f16768d868
# Objective A common pattern in Rust is the [newtype](https://doc.rust-lang.org/rust-by-example/generics/new_types.html). This is an especially useful pattern in Bevy as it allows us to give common/foreign types different semantics (such as allowing it to implement `Component` or `FromWorld`) or to simply treat them as a "new type" (clever). For example, it allows us to wrap a common `Vec<String>` and do things like: ```rust #[derive(Component)] struct Items(Vec<String>); fn give_sword(query: Query<&mut Items>) { query.single_mut().0.push(String::from("Flaming Poisoning Raging Sword of Doom")); } ``` > We could then define another struct that wraps `Vec<String>` without anything clashing in the query. However, one of the worst parts of this pattern is the ugly `.0` we have to write in order to access the type we actually care about. This is why people often implement `Deref` and `DerefMut` in order to get around this. Since it's such a common pattern, especially for Bevy, it makes sense to add a derive macro to automatically add those implementations. ## Solution Added a derive macro for `Deref` and another for `DerefMut` (both exported into the prelude). This works on all structs (including tuple structs) as long as they only contain a single field: ```rust #[derive(Deref)] struct Foo(String); #[derive(Deref, DerefMut)] struct Bar { name: String, } ``` This allows us to then remove that pesky `.0`: ```rust #[derive(Component, Deref, DerefMut)] struct Items(Vec<String>); fn give_sword(query: Query<&mut Items>) { query.single_mut().push(String::from("Flaming Poisoning Raging Sword of Doom")); } ``` ### Alternatives There are other alternatives to this such as by using the [`derive_more`](https://crates.io/crates/derive_more) crate. However, it doesn't seem like we need an entire crate just yet since we only need `Deref` and `DerefMut` (for now). ### Considerations One thing to consider is that the Rust std library recommends _not_ using `Deref` and `DerefMut` for things like this: "`Deref` should only be implemented for smart pointers to avoid confusion" ([reference](https://doc.rust-lang.org/std/ops/trait.Deref.html)). Personally, I believe it makes sense to use it in the way described above, but others may disagree. ### Additional Context Discord: https://discord.com/channels/691052431525675048/692572690833473578/956648422163746827 (controversiality discussed [here](https://discord.com/channels/691052431525675048/692572690833473578/956711911481835630)) --- ## Changelog - Add `Deref` derive macro (exported to prelude) - Add `DerefMut` derive macro (exported to prelude) - Updated most newtypes in examples to use one or both derives Co-authored-by: MrGVSV <49806985+MrGVSV@users.noreply.github.com>
70 lines
2.3 KiB
Rust
70 lines
2.3 KiB
Rust
use proc_macro::{Span, TokenStream};
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use quote::quote;
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use syn::{parse_macro_input, Data, DeriveInput, Index, Member, Type};
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pub fn derive_deref(input: TokenStream) -> TokenStream {
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let ast = parse_macro_input!(input as DeriveInput);
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let ident = &ast.ident;
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let (field_member, field_type) = match get_inner_field(&ast, false) {
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Ok(items) => items,
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Err(err) => {
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return err.into_compile_error().into();
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}
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};
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let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
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TokenStream::from(quote! {
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impl #impl_generics ::std::ops::Deref for #ident #ty_generics #where_clause {
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type Target = #field_type;
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fn deref(&self) -> &Self::Target {
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&self.#field_member
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}
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}
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})
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}
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pub fn derive_deref_mut(input: TokenStream) -> TokenStream {
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let ast = parse_macro_input!(input as DeriveInput);
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let ident = &ast.ident;
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let (field_member, _) = match get_inner_field(&ast, true) {
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Ok(items) => items,
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Err(err) => {
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return err.into_compile_error().into();
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}
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};
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let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
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TokenStream::from(quote! {
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impl #impl_generics ::std::ops::DerefMut for #ident #ty_generics #where_clause {
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fn deref_mut(&mut self) -> &mut Self::Target {
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&mut self.#field_member
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}
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}
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})
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}
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fn get_inner_field(ast: &DeriveInput, is_mut: bool) -> syn::Result<(Member, &Type)> {
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match &ast.data {
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Data::Struct(data_struct) if data_struct.fields.len() == 1 => {
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let field = data_struct.fields.iter().next().unwrap();
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let member = field
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.ident
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.as_ref()
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.map(|name| Member::Named(name.clone()))
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.unwrap_or_else(|| Member::Unnamed(Index::from(0)));
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Ok((member, &field.ty))
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}
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_ => {
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let msg = if is_mut {
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"DerefMut can only be derived for structs with a single field"
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} else {
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"Deref can only be derived for structs with a single field"
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};
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Err(syn::Error::new(Span::call_site().into(), msg))
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}
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}
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}
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