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Make the Pod/NoUninit derive macros not use transmute, to be more robust against possible future relaxations transmute's size check. #320

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Merged
merged 1 commit into from
Aug 6, 2025
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Make the Pod/NoUninit derive macros not use transmute, to be more robust against possible future relaxations transmute's size check. #320

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21 changes: 13 additions & 8 deletions derive/src/traits.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -80,7 +80,7 @@ impl Derivable for Pod {
match &input.data {
Data::Struct(_) => {
let assert_no_padding = if !completly_packed {
Some(generate_assert_no_padding(input, None)?)
Some(generate_assert_no_padding(input, None, "Pod")?)
} else {
None
};
Expand Down Expand Up @@ -263,7 +263,8 @@ impl Derivable for NoUninit {

match &input.data {
Data::Struct(DataStruct { .. }) => {
let assert_no_padding = generate_assert_no_padding(&input, None)?;
let assert_no_padding =
generate_assert_no_padding(&input, None, "NoUninit")?;
let assert_fields_are_no_padding = generate_fields_are_trait(
&input,
None,
Expand Down Expand Up @@ -312,7 +313,7 @@ impl Derivable for NoUninit {
.iter()
.map(|variant| {
let assert_no_padding =
generate_assert_no_padding(&input, Some(variant))?;
generate_assert_no_padding(&input, Some(variant), "NoUninit")?;
let assert_fields_are_no_padding = generate_fields_are_trait(
&input,
Some(variant),
Expand Down Expand Up @@ -1046,7 +1047,7 @@ fn generate_checked_bit_pattern_enum_with_fields(
/// the type is equal to the sum of the size of it's fields and discriminant
/// (for enums, this must be asserted for each variant).
fn generate_assert_no_padding(
input: &DeriveInput, enum_variant: Option<&Variant>,
input: &DeriveInput, enum_variant: Option<&Variant>, for_trait: &str,
) -> Result<TokenStream> {
let struct_type = &input.ident;
let fields = get_fields(input, enum_variant)?;
Expand All @@ -1073,10 +1074,14 @@ fn generate_assert_no_padding(
quote!(0)
};

Ok(quote! {const _: fn() = || {
#[doc(hidden)]
struct TypeWithoutPadding([u8; #size_sum]);
let _ = ::core::mem::transmute::<#struct_type, TypeWithoutPadding>;
let message =
format!("derive({for_trait}) was applied to a type with padding");

Ok(quote! {const _: () = {
assert!(
::core::mem::size_of::<#struct_type>() == (#size_sum),
#message,
);
};})
}

Expand Down
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