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// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
use proc_macro2::{Span, TokenStream};
use syn::{
parse_quote, Data, DataEnum, DataStruct, DataUnion, Error, Expr, ExprLit, ExprUnary, Lit, UnOp,
WherePredicate,
};
use crate::{
derive::try_from_bytes::derive_try_from_bytes,
repr::{CompoundRepr, EnumRepr, Repr, Spanned},
util::{enum_size_from_repr, Ctx, FieldBounds, ImplBlockBuilder, Trait, TraitBound},
};
/// Returns `Ok(index)` if variant `index` of the enum has a discriminant of
/// zero. If `Err(bool)` is returned, the boolean is true if the enum has
/// unknown discriminants (e.g. discriminants set to const expressions which we
/// can't evaluate in a proc macro). If the enum has unknown discriminants, then
/// it might have a zero variant that we just can't detect.
pub(crate) fn find_zero_variant(enm: &DataEnum) -> Result<usize, bool> {
// Discriminants can be anywhere in the range [i128::MIN, u128::MAX] because
// the discriminant type may be signed or unsigned. Since we only care about
// tracking the discriminant when it's less than or equal to zero, we can
// avoid u128 -> i128 conversions and bounds checking by making the "next
// discriminant" value implicitly negative.
// Technically 64 bits is enough, but 128 is better for future compatibility
// with https://github.com/rust-lang/rust/issues/56071
let mut next_negative_discriminant = Some(0);
// Sometimes we encounter explicit discriminants that we can't know the
// value of (e.g. a constant expression that requires evaluation). These
// could evaluate to zero or a negative number, but we can't assume that
// they do (no false positives allowed!). So we treat them like strictly-
// positive values that can't result in any zero variants, and track whether
// we've encountered any unknown discriminants.
let mut has_unknown_discriminants = false;
for (i, v) in enm.variants.iter().enumerate() {
match v.discriminant.as_ref() {
// Implicit discriminant
None => {
match next_negative_discriminant.as_mut() {
Some(0) => return Ok(i),
// n is nonzero so subtraction is always safe
Some(n) => *n -= 1,
None => (),
}
}
// Explicit positive discriminant
Some((_, Expr::Lit(ExprLit { lit: Lit::Int(int), .. }))) => {
match int.base10_parse::<u128>().ok() {
Some(0) => return Ok(i),
Some(_) => next_negative_discriminant = None,
None => {
// Numbers should never fail to parse, but just in case:
has_unknown_discriminants = true;
next_negative_discriminant = None;
}
}
}
// Explicit negative discriminant
Some((_, Expr::Unary(ExprUnary { op: UnOp::Neg(_), expr, .. }))) => match &**expr {
Expr::Lit(ExprLit { lit: Lit::Int(int), .. }) => {
match int.base10_parse::<u128>().ok() {
Some(0) => return Ok(i),
// x is nonzero so subtraction is always safe
Some(x) => next_negative_discriminant = Some(x - 1),
None => {
// Numbers should never fail to parse, but just in
// case:
has_unknown_discriminants = true;
next_negative_discriminant = None;
}
}
}
// Unknown negative discriminant (e.g. const repr)
_ => {
has_unknown_discriminants = true;
next_negative_discriminant = None;
}
},
// Unknown discriminant (e.g. const expr)
_ => {
has_unknown_discriminants = true;
next_negative_discriminant = None;
}
}
}
Err(has_unknown_discriminants)
}
pub(crate) fn derive_from_zeros(ctx: &Ctx, top_level: Trait) -> Result<TokenStream, Error> {
let try_from_bytes = derive_try_from_bytes(ctx, top_level)?;
let from_zeros = match &ctx.ast.data {
Data::Struct(strct) => derive_from_zeros_struct(ctx, strct),
Data::Enum(enm) => derive_from_zeros_enum(ctx, enm)?,
Data::Union(unn) => derive_from_zeros_union(ctx, unn),
};
Ok(IntoIterator::into_iter([try_from_bytes, from_zeros]).collect())
}
pub(crate) fn derive_from_bytes(ctx: &Ctx, top_level: Trait) -> Result<TokenStream, Error> {
let from_zeros = derive_from_zeros(ctx, top_level)?;
let from_bytes = match &ctx.ast.data {
Data::Struct(strct) => derive_from_bytes_struct(ctx, strct),
Data::Enum(enm) => derive_from_bytes_enum(ctx, enm)?,
Data::Union(unn) => derive_from_bytes_union(ctx, unn),
};
Ok(IntoIterator::into_iter([from_zeros, from_bytes]).collect())
}
fn derive_from_zeros_struct(ctx: &Ctx, strct: &DataStruct) -> TokenStream {
ImplBlockBuilder::new(ctx, strct, Trait::FromZeros, FieldBounds::ALL_SELF).build()
}
fn derive_from_zeros_enum(ctx: &Ctx, enm: &DataEnum) -> Result<TokenStream, Error> {
let repr = EnumRepr::from_attrs(&ctx.ast.attrs)?;
// We don't actually care what the repr is; we just care that it's one of
// the allowed ones.
