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Allow single-field named structs to be transparent #3971

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22 changes: 14 additions & 8 deletions sqlx-macros-core/src/derives/decode.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,18 +8,17 @@ use quote::quote;
use syn::punctuated::Punctuated;
use syn::token::Comma;
use syn::{
parse_quote, Arm, Data, DataEnum, DataStruct, DeriveInput, Field, Fields, FieldsNamed,
FieldsUnnamed, Stmt, TypeParamBound, Variant,
parse_quote, Arm, Data, DataEnum, DataStruct, DeriveInput, Field, Fields, FieldsNamed, Stmt,
TypeParamBound, Variant,
};

pub fn expand_derive_decode(input: &DeriveInput) -> syn::Result<TokenStream> {
let attrs = parse_container_attributes(&input.attrs)?;
match &input.data {
Data::Struct(DataStruct {
fields: Fields::Unnamed(FieldsUnnamed { unnamed, .. }),
..
}) if unnamed.len() == 1 => {
expand_derive_decode_transparent(input, unnamed.first().unwrap())
Data::Struct(DataStruct { fields, .. })
if fields.len() == 1 && (matches!(fields, Fields::Unnamed(_)) || attrs.transparent) =>
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@abonander abonander Aug 18, 2025 •
edited
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You should change the following assert to mention that #[sqlx(transparent)] is only allowed on tuple structs or structs with a single named field:

sqlx/sqlx-macros-core/src/derives/attributes.rs

Lines 282 to 286 in e77f32e

assert_attribute!(
!attributes.transparent,
"unexpected #[sqlx(transparent)]",
input
);

Otherwise if the user adds one field, they'll just get a confusing unexpected #[sqlx(transparent)] error.

{
expand_derive_decode_transparent(input, fields.iter().next().unwrap())
}
Data::Enum(DataEnum { variants, .. }) => match attrs.repr {
Some(_) => expand_derive_decode_weak_enum(input, variants),
Expand Down Expand Up @@ -72,6 +71,12 @@ fn expand_derive_decode_transparent(
.push(parse_quote!(#ty: ::sqlx::decode::Decode<'r, DB>));
let (impl_generics, _, where_clause) = generics.split_for_impl();

let field_ident = if let Some(ident) = &field.ident {
quote! { #ident }
} else {
quote! { 0 }
};

let tts = quote!(
#[automatically_derived]
impl #impl_generics ::sqlx::decode::Decode<'r, DB> for #ident #ty_generics #where_clause {
Expand All @@ -83,7 +88,8 @@ fn expand_derive_decode_transparent(
dyn ::std::error::Error + 'static + ::std::marker::Send + ::std::marker::Sync,
>,
> {
<#ty as ::sqlx::decode::Decode<'r, DB>>::decode(value).map(Self)
<#ty as ::sqlx::decode::Decode<'r, DB>>::decode(value)
.map(|val| Self { #field_ident: val })
}
}
);
Expand Down
23 changes: 14 additions & 9 deletions sqlx-macros-core/src/derives/encode.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,18 +9,17 @@ use syn::punctuated::Punctuated;
use syn::token::Comma;
use syn::{
parse_quote, Data, DataEnum, DataStruct, DeriveInput, Expr, Field, Fields, FieldsNamed,
FieldsUnnamed, Lifetime, LifetimeParam, Stmt, TypeParamBound, Variant,
Lifetime, LifetimeParam, Stmt, TypeParamBound, Variant,
};

pub fn expand_derive_encode(input: &DeriveInput) -> syn::Result<TokenStream> {
let args = parse_container_attributes(&input.attrs)?;

match &input.data {
Data::Struct(DataStruct {
fields: Fields::Unnamed(FieldsUnnamed { unnamed, .. }),
..
}) if unnamed.len() == 1 => {
expand_derive_encode_transparent(input, unnamed.first().unwrap())
Data::Struct(DataStruct { fields, .. })
if fields.len() == 1 && (matches!(fields, Fields::Unnamed(_)) || args.transparent) =>
{
expand_derive_encode_transparent(input, fields.iter().next().unwrap())
}
Data::Enum(DataEnum { variants, .. }) => match args.repr {
Some(_) => expand_derive_encode_weak_enum(input, variants),
Expand Down Expand Up @@ -77,6 +76,12 @@ fn expand_derive_encode_transparent(
.push(parse_quote!(#ty: ::sqlx::encode::Encode<#lifetime, DB>));
let (impl_generics, _, where_clause) = generics.split_for_impl();

let field_ident = if let Some(ident) = &field.ident {
quote! { #ident }
} else {
quote! { 0 }
};

Ok(quote!(
#[automatically_derived]
impl #impl_generics ::sqlx::encode::Encode<#lifetime, DB> for #ident #ty_generics
Expand All @@ -86,15 +91,15 @@ fn expand_derive_encode_transparent(
&self,
buf: &mut <DB as ::sqlx::database::Database>::ArgumentBuffer<#lifetime>,
) -> ::std::result::Result<::sqlx::encode::IsNull, ::sqlx::error::BoxDynError> {
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::encode_by_ref(&self.0, buf)
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::encode_by_ref(&self.#field_ident, buf)
}

fn produces(&self) -> Option<DB::TypeInfo> {
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::produces(&self.0)
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::produces(&self.#field_ident)
}

fn size_hint(&self) -> usize {
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::size_hint(&self.0)
<#ty as ::sqlx::encode::Encode<#lifetime, DB>>::size_hint(&self.#field_ident)
}
}
))
Expand Down
26 changes: 12 additions & 14 deletions sqlx-macros-core/src/derives/type.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,34 +7,32 @@ use quote::{quote, quote_spanned};
use syn::punctuated::Punctuated;
use syn::token::Comma;
use syn::{
parse_quote, Data, DataEnum, DataStruct, DeriveInput, Field, Fields, FieldsNamed,
FieldsUnnamed, Variant,
parse_quote, Data, DataEnum, DataStruct, DeriveInput, Field, Fields, FieldsNamed, Variant,
};

pub fn expand_derive_type(input: &DeriveInput) -> syn::Result<TokenStream> {
let attrs = parse_container_attributes(&input.attrs)?;
match &input.data {
// Newtype structs:
// struct Foo(i32);
Data::Struct(DataStruct {
fields: Fields::Unnamed(FieldsUnnamed { unnamed, .. }),
..
}) => {
if unnamed.len() == 1 {
expand_derive_has_sql_type_transparent(input, unnamed.first().unwrap())
} else {
Err(syn::Error::new_spanned(
input,
"structs with zero or more than one unnamed field are not supported",
))
}
Data::Struct(DataStruct { fields, .. })
if fields.len() == 1 && (matches!(fields, Fields::Unnamed(_)) || attrs.transparent) =>
{
expand_derive_has_sql_type_transparent(input, fields.iter().next().unwrap())
}
// Record types
// struct Foo { foo: i32, bar: String }
Data::Struct(DataStruct {
fields: Fields::Named(FieldsNamed { named, .. }),
..
}) => expand_derive_has_sql_type_struct(input, named),
Data::Struct(DataStruct {
fields: Fields::Unnamed(..),
..
}) => Err(syn::Error::new_spanned(
input,
"structs with zero or more than one unnamed field are not supported",
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In hindsight, this wording is really clunky.

How about: tuple structs may only have a single field

)),
Data::Struct(DataStruct {
fields: Fields::Unit,
..
Expand Down
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