bevy/crates/bevy_macro_utils/src/label.rs

use proc_macro::{TokenStream, TokenTree};
use quote::{quote, quote_spanned};
use rustc_hash::FxHashSet;
use syn::{spanned::Spanned, Ident};

use crate::BevyManifest;

/// Finds an identifier that will not conflict with the specified set of tokens.
/// If the identifier is present in `haystack`, extra characters will be added
/// to it until it no longer conflicts with anything.
///
/// Note that the returned identifier can still conflict in niche cases,
/// such as if an identifier in `haystack` is hidden behind an un-expanded macro.
pub fn ensure_no_collision(value: Ident, haystack: TokenStream) -> Ident {
    // Collect all the identifiers in `haystack` into a set.
    let idents = {
        // List of token streams that will be visited in future loop iterations.
        let mut unvisited = vec![haystack];
        // Identifiers we have found while searching tokens.
        let mut found = FxHashSet::default();
        while let Some(tokens) = unvisited.pop() {
            for t in tokens {
                match t {
                    // Collect any identifiers we encounter.
                    TokenTree::Ident(ident) => {
                        found.insert(ident.to_string());
                    }
                    // Queue up nested token streams to be visited in a future loop iteration.
                    TokenTree::Group(g) => unvisited.push(g.stream()),
                    TokenTree::Punct(_) | TokenTree::Literal(_) => {}
                }
            }
        }

        found
    };

    let span = value.span();

    // If there's a collision, add more characters to the identifier
    // until it doesn't collide with anything anymore.
    let mut value = value.to_string();
    while idents.contains(&value) {
        value.push('X');
    }

    Ident::new(&value, span)
}

/// Derive a label trait
///
/// # Args
///
/// - `input`: The [`syn::DeriveInput`] for struct that is deriving the label trait
/// - `trait_path`: The path [`syn::Path`] to the label trait
pub fn derive_boxed_label(input: syn::DeriveInput, trait_path: &syn::Path) -> TokenStream {
    let bevy_utils_path = BevyManifest::default().get_path("bevy_utils");

    let ident = input.ident;
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
    let mut where_clause = where_clause.cloned().unwrap_or_else(|| syn::WhereClause {
        where_token: Default::default(),
        predicates: Default::default(),
    });
    where_clause.predicates.push(
        syn::parse2(quote! {
            Self: 'static + Send + Sync + Clone + Eq + ::std::fmt::Debug + ::std::hash::Hash
        })
        .unwrap(),
    );

    (quote! {
        impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
            fn dyn_clone(&self) -> std::boxed::Box<dyn #trait_path> {
                std::boxed::Box::new(std::clone::Clone::clone(self))
            }

            fn as_dyn_eq(&self) -> &dyn #bevy_utils_path::label::DynEq {
                self
            }

            fn dyn_hash(&self, mut state: &mut dyn ::std::hash::Hasher) {
                let ty_id = #trait_path::inner_type_id(self);
                ::std::hash::Hash::hash(&ty_id, &mut state);
                ::std::hash::Hash::hash(self, &mut state);
            }
        }

        impl #impl_generics ::std::convert::AsRef<dyn #trait_path> for #ident #ty_generics #where_clause {
            fn as_ref(&self) -> &dyn #trait_path {
                self
            }
        }
    })
    .into()
}

/// Derive a label trait
///
/// # Args
///
/// - `input`: The [`syn::DeriveInput`] for struct that is deriving the label trait
/// - `trait_path`: The path [`syn::Path`] to the label trait
pub fn derive_label(
    input: syn::DeriveInput,
    trait_path: &syn::Path,
    attr_name: &str,
) -> TokenStream {
    // return true if the variant specified is an `ignore_fields` attribute
    fn is_ignore(attr: &syn::Attribute, attr_name: &str) -> bool {
        if attr.path().get_ident().as_ref().unwrap() != &attr_name {
            return false;
        }

        syn::custom_keyword!(ignore_fields);
        attr.parse_args_with(|input: syn::parse::ParseStream| {
            let ignore = input.parse::<Option<ignore_fields>>()?.is_some();
            Ok(ignore)
        })
        .unwrap()
    }

    let ident = input.ident.clone();

    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
    let mut where_clause = where_clause.cloned().unwrap_or_else(|| syn::WhereClause {
        where_token: Default::default(),
        predicates: Default::default(),
    });
    where_clause
        .predicates
        .push(syn::parse2(quote! { Self: 'static }).unwrap());

    let as_str = match input.data {
        syn::Data::Struct(d) => {
            // see if the user tried to ignore fields incorrectly
            if let Some(attr) = d
                .fields
                .iter()
                .flat_map(|f| &f.attrs)
                .find(|a| is_ignore(a, attr_name))
            {
                let err_msg = format!("`#[{attr_name}(ignore_fields)]` cannot be applied to fields individually: add it to the struct declaration");
                return quote_spanned! {
                    attr.span() => compile_error!(#err_msg);
                }
                .into();
            }
            // Structs must either be fieldless, or explicitly ignore the fields.
            let ignore_fields = input.attrs.iter().any(|a| is_ignore(a, attr_name));
            if matches!(d.fields, syn::Fields::Unit) || ignore_fields {
                let lit = ident.to_string();
                quote! { #lit }
            } else {
                let err_msg = format!("Labels cannot contain data, unless explicitly ignored with `#[{attr_name}(ignore_fields)]`");
                return quote_spanned! {
                    d.fields.span() => compile_error!(#err_msg);
                }
                .into();
            }
        }
        syn::Data::Enum(d) => {
            // check if the user put #[label(ignore_fields)] in the wrong place
            if let Some(attr) = input.attrs.iter().find(|a| is_ignore(a, attr_name)) {
                let err_msg = format!("`#[{attr_name}(ignore_fields)]` can only be applied to enum variants or struct declarations");
                return quote_spanned! {
                    attr.span() => compile_error!(#err_msg);
                }
                .into();
            }
            let arms = d.variants.iter().map(|v| {
                // Variants must either be fieldless, or explicitly ignore the fields.
                let ignore_fields = v.attrs.iter().any(|a| is_ignore(a, attr_name));
                if matches!(v.fields, syn::Fields::Unit) | ignore_fields {
                    let mut path = syn::Path::from(ident.clone());
                    path.segments.push(v.ident.clone().into());
                    let lit = format!("{ident}::{}", v.ident.clone());
                    quote! { #path { .. } => #lit }
                } else {
                    let err_msg = format!("Label variants cannot contain data, unless explicitly ignored with `#[{attr_name}(ignore_fields)]`");
                    quote_spanned! {
                        v.fields.span() => _ => { compile_error!(#err_msg); }
                    }
                }
            });
            quote! {
                match self {
                    #(#arms),*
                }
            }
        }
        syn::Data::Union(_) => {
            return quote_spanned! {
                input.span() => compile_error!("Unions cannot be used as labels.");
            }
            .into();
        }
    };

    (quote! {
        impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
            fn as_str(&self) -> &'static str {
                #as_str
            }
        }
    })
    .into()
}