zerofrom_derive/
lib.rs

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1// This file is part of ICU4X. For terms of use, please see the file
2// called LICENSE at the top level of the ICU4X source tree
3// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
4
5//! Custom derives for `ZeroFrom` from the `zerofrom` crate.
6
7// https://github.com/unicode-org/icu4x/blob/main/documents/process/boilerplate.md#library-annotations
8#![cfg_attr(
9    not(test),
10    deny(
11        clippy::indexing_slicing,
12        clippy::unwrap_used,
13        clippy::expect_used,
14        clippy::panic,
15        clippy::exhaustive_structs,
16        clippy::exhaustive_enums,
17        clippy::trivially_copy_pass_by_ref,
18        missing_debug_implementations,
19    )
20)]
21
22use core::mem;
23use proc_macro::TokenStream;
24use proc_macro2::{Span, TokenStream as TokenStream2};
25use quote::quote;
26use std::collections::{HashMap, HashSet};
27use syn::fold::{self, Fold};
28use syn::punctuated::Punctuated;
29use syn::spanned::Spanned;
30use syn::{
31    parse_macro_input, parse_quote, DeriveInput, Ident, Lifetime, MetaList, Token,
32    TraitBoundModifier, Type, TypeParamBound, TypePath, WherePredicate,
33};
34use synstructure::Structure;
35mod visitor;
36
37/// Custom derive for `zerofrom::ZeroFrom`,
38///
39/// This implements `ZeroFrom<Ty> for Ty` for types
40/// without a lifetime parameter, and `ZeroFrom<Ty<'data>> for Ty<'static>`
41/// for types with a lifetime parameter.
42///
43/// Apply the `#[zerofrom(clone)]` attribute to a field if it doesn't implement
44/// Copy or ZeroFrom; this data will be cloned when the struct is zero_from'ed.
45///
46/// Apply the `#[zerofrom(maybe_borrow(T, U, V))]` attribute to the struct to indicate
47/// that certain type parameters may themselves contain borrows (by default
48/// the derives assume that type parameters perform no borrows and can be copied or cloned).
49///
50/// In rust versions where [this issue](https://github.com/rust-lang/rust/issues/114393) is fixed,
51/// `#[zerofrom(may_borrow)]` can be applied directly to type parameters.
52#[proc_macro_derive(ZeroFrom, attributes(zerofrom))]
53pub fn zf_derive(input: TokenStream) -> TokenStream {
54    let input = parse_macro_input!(input as DeriveInput);
55    TokenStream::from(zf_derive_impl(&input))
56}
57
58fn has_attr(attrs: &[syn::Attribute], name: &str) -> bool {
59    attrs.iter().any(|a| {
60        if let Ok(i) = a.parse_args::<Ident>() {
61            if i == name {
62                return true;
63            }
64        }
65        false
66    })
67}
68
69// Collects all idents from #[zerofrom(may_borrow(A, B, C, D))]
70// needed since #[zerofrom(may_borrow)] doesn't work yet
71// (https://github.com/rust-lang/rust/issues/114393)
72fn get_may_borrow_attr(attrs: &[syn::Attribute]) -> Result<HashSet<Ident>, Span> {
73    let mut params = HashSet::new();
74    for attr in attrs {
75        if let Ok(list) = attr.parse_args::<MetaList>() {
76            if list.path.is_ident("may_borrow") {
77                if let Ok(list) =
78                    list.parse_args_with(Punctuated::<Ident, Token![,]>::parse_terminated)
79                {
80                    params.extend(list)
81                } else {
82                    return Err(attr.span());
83                }
84            }
85        }
86    }
87    Ok(params)
88}
89
90fn zf_derive_impl(input: &DeriveInput) -> TokenStream2 {
91    let mut tybounds = input
92        .generics
93        .type_params()
94        .map(|ty| {
95            // Strip out param defaults, we don't need them in the impl
96            let mut ty = ty.clone();
97            ty.eq_token = None;
98            ty.default = None;
99            ty
100        })
101        .collect::<Vec<_>>();
102    let typarams = tybounds
103        .iter()
104        .map(|ty| ty.ident.clone())
105        .collect::<Vec<_>>();
106    let lts = input.generics.lifetimes().count();
107    let name = &input.ident;
108    let structure = Structure::new(input);
109
110    let may_borrow_attrs = match get_may_borrow_attr(&input.attrs) {
111        Ok(mb) => mb,
112        Err(span) => {
113            return syn::Error::new(
114            span,
115            "#[zerofrom(may_borrow)] on the struct takes in a comma separated list of type parameters, like so: `#[zerofrom(may_borrow(A, B, C, D)]`",
116        ).to_compile_error();
117        }
118    };
119
120    // This contains every generic type introduced in this code.