match repr {
Repr::Compound(Spanned { t: CompoundRepr::C | CompoundRepr::Primitive(_), span: _ }, _) => {
}
Repr::Transparent(_) | Repr::Compound(Spanned { t: CompoundRepr::Rust, span: _ }, _) => {
return ctx.error_or_skip(
Error::new(
Span::call_site(),
"must have #[repr(C)] or #[repr(Int)] attribute in order to guarantee this type's memory layout",
),
);
}
}
let zero_variant = match find_zero_variant(enm) {
Ok(index) => enm.variants.iter().nth(index).unwrap(),
// Has unknown variants
Err(true) => {
return ctx.error_or_skip(Error::new_spanned(
&ctx.ast,
"FromZeros only supported on enums with a variant that has a discriminant of `0`\n\
help: This enum has discriminants which are not literal integers. One of those may \
define or imply which variant has a discriminant of zero. Use a literal integer to \
define or imply the variant with a discriminant of zero.",
));
}
// Does not have unknown variants
Err(false) => {
return ctx.error_or_skip(Error::new_spanned(
&ctx.ast,
"FromZeros only supported on enums with a variant that has a discriminant of `0`",
));
}
};
let zerocopy_crate = &ctx.zerocopy_crate;
let explicit_bounds = zero_variant
.fields
.iter()
.map(|field| {
let ty = &field.ty;
parse_quote! { #ty: #zerocopy_crate::FromZeros }
})
.collect::<Vec<WherePredicate>>();
Ok(ImplBlockBuilder::new(ctx, enm, Trait::FromZeros, FieldBounds::Explicit(explicit_bounds))
.build())
}
fn derive_from_zeros_union(ctx: &Ctx, unn: &DataUnion) -> TokenStream {
let field_type_trait_bounds = FieldBounds::All(&[TraitBound::Slf]);
ImplBlockBuilder::new(ctx, unn, Trait::FromZeros, field_type_trait_bounds).build()
}
fn derive_from_bytes_struct(ctx: &Ctx, strct: &DataStruct) -> TokenStream {
ImplBlockBuilder::new(ctx, strct, Trait::FromBytes, FieldBounds::ALL_SELF).build()
}
fn derive_from_bytes_enum(ctx: &Ctx, enm: &DataEnum) -> Result<TokenStream, Error> {
let repr = EnumRepr::from_attrs(&ctx.ast.attrs)?;
let variants_required = 1usize << enum_size_from_repr(&repr)?;
if enm.variants.len() != variants_required {
return ctx.error_or_skip(Error::new_spanned(
&ctx.ast,
format!(
"FromBytes only supported on {} enum with {} variants",
repr.repr_type_name(),
variants_required
),
));
}
Ok(ImplBlockBuilder::new(ctx, enm, Trait::FromBytes, FieldBounds::ALL_SELF).build())
}
fn derive_from_bytes_union(ctx: &Ctx, unn: &DataUnion) -> TokenStream {
let field_type_trait_bounds = FieldBounds::All(&[TraitBound::Slf]);
ImplBlockBuilder::new(ctx, unn, Trait::FromBytes, field_type_trait_bounds).build()
}
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