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feat(from_str): support rename attribute #536

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28b396b
feat(from_str): support rename attribute
tisonkun Jan 15, 2026
7df1200
test(from_str): add compile-fail tests for rename attribute
tisonkun Jan 15, 2026
3e2471b
feat(from_str): support rename attribute on variants and flat structs
tisonkun Jan 15, 2026
6615354
fixup
tisonkun Jan 15, 2026
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172 changes: 150 additions & 22 deletions impl/src/from_str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -133,9 +133,11 @@ struct FlatExpansion<'i> {
/// a value-specific [`attr::RenameAll`] overriding [`FlatExpansion::rename_all`], if any.
///
/// [`syn::Ident`]: struct@syn::Ident
#[allow(clippy::type_complexity)]
matches: Vec<(
&'i syn::Ident,
Either<&'i syn::DataStruct, &'i syn::Variant>,
Option<attr::Rename>,
Option<attr::RenameAll>,
)>,

Expand All @@ -157,6 +159,14 @@ impl<'i> TryFrom<&'i syn::DeriveInput> for FlatExpansion<'i> {
fn try_from(input: &'i syn::DeriveInput) -> syn::Result<Self> {
let attr_ident = &format_ident!("from_str");

let FlatContainerAttributes {
rename,
rename_all,
error: custom_error,
} = FlatContainerAttributes::parse_attrs(&input.attrs, attr_ident)?
.map(Spanning::into_inner)
.unwrap_or_default();

let matches = match &input.data {
syn::Data::Struct(data) => {
if !data.fields.is_empty() {
Expand All @@ -165,7 +175,7 @@ impl<'i> TryFrom<&'i syn::DeriveInput> for FlatExpansion<'i> {
"only structs with no fields can derive `FromStr`",
));
}
vec![(&input.ident, Either::Left(data), None)]
vec![(&input.ident, Either::Left(data), rename, rename_all)]
}
syn::Data::Enum(data) => data
.variants
Expand All @@ -177,10 +187,16 @@ impl<'i> TryFrom<&'i syn::DeriveInput> for FlatExpansion<'i> {
"only enums with no fields can derive `FromStr`",
));
}
let attr =
attr::RenameAll::parse_attrs(&variant.attrs, attr_ident)?
.map(Spanning::into_inner);
Ok((&variant.ident, Either::Right(variant), attr))
let attrs =
FlatVariantAttributes::parse_attrs(&variant.attrs, attr_ident)?
.map(Spanning::into_inner)
.unwrap_or_default();
Ok((
&variant.ident,
Either::Right(variant),
attrs.rename,
attrs.rename_all,
))
})
.collect::<syn::Result<_>>()?,
syn::Data::Union(_) => {
Expand All @@ -191,19 +207,20 @@ impl<'i> TryFrom<&'i syn::DeriveInput> for FlatExpansion<'i> {
}
};

let FlatContainerAttributes {
rename_all,
error: custom_error,
} = FlatContainerAttributes::parse_attrs(&input.attrs, attr_ident)?
.map(Spanning::into_inner)
.unwrap_or_default();

let mut similar_matches = <HashMap<_, Vec<_>>>::new();
if rename_all.is_none() {
for (ident, _, renaming) in &matches {
for (ident, _, rename, renaming) in &matches {
let name = ident.to_string();
let lowercased = name.to_lowercase();
if let Some(rename) = renaming {
if let Some(rename) = rename {
let renamed_lowercased = rename.as_str().to_lowercase();
if renamed_lowercased != lowercased {
similar_matches
.entry(renamed_lowercased)
.or_default()
.push(*ident);
}
} else if let Some(rename) = renaming {
let renamed_lowercased = rename.convert_case(&name);
if renamed_lowercased != lowercased {
similar_matches
Expand All @@ -217,9 +234,11 @@ impl<'i> TryFrom<&'i syn::DeriveInput> for FlatExpansion<'i> {
}

let mut exact_matches = <HashMap<String, Vec<String>>>::new();
for (ident, _, renaming) in &matches {
for (ident, _, rename, renaming) in &matches {
let name = ident.to_string();
let exact = if let Some(default_renaming) = &rename_all {
let exact = if let Some(rename) = rename {
rename.as_str().to_owned()
} else if let Some(default_renaming) = &rename_all {
renaming
.as_ref()
.unwrap_or(default_renaming)
Expand Down Expand Up @@ -273,10 +292,14 @@ impl ToTokens for FlatExpansion<'_> {
let match_arms = if let Some(default_renaming) = self.rename_all {
self.matches
.iter()
.map(|(ident, value, renaming)| {
let converted = renaming
.unwrap_or(default_renaming)
.convert_case(&ident.to_string());
.map(|(ident, value, rename, renaming)| {
let converted = if let Some(rename) = rename {
rename.as_str().to_owned()
} else {
renaming
.unwrap_or(default_renaming)
.convert_case(&ident.to_string())
};
let constructor = value.self_constructor_empty();