121    // If the gneeric type is may_borrow, this additionally contains the identifier corresponding to
122    // a newly introduced mirror type parameter that we are borrowing from, similar to C in the original trait.
123    // For convenience, we are calling these "C types"
124    let generics_env: HashMap<Ident, Option<Ident>> = tybounds
125        .iter_mut()
126        .map(|param| {
127            // First one doesn't work yet https://github.com/rust-lang/rust/issues/114393
128            let maybe_new_param = if has_attr(&param.attrs, "may_borrow")
129                || may_borrow_attrs.contains(&param.ident)
130            {
131                // Remove `?Sized`` bound because we need a param to be Sized in order to take a ZeroFrom of it.
132                // This only applies to fields marked as `may_borrow`.
133                let mut bounds = core::mem::take(&mut param.bounds);
134                while let Some(bound_pair) = bounds.pop() {
135                    let bound = bound_pair.into_value();
136                    if let TypeParamBound::Trait(ref trait_bound) = bound {
137                        if trait_bound.path.get_ident().map(|ident| ident == "Sized") == Some(true)
138                            && matches!(trait_bound.modifier, TraitBoundModifier::Maybe(_))
139                        {
140                            continue;
141                        }
142                    }
143                    param.bounds.push(bound);
144                }
145                Some(Ident::new(
146                    &format!("{}ZFParamC", param.ident),
147                    param.ident.span(),
148                ))
149            } else {
150                None
151            };
152            (param.ident.clone(), maybe_new_param)
153        })
154        .collect();
155
156    // Do any of the generics potentially borrow?
157    let generics_may_borrow = generics_env.values().any(|x| x.is_some());
158
159    if lts == 0 && !generics_may_borrow {
160        let has_clone = structure
161            .variants()
162            .iter()
163            .flat_map(|variant| variant.bindings().iter())
164            .any(|binding| has_attr(&binding.ast().attrs, "clone"));
165        let (clone, clone_trait) = if has_clone {
166            (quote!(this.clone()), quote!(Clone))
167        } else {
168            (quote!(*this), quote!(Copy))
169        };
170        let bounds: Vec<WherePredicate> = typarams
171            .iter()
172            .map(|ty| parse_quote!(#ty: #clone_trait + 'static))
173            .collect();
174        quote! {
175            impl<'zf, #(#tybounds),*> zerofrom::ZeroFrom<'zf, #name<#(#typarams),*>> for #name<#(#typarams),*> where #(#bounds),* {
176                fn zero_from(this: &'zf Self) -> Self {
177                    #clone
178                }
179            }
180        }
181    } else {
182        if lts > 1 {
183            return syn::Error::new(
184                input.generics.span(),
185                "derive(ZeroFrom) cannot have multiple lifetime parameters",
186            )
187            .to_compile_error();
188        }
189
190        let mut zf_bounds: Vec<WherePredicate> = vec![];
191        let body = structure.each_variant(|vi| {
192            vi.construct(|f, i| {
193                let binding = format!("__binding_{i}");
194                let field = Ident::new(&binding, Span::call_site());
195
196                if has_attr(&f.attrs, "clone") {
197                    quote! {
198                        #field.clone()
199                    }
200                } else {
201                    // the field type
202                    let fty = replace_lifetime(&f.ty, custom_lt("'zf"));
203                    // the corresponding lifetimey type we are borrowing from (effectively, the C type)
204                    let lifetime_ty =
205                        replace_lifetime_and_type(&f.ty, custom_lt("'zf_inner"), &generics_env);
206
207                    let (has_ty, has_lt) = visitor::check_type_for_parameters(&f.ty, &generics_env);
208                    if has_ty {
209                        // For types without type parameters, the compiler can figure out that the field implements
210                        // ZeroFrom on its own. However, if there are type parameters, there may be complex preconditions
