_ascribe! not a built-in #122806
• Mark builtin_syntax as internal #127622
• Add AST support for unsafe binders #134140
• add field representing types #152730
• Replace builtin # with k# #162232