compiler/rustc_macros/src/extension.rs

use proc_macro2::Ident;
use quote::quote;
use syn::parse::{Parse, ParseStream};
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::{
    Attribute, Generics, ImplItem, Pat, PatIdent, Path, Signature, Token, TraitItem,
    TraitItemConst, TraitItemFn, TraitItemMacro, TraitItemType, Type, Visibility, WhereClause,
    braced, parse_macro_input,
};

pub(crate) fn extension(
    attr: proc_macro::TokenStream,
    input: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let ExtensionAttr { vis, trait_ } = parse_macro_input!(attr as ExtensionAttr);
    let Impl { attrs, generics, self_ty, items, wc } = parse_macro_input!(input as Impl);
    let headers: Vec<_> = items
        .iter()
        .map(|item| match item {
            ImplItem::Fn(f) => TraitItem::Fn(TraitItemFn {
                attrs: scrub_attrs(&f.attrs),
                sig: scrub_header(f.sig.clone()),
                default: None,
                semi_token: Some(Token![;](f.block.span())),
            }),
            ImplItem::Const(ct) => TraitItem::Const(TraitItemConst {
                attrs: scrub_attrs(&ct.attrs),
                const_token: ct.const_token,
                ident: ct.ident.clone(),
                generics: ct.generics.clone(),
                colon_token: ct.colon_token,
                ty: ct.ty.clone(),
                default: None,
                semi_token: ct.semi_token,
            }),
            ImplItem::Type(ty) => TraitItem::Type(TraitItemType {
                attrs: scrub_attrs(&ty.attrs),
                type_token: ty.type_token,
                ident: ty.ident.clone(),
                generics: ty.generics.clone(),
                colon_token: None,
                bounds: Punctuated::new(),
                default: None,
                semi_token: ty.semi_token,
            }),
            ImplItem::Macro(mac) => TraitItem::Macro(TraitItemMacro {
                attrs: scrub_attrs(&mac.attrs),
                mac: mac.mac.clone(),
                semi_token: mac.semi_token,
            }),
            ImplItem::Verbatim(stream) => TraitItem::Verbatim(stream.clone()),
            _ => unimplemented!(),
        })
        .collect();

    quote! {
        #(#attrs)*
        #vis trait #trait_ {
            #(#headers)*
        }

        impl #generics #trait_ for #self_ty #wc {
            #(#items)*
        }
    }
    .into()
}

/// Only keep `#[doc]` attrs.
fn scrub_attrs(attrs: &[Attribute]) -> Vec<Attribute> {
    attrs
        .into_iter()
        .cloned()
        .filter(|attr| {
            let ident = &attr.path().segments[0].ident;
            ident == "doc" || ident == "must_use"
        })
        .collect()
}

/// Scrub arguments so that they're valid for trait signatures.
fn scrub_header(mut sig: Signature) -> Signature {
    for (idx, input) in sig.inputs.iter_mut().enumerate() {
        match input {
            syn::FnArg::Receiver(rcvr) => {
                // `mut self` -> `self`
                if rcvr.reference.is_none() {
                    rcvr.mutability.take();
                }
            }
            syn::FnArg::Typed(arg) => match &mut *arg.pat {
                Pat::Ident(arg) => {
                    // `ref mut ident @ pat` -> `ident`
                    arg.by_ref.take();
                    arg.mutability.take();
                    arg.subpat.take();
                }
                _ => {
                    // `pat` -> `__arg0`
                    arg.pat = Box::new(
                        PatIdent {
                            attrs: vec![],
                            by_ref: None,
                            mutability: None,
                            ident: Ident::new(&format!("__arg{idx}"), arg.pat.span()),
                            subpat: None,
                        }
                        .into(),
                    )
                }
            },
        }
    }
    sig
}

struct ExtensionAttr {
    vis: Visibility,
    trait_: Path,
}

impl Parse for ExtensionAttr {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let vis = input.parse()?;
        let _: Token![trait] = input.parse()?;
        let trait_ = input.parse()?;
        Ok(ExtensionAttr { vis, trait_ })
    }
}

struct Impl {
    attrs: Vec<Attribute>,
    generics: Generics,
    self_ty: Type,
    items: Vec<ImplItem>,
    wc: Option<WhereClause>,
}

impl Parse for Impl {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let attrs = input.call(Attribute::parse_outer)?;
        let _: Token![impl] = input.parse()?;
        let generics = input.parse()?;
        let self_ty = input.parse()?;
        let wc = input.parse()?;

        let content;
        let _brace_token = braced!(content in input);
        let mut items = Vec::new();
        while !content.is_empty() {
            items.push(content.parse()?);
        }

        Ok(Impl { attrs, generics, self_ty, items, wc })
    }
}