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// Copyright (C) 2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#![cfg(feature = "max-encoded-len")]
use crate::{
trait_bounds,
utils::{codec_crate_path, custom_mel_trait_bound, has_dumb_trait_bound, should_skip},
};
use quote::{quote, quote_spanned};
use syn::{parse_quote, spanned::Spanned, Data, DeriveInput, Fields, Type};
/// impl for `#[derive(MaxEncodedLen)]`
pub fn derive_max_encoded_len(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
let mut input: DeriveInput = match syn::parse(input) {
Ok(input) => input,
Err(e) => return e.to_compile_error().into(),
};
let crate_path = match codec_crate_path(&input.attrs) {
Ok(crate_path) => crate_path,
Err(error) => return error.into_compile_error().into(),
};
let name = &input.ident;
if let Err(e) = trait_bounds::add(
&input.ident,
&mut input.generics,
&input.data,
custom_mel_trait_bound(&input.attrs),
parse_quote!(#crate_path::MaxEncodedLen),
None,
has_dumb_trait_bound(&input.attrs),
&crate_path
) {
return e.to_compile_error().into()
}
let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
let data_expr = data_length_expr(&input.data);
quote::quote!(
const _: () = {
impl #impl_generics #crate_path::MaxEncodedLen for #name #ty_generics #where_clause {
fn max_encoded_len() -> ::core::primitive::usize {
#data_expr
}
}
};
)
.into()
}
/// generate an expression to sum up the max encoded length from several fields
fn fields_length_expr(fields: &Fields) -> proc_macro2::TokenStream {
let type_iter: Box<dyn Iterator<Item = &Type>> = match fields {
Fields::Named(ref fields) => Box::new(
fields.named.iter().filter_map(|field| if should_skip(&field.attrs) {
None
} else {
Some(&field.ty)
})
),
Fields::Unnamed(ref fields) => Box::new(
fields.unnamed.iter().filter_map(|field| if should_skip(&field.attrs) {
None
} else {
Some(&field.ty)
})
),
Fields::Unit => Box::new(std::iter::empty()),
};
// expands to an expression like
//
// 0
// .saturating_add(<type of first field>::max_encoded_len())
// .saturating_add(<type of second field>::max_encoded_len())
//
// We match the span of each field to the span of the corresponding
// `max_encoded_len` call. This way, if one field's type doesn't implement
// `MaxEncodedLen`, the compiler's error message will underline which field
// caused the issue.
let expansion = type_iter.map(|ty| {
quote_spanned! {
ty.span() => .saturating_add(<#ty>::max_encoded_len())
}
});
quote! {
0_usize #( #expansion )*
}
}
// generate an expression to sum up the max encoded length of each field
fn data_length_expr(data: &Data) -> proc_macro2::TokenStream {
match *data {
Data::Struct(ref data) => fields_length_expr(&data.fields),
Data::Enum(ref data) => {
// We need an expression expanded for each variant like
//
// 0
// .max(<variant expression>)
// .max(<variant expression>)
// .saturating_add(1)
//
// The 1 derives from the discriminant; see
// https://github.com/paritytech/parity-scale-codec/
// blob/f0341dabb01aa9ff0548558abb6dcc5c31c669a1/derive/src/encode.rs#L211-L216
//
// Each variant expression's sum is computed the way an equivalent struct's would be.
let expansion = data.variants.iter().map(|variant| {
let variant_expression = fields_length_expr(&variant.fields);
quote! {
.max(#variant_expression)
}
});
quote! {
0_usize #( #expansion )* .saturating_add(1)
}
},
Data::Union(ref data) => {
// https://github.com/paritytech/parity-scale-codec/
// blob/f0341dabb01aa9ff0548558abb6dcc5c31c669a1/derive/src/encode.rs#L290-L293
syn::Error::new(data.union_token.span(), "Union types are not supported.")
.to_compile_error()
},
}
}