quote! { #converted => #constructor, }
Expand All @@ -285,10 +308,14 @@ impl ToTokens for FlatExpansion<'_> {
} else {
self.matches
.iter()
.map(|(ident, value, renaming)| {
.map(|(ident, value, rename, renaming)| {
let name = ident.to_string();
let constructor = value.self_constructor_empty();
if let Some(rename) = renaming {
if let Some(rename) = rename {
let exact_name = rename.as_str();

quote! { _ if s == #exact_name => #constructor, }
} else if let Some(rename) = renaming {
let exact_name = rename.convert_case(&name);

quote! { _ if s == #exact_name => #constructor, }
Expand Down Expand Up @@ -465,6 +492,89 @@ impl<L: FieldsExt, R: FieldsExt> FieldsExt for Either<&L, &R> {
}
}

/// Representation of possible [`FromStr`] derive macro attributes placed on a variant.
///
/// ```rust,ignore
/// #[<attribute>(rename = "...")]
/// #[<attribute>(rename_all = "<casing>")]
/// ```
#[derive(Default)]
struct FlatVariantAttributes {
/// [`attr::Rename`] for custom renaming.
rename: Option<attr::Rename>,

/// [`attr::RenameAll`] for case conversion.
rename_all: Option<attr::RenameAll>,
}

impl Parse for FlatVariantAttributes {
fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
mod ident {
use syn::custom_keyword;

custom_keyword!(rename);
custom_keyword!(rename_all);
}

let ahead = input.lookahead1();
if ahead.peek(ident::rename) {
Ok(Self {
rename: Some(input.parse()?),
..Default::default()
})
} else if ahead.peek(ident::rename_all) {
Ok(Self {
rename_all: Some(input.parse()?),
..Default::default()
})
} else {
Err(ahead.error())
}
}
}

impl attr::ParseMultiple for FlatVariantAttributes {
fn merge_attrs(
prev: Spanning<Self>,
new: Spanning<Self>,
name: &syn::Ident,
) -> syn::Result<Spanning<Self>> {
let Spanning {
span: prev_span,
item: mut prev,
} = prev;
let Spanning {
span: new_span,
item: new,
} = new;

if new.rename.and_then(|n| prev.rename.replace(n)).is_some() {
return Err(syn::Error::new(
new_span,
format!(
"multiple `#[{name}(rename=\"...\")]` attributes aren't allowed"
),
));
}

if new
.rename_all
.and_then(|n| prev.rename_all.replace(n))
.is_some()
{
return Err(syn::Error::new(
new_span,
format!("multiple `#[{name}(rename_all=\"...\")]` attributes aren't allowed"),
));
}

Ok(Spanning::new(
prev,
prev_span.join(new_span).unwrap_or(prev_span),
))
}
}

/// Representation of possible [`FromStr`] derive macro attributes placed on an enum or a struct for
/// a [`FlatExpansion`].
///
Expand All @@ -478,6 +588,9 @@ impl<L: FieldsExt, R: FieldsExt> FieldsExt for Either<&L, &R> {
/// be specified only once.
#[derive(Default)]
struct FlatContainerAttributes {
/// [`attr::Rename`] for custom renaming.
rename: Option<attr::Rename>,

/// [`attr::RenameAll`] for case conversion.
rename_all: Option<attr::RenameAll>,

Expand All @@ -491,6 +604,7 @@ impl Parse for FlatContainerAttributes {
use syn::custom_keyword;

custom_keyword!(error);
custom_keyword!(rename);
custom_keyword!(rename_all);
}

Expand All @@ -500,6 +614,11 @@ impl Parse for FlatContainerAttributes {
error: Some(input.parse()?),
..Default::default()
})
} else if ahead.peek(ident::rename) {
Ok(Self {
rename: Some(input.parse()?),
..Default::default()
})
} else if ahead.peek(ident::rename_all) {
Ok(Self {
rename_all: Some(input.parse()?),
Expand All @@ -526,6 +645,15 @@ impl attr::ParseMultiple for FlatContainerAttributes {
item: new,
} = new;

if new.rename.and_then(|n| prev.rename.replace(n)).is_some() {
return Err(syn::Error::new(
new_span,
format!(
"multiple `#[{name}(rename=\"...\")]` attributes aren't allowed"
),
));
}

