Use this skill when building procedural macros for Topcoat in Rust. It explains how to preserve parsed syntax in AST nodes, separate parsing from meaning, and keep macro entry points focused on parsing input, expanding valid code, and reporting errors.
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Always use this skill before writing procedural macros for Topcoat
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syn nodes, an AST node is a lossless record of the syntax it matched: every field is pub and holds the tokens as written, spans included. The original source should be reconstructible from the node. Field order does not have to match the source, but spans must be preserved.Parse impl rejects what is not valid syntax and stores everything else verbatim in token fields; it never normalizes, resolves, or drops information on the way in.Parse impls, parse directly into the Self { ... } fields (Self { x: input.parse()? }) rather than through let bindings. Use a let only when a parsed value must be inspected to decide how to parse a later field.mod kw with syn::custom_keyword! invocations instead of parsing syn::Ident. Use input.lookahead1() if it makes sense.ParseOption and parse them with input.call(Node::parse_option)?.The macro/ crate only bridges proc_macro::TokenStream to the grammar crate that holds the AST and the codegen. Every entry point is a parse, a match, and a to_compile_error() on the error arm; no logic lives in lib.rs.
An attribute macro receives two token streams, so it gets three types: one Parse node per stream and a tuple struct pairing them, which is the node the codegen hangs off.
// grammar crate
pub struct ProcedureAttr {}
pub struct ProcedureItem { ... }
pub struct Procedure(ProcedureAttr, ProcedureItem);
impl Procedure {
#[must_use]
pub fn new(attr: ProcedureAttr, item: ProcedureItem) -> Self {
Self(attr, item)
}
pub fn parse(attr: TokenStream, item: TokenStream) -> syn::Result<Self> {
Ok(Self::new(syn::parse2(attr)?, syn::parse2(item)?))
}
}
impl ToTokens for Procedure { ... }
Parse stays per stream, since neither stream alone is the macro input. The pairing type owns the inherent parse that joins them and the ToTokens impl that expands them, which keeps the entry point a one-liner:
// macro crate
#[doc = include_str!("../docs/procedure.md")]
#[proc_macro_attribute]
pub fn procedure(attr: TokenStream, item: TokenStream) -> TokenStream {
match topcoat_runtime_grammar::procedure::Procedure::parse(attr.into(), item.into()) {
Ok(value) => quote! { #value }.into(),
Err(error) => error.to_compile_error().into(),
}
}
A function-like macro has a single input, so it needs no pairing type: syn::parse_macro_input! straight into its AST node, then expand.
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Use this skill to check Topcoat changes locally before committing or opening a pull request. It runs the project’s Rust formatting, lint, test, and documentation commands, plus browser project checks when relevant, so problems can be caught before review.
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Use this skill to write concise commit messages in the Topcoat project’s Conventional Commits format. It helps choose a change type and scope, describe the update clearly, and mark breaking changes so release notes and version bumps can be based on commit history.
中文简介正在准备中。当前显示英语简介。
Use this skill to prepare a Topcoat pull request from the full branch diff. It guides you to write a Conventional Commits title and a concise body covering the change, verification performed, and an AI-use disclaimer when an agent creates the PR.
中文简介正在准备中。当前显示英语简介。
Use this skill to write long-form Markdown guides for Topcoat. It covers where documentation sources belong, how to structure explanations from basic to advanced, and how to keep prose plain, concise, and easier to maintain as implementations change.
中文简介正在准备中。当前显示英语简介。
Apply the project’s Rust conventions when editing code or documentation. The guidance covers module and type organization, workspace dependency declarations, plain English documentation, and tests that check meaningful behavior rather than fixed text.
中文简介正在准备中。当前显示英语简介。