bevy/crates/bevy_reflect/bevy_reflect_derive/src/utility.rs

//! General-purpose utility functions for internal usage within this crate.

use crate::field_attributes::ReflectIgnoreBehavior;
use bevy_macro_utils::BevyManifest;
use bit_set::BitSet;
use proc_macro2::{Ident, Span};
use quote::quote;
use syn::{Member, Path, Type, WhereClause};

/// Returns the correct path for `bevy_reflect`.
pub(crate) fn get_bevy_reflect_path() -> Path {
    BevyManifest::get_path_direct("bevy_reflect")
}

/// Returns the "reflected" ident for a given string.
///
/// # Example
///
/// ```ignore
/// let reflected: Ident = get_reflect_ident("Hash");
/// assert_eq!("ReflectHash", reflected.to_string());
/// ```
pub(crate) fn get_reflect_ident(name: &str) -> Ident {
    let reflected = format!("Reflect{name}");
    Ident::new(&reflected, Span::call_site())
}

/// Helper struct used to process an iterator of `Result<Vec<T>, syn::Error>`,
/// combining errors into one along the way.
pub(crate) struct ResultSifter<T> {
    items: Vec<T>,
    errors: Option<syn::Error>,
}

/// Returns a `Member` made of `ident` or `index` if `ident` is None.
///
/// Rust struct syntax allows for `Struct { foo: "string" }` with explicitly
/// named fields. It allows the `Struct { 0: "string" }` syntax when the struct
/// is declared as a tuple struct.
///
/// ```
/// # fn main() {
/// struct Foo { field: &'static str }
/// struct Bar(&'static str);
/// let Foo { field } = Foo { field: "hi" };
/// let Bar { 0: field } = Bar { 0: "hello" };
/// let Bar(field) = Bar("hello"); // more common syntax
/// # }
/// ```
///
/// This function helps field access in context where you are declaring either
/// a tuple struct or a struct with named fields. If you don't have a field name,
/// it means you need to access the struct through an index.
pub(crate) fn ident_or_index(ident: Option<&Ident>, index: usize) -> Member {
    ident.map_or_else(
        || Member::Unnamed(index.into()),
        |ident| Member::Named(ident.clone()),
    )
}

/// Options defining how to extend the `where` clause in reflection with any additional bounds needed.
pub(crate) struct WhereClauseOptions {
    /// Any types that will be reflected and need an extra trait bound
    pub(crate) active_types: Box<[Type]>,
    /// Trait bounds to add to the active types
    pub(crate) active_trait_bounds: proc_macro2::TokenStream,
    /// Any types that won't be reflected and need an extra trait bound
    pub(crate) ignored_types: Box<[Type]>,
    /// Trait bounds to add to the ignored types
    pub(crate) ignored_trait_bounds: proc_macro2::TokenStream,
}

impl Default for WhereClauseOptions {
    /// By default, don't add any additional bounds to the `where` clause
    fn default() -> Self {
        Self {
            active_types: Box::new([]),
            ignored_types: Box::new([]),
            active_trait_bounds: quote! {},
            ignored_trait_bounds: quote! {},
        }
    }
}

/// Extends the `where` clause in reflection with any additional bounds needed.
///
/// This is mostly used to add additional bounds to reflected objects with generic types.
/// For reflection purposes, we usually have:
/// * `active_trait_bounds: Reflect`
/// * `ignored_trait_bounds: Any + Send + Sync`
///
/// # Arguments
///
/// * `where_clause`: existing `where` clause present on the object to be derived
/// * `where_clause_options`: additional parameters defining which trait bounds to add to the `where` clause
///
/// # Example
///
/// The struct:
/// ```ignore
/// #[derive(Reflect)]
/// struct Foo<T, U> {
///     a: T,
///     #[reflect(ignore)]
///     b: U
/// }
/// ```
/// will have active types: `[T]` and ignored types: `[U]`
///
/// The `extend_where_clause` function will yield the following `where` clause:
/// ```ignore
/// where
///     T: Reflect,  // active_trait_bounds
///     U: Any + Send + Sync,  // ignored_trait_bounds
/// ```
pub(crate) fn extend_where_clause(
    where_clause: Option<&WhereClause>,
    where_clause_options: &WhereClauseOptions,
) -> proc_macro2::TokenStream {
    let active_types = &where_clause_options.active_types;
    let ignored_types = &where_clause_options.ignored_types;
    let active_trait_bounds = &where_clause_options.active_trait_bounds;
    let ignored_trait_bounds = &where_clause_options.ignored_trait_bounds;

    let mut generic_where_clause = if let Some(where_clause) = where_clause {
        let predicates = where_clause.predicates.iter();
        quote! {where #(#predicates,)*}
    } else if !(active_types.is_empty() && ignored_types.is_empty()) {
        quote! {where}
    } else {
        quote! {}
    };
    generic_where_clause.extend(quote! {
        #(#active_types: #active_trait_bounds,)*
        #(#ignored_types: #ignored_trait_bounds,)*
    });
    generic_where_clause
}

impl<T> Default for ResultSifter<T> {
    fn default() -> Self {
        Self {
            items: Vec::new(),
            errors: None,
        }
    }
}

impl<T> ResultSifter<T> {
    /// Sift the given result, combining errors if necessary.
    pub fn sift(&mut self, result: Result<T, syn::Error>) {
        match result {
            Ok(data) => self.items.push(data),
            Err(err) => {
                if let Some(ref mut errors) = self.errors {
                    errors.combine(err);
                } else {
                    self.errors = Some(err);
                }
            }
        }
    }

    /// Associated method that provides a convenient implementation for [`Iterator::fold`].
    pub fn fold(mut sifter: Self, result: Result<T, syn::Error>) -> Self {
        sifter.sift(result);
        sifter
    }

    /// Complete the sifting process and return the final result.
    pub fn finish(self) -> Result<Vec<T>, syn::Error> {
        if let Some(errors) = self.errors {
            Err(errors)
        } else {
            Ok(self.items)
        }
    }
}

/// Converts an iterator over ignore behavior of members to a bitset of ignored members.
///
/// Takes into account the fact that always ignored (non-reflected) members are skipped.
///
/// # Example
/// ```rust,ignore
/// pub struct HelloWorld {
///     reflected_field: u32      // index: 0
///
///     #[reflect(ignore)]
///     non_reflected_field: u32  // index: N/A (not 1!)
///
///     #[reflect(skip_serializing)]
///     non_serialized_field: u32 // index: 1
/// }
/// ```
/// Would convert to the `0b01` bitset (i.e second field is NOT serialized)
///
pub(crate) fn members_to_serialization_denylist<T>(member_iter: T) -> BitSet<u32>
where
    T: Iterator<Item = ReflectIgnoreBehavior>,
{
    let mut bitset = BitSet::default();

    member_iter.fold(0, |next_idx, member| match member {
        ReflectIgnoreBehavior::IgnoreAlways => next_idx,
        ReflectIgnoreBehavior::IgnoreSerialization => {
            bitset.insert(next_idx);
            next_idx + 1
        }
        ReflectIgnoreBehavior::None => next_idx + 1,
    });

    bitset
}