if new
.rename_all
.and_then(|n| prev.rename_all.replace(n))
Expand Down
49 changes: 49 additions & 0 deletions impl/src/utils.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1539,6 +1539,8 @@ pub(crate) mod attr {
feature = "mul_assign",
))]
pub(crate) use self::forward::Forward;
#[cfg(feature = "from_str")]
pub(crate) use self::rename::Rename;
#[cfg(any(feature = "display", feature = "from_str"))]
pub(crate) use self::rename_all::RenameAll;
#[cfg(any(
Expand Down Expand Up @@ -2389,6 +2391,53 @@ pub(crate) mod attr {

impl ParseMultiple for RenameAll {}
}

#[cfg(any(feature = "display", feature = "from_str"))]
mod rename {
use syn::{
parse::{Parse, ParseStream},
spanned::Spanned as _,
token,
};

use super::ParseMultiple;

/// Representation of a `rename` macro attribute.
///
/// ```rust,ignore
/// #[<attribute>(rename = "...")]
/// ```
#[derive(Clone, Debug)]
pub(crate) struct Rename(String);

impl Rename {
pub(crate) fn as_str(&self) -> &str {
&self.0
}
}

impl Parse for Rename {
fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
let _ = input.parse::<syn::Path>().and_then(|p| {
if p.is_ident("rename") {
Ok(p)
} else {
Err(syn::Error::new(
p.span(),
"unknown attribute argument, expected `rename = \"...\"`",
))
}
})?;

input.parse::<token::Eq>()?;

let lit: syn::LitStr = input.parse()?;
Ok(Self(lit.value()))
}
}

impl ParseMultiple for Rename {}
}
}

#[cfg(any(feature = "from", feature = "into"))]
Expand Down
3 changes: 3 additions & 0 deletions tests/compile_fail/as_mut/renamed_generic.stderr
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@tisonkun tisonkun Jan 15, 2026

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compiler error output diverges between stable and nightly. Not sure what we can do.

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@tisonkun tisonkun Jan 15, 2026

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Weird. I run cargo +stable test --workspace --features full,testing-helpers locally and it doesn't fail:

rustc --version --verbose
rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: aarch64-apple-darwin
release: 1.92.0
LLVM version: 21.1.3

Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,9 @@ error[E0599]: the method `as_mut` exists for struct `Baz<i32>`, but its trait bo
= note: trait bound `Foo<i32>: AsMut<Foo<i32>>` was not satisfied
note: the trait `AsMut` must be implemented
--> $RUST/core/src/convert/mod.rs
|
| pub const trait AsMut<T: PointeeSized>: PointeeSized {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: items from traits can only be used if the trait is implemented and in scope
= note: the following trait defines an item `as_mut`, perhaps you need to implement it:
candidate #1: `AsMut`
3 changes: 3 additions & 0 deletions tests/compile_fail/as_ref/renamed_generic.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,9 @@ error[E0599]: the method `as_ref` exists for struct `Baz<i32>`, but its trait bo
= note: trait bound `Foo<i32>: AsRef<Foo<i32>>` was not satisfied
note: the trait `AsRef` must be implemented
--> $RUST/core/src/convert/mod.rs
|
| pub const trait AsRef<T: PointeeSized>: PointeeSized {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: items from traits can only be used if the trait is implemented and in scope
= note: the following trait defines an item `as_ref`, perhaps you need to implement it:
candidate #1: `AsRef`
2 changes: 1 addition & 1 deletion tests/compile_fail/from_str/enum_unknown_attribute.stderr
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
error: expected `error` or `rename_all`
error: expected one of: `error`, `rename`, `rename_all`
--> tests/compile_fail/from_str/enum_unknown_attribute.rs:2:12
|
2 | #[from_str(unknown = "unknown")]
Expand Down
11 changes: 11 additions & 0 deletions tests/compile_fail/from_str/rename_collision.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
use derive_more::FromStr;

#[derive(FromStr)]
#[from_str(rename_all = "lowercase")]
enum E {
#[from_str(rename = "b")]
A,
B,
}

fn main() {}
5 changes: 5 additions & 0 deletions tests/compile_fail/from_str/rename_collision.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
error: `A`, `B` variants cannot have the same "b" string representation
--> tests/compile_fail/from_str/rename_collision.rs:5:6
|
5 | enum E {
| ^
11 changes: 11 additions & 0 deletions tests/compile_fail/from_str/rename_duplicate.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
use derive_more::FromStr;

#[derive(FromStr)]
enum E {
#[from_str(rename = "a")]
A,
#[from_str(rename = "a")]
B,
}

fn main() {}
5 changes: 5 additions & 0 deletions tests/compile_fail/from_str/rename_duplicate.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
error: `A`, `B` variants cannot have the same "a" string representation
--> tests/compile_fail/from_str/rename_duplicate.rs:4:6
|
4 | enum E {
| ^
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