rust-lang/rust (original) (raw)

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Tracking Issue for proc_macro_expand #90765

Description

Feature gate: #![feature(proc_macro_expand)]

This is a tracking issue for the TokenStream::expand_expr function for use in proc-macros. This function allows eagerly expanding TokenStreams containing expressions from within a proc-macro, such that they can be modified, giving proc-macros similar functionality to built-in macros like format_args!, concat! and include!.

Public API

impl TokenStream { pub fn expand_expr(&self) -> Result<TokenStream, ExpandError>; }

#[non_exhaustive] pub struct ExpandError;

impl Debug for ExpandError { ... } impl Display for ExpandError { ... } impl Error for ExpandError {} impl !Send for ExpandError {} impl !Sync for ExpandError {}

Steps / History

• Implementation for literal expressions: proc_macro: Add an expand_expr method to TokenStream #87264
• Generalization for other expression types: TODO
• (optional) Generalization to other AST types: TODO
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• What types of expressions beyond literals should be supported?
• Should expanding non-expression macros (e.g. types, items, etc.) be supported?
• Should certain macros (e.g. those with #[allow_internal_unstable]) be expandable or left unexpanded?
• How can we evolve which macros are expanded or not within the rustc stability model? Can downstream crates opt-in to additional macro expansion on nightly channels? Should an additional argument be provided to customize what should be expanded?
• Should compilation be allowed to proceed after an expand method errors?
• Are there concerns about stabilizing the span information generated by internal macros like include!?
• How can we properly preserve span information when expanding expression macros?