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[derive] Move find_zero_variant to enum module #2889

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Merged
joshlf merged 1 commit into from
Jan 20, 2026
Merged

[derive] Move find_zero_variant to enum module #2889

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82 changes: 80 additions & 2 deletions zerocopy-derive/src/enum.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,8 +9,8 @@
use proc_macro2::TokenStream;
use quote::quote;
use syn::{
parse_quote, spanned::Spanned as _, DataEnum, DeriveInput, Error, Fields, Generics, Ident,
Index, Path,
parse_quote, spanned::Spanned as _, DataEnum, DeriveInput, Error, Expr, ExprLit, ExprUnary,
Fields, Generics, Ident, Index, Lit, Path, UnOp,
};

use crate::{
Expand Down Expand Up @@ -450,3 +450,81 @@ pub(crate) fn derive_is_bit_valid(
}
})
}

/// 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;
}
}
}
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Err(has_unknown_discriminants)
}
83 changes: 2 additions & 81 deletions zerocopy-derive/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -53,8 +53,7 @@ use proc_macro2::{Span, TokenStream};
use quote::{quote, ToTokens};
use syn::{
parse_quote, spanned::Spanned as _, Attribute, Data, DataEnum, DataStruct, DataUnion,
DeriveInput, Error, Expr, ExprLit, ExprUnary, GenericParam, Ident, Lit, Meta, Path, Type, UnOp,
WherePredicate,
DeriveInput, Error, Expr, ExprLit, GenericParam, Ident, Lit, Meta, Path, Type, WherePredicate,
};

use crate::{repr::*, util::*};
Expand Down Expand Up @@ -1100,84 +1099,6 @@ fn derive_from_zeros_struct(
.build()
}

/// 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.
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)
}

/// An enum is `FromZeros` if:
/// - one of the variants has a discriminant of `0`
/// - that variant's fields are all `FromZeros`
Expand All @@ -1199,7 +1120,7 @@ fn derive_from_zeros_enum(
| Repr::Compound(Spanned { t: CompoundRepr::Rust, span: _ }, _) => 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 zero_variant = match find_zero_variant(enm) {
let zero_variant = match r#enum::find_zero_variant(enm) {
Ok(index) => enm.variants.iter().nth(index).unwrap(),
// Has unknown variants
Err(true) => {
Expand Down
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