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[derive] Overhaul repr parsing #1752

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Sep 30, 2024
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[derive] Overhaul repr parsing #1752

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64 changes: 18 additions & 46 deletions zerocopy-derive/src/enum.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,42 +6,11 @@
// This file may not be copied, modified, or distributed except according to
// those terms.

use ::proc_macro2::TokenStream;
use ::quote::quote;
use ::syn::{DataEnum, Fields, Generics, Ident};
use syn::parse_quote;
use proc_macro2::{Span, TokenStream};
use quote::quote;
use syn::{parse_quote, DataEnum, Error, Fields, Generics, Ident};

use crate::{derive_try_from_bytes_inner, EnumRepr, Trait};

/// Returns the repr for the tag enum, given the collection of reprs on the
/// enum.
///
/// This function returns:
/// - `Some(C)` for `repr(C)`
/// - `Some(int)` for `repr(int)` and `repr(C, int)`
/// - `None` for all other reprs
pub(crate) fn tag_repr(reprs: &[EnumRepr]) -> Option<&EnumRepr> {
let mut result = None;
for repr in reprs {
match repr {
EnumRepr::C => result = Some(repr),
EnumRepr::U8
| EnumRepr::U16
| EnumRepr::U32
| EnumRepr::U64
| EnumRepr::Usize
| EnumRepr::I8
| EnumRepr::I16
| EnumRepr::I32
| EnumRepr::I64
| EnumRepr::Isize => {
return Some(repr);
}
_ => (),
}
}
result
}
use crate::{derive_try_from_bytes_inner, repr::EnumRepr, Trait};

/// Generates a tag enum for the given enum. This generates an enum with the
/// same `repr`s, variants, and corresponding discriminants, but none of the
Expand All @@ -57,7 +26,7 @@ pub(crate) fn generate_tag_enum(repr: &EnumRepr, data: &DataEnum) -> TokenStream
});

quote! {
#[repr(#repr)]
#repr
#[allow(dead_code)]
enum ___ZerocopyTag {
#(#variants,)*
Expand Down Expand Up @@ -157,7 +126,8 @@ fn generate_variant_structs(

// We do this rather than emitting `#[derive(::zerocopy::TryFromBytes)]`
// because that is not hygienic, and this is also more performant.
let try_from_bytes_impl = derive_try_from_bytes_inner(&variant_struct);
let try_from_bytes_impl = derive_try_from_bytes_inner(&variant_struct)
.expect("derive_try_from_bytes_inner should not fail on synthesized type");

Some(quote! {
#variant_struct
Expand Down Expand Up @@ -231,21 +201,23 @@ fn generate_variants_union(generics: &Generics, data: &DataEnum) -> TokenStream
/// - `repr(C, int)`: <https://doc.rust-lang.org/reference/type-layout.html#combining-primitive-representations-of-enums-with-fields-and-reprc>
pub(crate) fn derive_is_bit_valid(
enum_ident: &Ident,
reprs: &[EnumRepr],
repr: &EnumRepr,
generics: &Generics,
data: &DataEnum,
) -> TokenStream {
let repr =
tag_repr(reprs).expect("cannot derive is_bit_valid for enum without a well-defined repr");

) -> Result<TokenStream, Error> {
let trait_path = Trait::TryFromBytes.crate_path();
let tag_enum = generate_tag_enum(repr, data);
let tag_consts = generate_tag_consts(data);

let (outer_tag_type, inner_tag_type) = if matches!(repr, EnumRepr::C) {
let (outer_tag_type, inner_tag_type) = if repr.is_c() {
(quote! { ___ZerocopyTag }, quote! { () })
} else {
} else if repr.is_primitive() {
(quote! { () }, quote! { ___ZerocopyTag })
} else {
return Err(Error::new(
Span::call_site(),
"must have #[repr(C)] or #[repr(Int)] attribute in order to guarantee this type's memory layout",
));
};

let variant_structs = generate_variant_structs(enum_ident, generics, data);
Expand Down Expand Up @@ -297,7 +269,7 @@ pub(crate) fn derive_is_bit_valid(
}
});

quote! {
Ok(quote! {
// SAFETY: We use `is_bit_valid` to validate that the bit pattern of the
// enum's tag corresponds to one of the enum's discriminants. Then, we
// check the bit validity of each field of the corresponding variant.
Expand Down Expand Up @@ -392,5 +364,5 @@ pub(crate) fn derive_is_bit_valid(
_ => false,
}
}
}
})
}
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