211                        // to `FieldTy: ZeroFrom` that need to be satisfied. We get them to be satisfied by requiring
212                        // `FieldTy<'zf>: ZeroFrom<'zf, FieldTy<'zf_inner>>`
213                        if has_lt {
214                            zf_bounds
215                                .push(parse_quote!(#fty: zerofrom::ZeroFrom<'zf, #lifetime_ty>));
216                        } else {
217                            zf_bounds.push(parse_quote!(#fty: zerofrom::ZeroFrom<'zf, #fty>));
218                        }
219                    }
220                    if has_ty || has_lt {
221                        // By doing this we essentially require ZF to be implemented
222                        // on all fields
223                        quote! {
224                            <#fty as zerofrom::ZeroFrom<'zf, #lifetime_ty>>::zero_from(#field)
225                        }
226                    } else {
227                        // No lifetimes, so we can just copy
228                        quote! { *#field }
229                    }
230                }
231            })
232        });
233        // Due to the possibility of generics_may_borrow, we might reach here with no lifetimes on self,
234        // don't accidentally feed them to self later
235        let (maybe_zf_lifetime, maybe_zf_inner_lifetime) = if lts == 0 {
236            (quote!(), quote!())
237        } else {
238            (quote!('zf,), quote!('zf_inner,))
239        };
240
241        // Array of C types. Only different if generics are allowed to borrow
242        let mut typarams_c = typarams.clone();
243
244        if generics_may_borrow {
245            for typaram_c in &mut typarams_c {
246                if let Some(Some(replacement)) = generics_env.get(typaram_c) {
247                    // we use mem::replace here so we can be really clear about the C vs the T type
248                    let typaram_t = mem::replace(typaram_c, replacement.clone());
249                    zf_bounds
250                        .push(parse_quote!(#typaram_c: zerofrom::ZeroFrom<'zf_inner, #typaram_t>));
251                    tybounds.push(parse_quote!(#typaram_c));
252                }
253            }
254        }
255
256        quote! {
257            impl<'zf, 'zf_inner, #(#tybounds),*> zerofrom::ZeroFrom<'zf, #name<#maybe_zf_inner_lifetime #(#typarams_c),*>> for #name<#maybe_zf_lifetime #(#typarams),*>
258                where
259                #(#zf_bounds,)* {
260                fn zero_from(this: &'zf #name<#maybe_zf_inner_lifetime #(#typarams_c),*>) -> Self {
261                    match *this { #body }
262                }
263            }
264        }
265    }
266}
267
268fn custom_lt(s: &str) -> Lifetime {
269    Lifetime::new(s, Span::call_site())
270}
271
272/// Replace all lifetimes in a type with a specified one
273fn replace_lifetime(x: &Type, lt: Lifetime) -> Type {
274    struct ReplaceLifetime(Lifetime);
275
276    impl Fold for ReplaceLifetime {
277        fn fold_lifetime(&mut self, _: Lifetime) -> Lifetime {
278            self.0.clone()
279        }
280    }
281    ReplaceLifetime(lt).fold_type(x.clone())
282}
283
284/// Replace all lifetimes in a type with a specified one, AND replace all types that have a corresponding C type
285/// with the C type
286fn replace_lifetime_and_type(
287    x: &Type,
288    lt: Lifetime,
289    generics_env: &HashMap<Ident, Option<Ident>>,
290) -> Type {
291    struct ReplaceLifetimeAndTy<'a>(Lifetime, &'a HashMap<Ident, Option<Ident>>);
292
293    impl Fold for ReplaceLifetimeAndTy<'_> {
294        fn fold_lifetime(&mut self, _: Lifetime) -> Lifetime {
295            self.0.clone()
296        }
297        fn fold_type_path(&mut self, i: TypePath) -> TypePath {
298            if i.qself.is_none() {
299                if let Some(ident) = i.path.get_ident() {
300                    if let Some(Some(replacement)) = self.1.get(ident) {
301                        return parse_quote!(#replacement);
302                    }
303                }
304            }
305            fold::fold_type_path(self, i)
306        }
307    }
308    ReplaceLifetimeAndTy(lt, generics_env).fold_type(x.clone())
309}
zerofrom_derive/lib.rs

zerofrom_derive/
lib.rs
