impl rename_all for #[derive(Display)] (JelteF#216)

impl/Cargo.toml

@@ -53,10 +53,10 @@ add = []
 add_assign = []
 as_ref = ["syn/extra-traits", "syn/visit"]
 constructor = []
-debug = ["syn/extra-traits", "dep:unicode-xid"]
+debug = ["syn/extra-traits", "dep:unicode-xid", "dep:convert_case"]
 deref = []
 deref_mut = []
-display = ["syn/extra-traits", "dep:unicode-xid"]
+display = ["syn/extra-traits", "dep:unicode-xid", "dep:convert_case"]
 error = ["syn/extra-traits"]
 from = ["syn/extra-traits"]
 from_str = []

impl/src/fmt/display.rs

@@ -11,7 +11,7 @@ use crate::utils::{attr::ParseMultiple as _, Spanning};
 
 use super::{
     trait_name_to_attribute_name, ContainerAttributes, ContainsGenericsExt as _,
-    FmtAttribute,
+    FmtAttribute, RenameAllAttribute,
 };
 
 /// Expands a [`fmt::Display`]-like derive macro.
@@ -29,9 +29,13 @@ pub fn expand(input: &syn::DeriveInput, trait_name: &str) -> syn::Result<TokenSt
     let trait_name = normalize_trait_name(trait_name);
     let attr_name = format_ident!("{}", trait_name_to_attribute_name(trait_name));
 
-    let attrs = ContainerAttributes::parse_attrs(&input.attrs, &attr_name)?
-        .map(Spanning::into_inner)
-        .unwrap_or_default();
+    let attrs = ContainerAttributes::parse_attrs(&input.attrs, &attr_name)?;
+    if let Some(attrs) = &attrs {
+        if matches!(input.data, syn::Data::Struct(_)) {
+            attrs.validate_for_struct(&attr_name)?;
+        }
+    }
+    let attrs = attrs.map(Spanning::into_inner).unwrap_or_default();
     let trait_ident = format_ident!("{trait_name}");
     let ident = &input.ident;
 
@@ -97,6 +101,7 @@ fn expand_struct(
 ) -> syn::Result<(Vec<syn::WherePredicate>, TokenStream)> {
     let s = Expansion {
         shared_attr: None,
+        rename_all: None,
         attrs,
         fields: &s.fields,
         type_params,
@@ -148,14 +153,18 @@ fn expand_enum(
     let (bounds, match_arms) = e.variants.iter().try_fold(
         (Vec::new(), TokenStream::new()),
         |(mut bounds, mut arms), variant| {
-            let attrs = ContainerAttributes::parse_attrs(&variant.attrs, attr_name)?
-                .map(Spanning::into_inner)
-                .unwrap_or_default();
+            let attrs = ContainerAttributes::parse_attrs(&variant.attrs, attr_name)?;
+            if let Some(attrs) = &attrs {
+                attrs.validate_for_struct(attr_name)?;
+            };
+            let attrs = attrs.map(Spanning::into_inner).unwrap_or_default();
+
             let ident = &variant.ident;
 
             if attrs.fmt.is_none()
                 && variant.fields.is_empty()
                 && attr_name != "display"
+                && container_attrs.rename_all.is_none()
             {
                 return Err(syn::Error::new(
                     e.variants.span(),
@@ -168,6 +177,7 @@ fn expand_enum(
 
             let v = Expansion {
                 shared_attr: container_attrs.fmt.as_ref(),
+                rename_all: container_attrs.rename_all,
                 attrs: &attrs,
                 fields: &variant.fields,
                 type_params,
@@ -234,6 +244,11 @@ struct Expansion<'a> {
     /// [`None`] for a struct.
     shared_attr: Option<&'a FmtAttribute>,
 
+    /// [`RenameAllAttribute`] placed on enum.
+    ///
+    /// [`None`] for a struct.
+    rename_all: Option<RenameAllAttribute>,
+
     /// Derive macro [`ContainerAttributes`].
     attrs: &'a ContainerAttributes,
 
@@ -305,7 +320,11 @@ impl Expansion<'_> {
             None => {
                 if shared_attr_is_wrapping || !has_shared_attr {
                     body = if self.fields.is_empty() {
-                        let ident_str = self.ident.unraw().to_string();
+                        let ident_str = if let Some(rename_all) = &self.rename_all {
+                            rename_all.convert_case(self.ident)
+                        } else {
+                            self.ident.unraw().to_string()
+                        };
 
                         if shared_attr_is_wrapping {
                             quote! { #ident_str }

impl/src/fmt/mod.rs

@@ -8,6 +8,9 @@ pub(crate) mod debug;
 pub(crate) mod display;
 mod parsing;
 
+use std::fmt::Display;
+
+use convert_case::{Case, Casing as _};
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, ToTokens};
 use syn::{
@@ -16,7 +19,7 @@ use syn::{
     parse_quote,
     punctuated::Punctuated,
     spanned::Spanned as _,
-    token,
+    token, LitStr, Token,
 };
 
 use crate::{
@@ -88,6 +91,62 @@ impl BoundsAttribute {
     }
 }
 
+/// Representation of a `rename_all` macro attribute.
+///
+/// ```rust,ignore
+/// #[<attribute>(rename_all = "...")]
+/// ```
+///
+/// Possible Cases:
+/// - `lowercase`
+/// - `UPPERCASE`
+/// - `PascalCase`
+/// - `camelCase`
+/// - `snake_case`
+/// - `SCREAMING_SNAKE_CASE`
+/// - `kebab-case`
+/// - `SCREAMING-KEBAB-CASE`
+#[derive(Debug, Clone, Copy)]
+struct RenameAllAttribute(Case);
+
+impl RenameAllAttribute {
+    fn convert_case(&self, ident: &syn::Ident) -> String {
+        ident.unraw().to_string().to_case(self.0)
+    }
+}
+
+impl Parse for RenameAllAttribute {
+    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
+        let _ = input.parse::<syn::Path>().and_then(|p| {
+            if p.is_ident("rename_all") {
+                Ok(p)
+            } else {
+                Err(syn::Error::new(
+                    p.span(),
+                    "unknown attribute argument, expected `rename_all = \"...\"`",
+                ))
+            }
+        })?;
+
+        input.parse::<Token![=]>()?;
+
+        let value: LitStr = input.parse()?;
+
+        // TODO should we really do a case insensitive comparision here?
+        Ok(Self(match value.value().replace(['-', '_'], "").to_lowercase().as_str() {
+            "lowercase" => Case::Flat,
+            "uppercase" => Case::UpperFlat,
+            "pascalcase" => Case::Pascal,
+            "camelcase" => Case::Camel,
+            "snakecase" => Case::Snake,
+            "screamingsnakecase" => Case::UpperSnake,
+            "kebabcase" => Case::Kebab,
+            "screamingkebabcase" => Case::UpperKebab,
+            _ => return Err(syn::Error::new_spanned(value, "unexpected casing expected one of: \"lowercase\", \"UPPERCASE\", \"PascalCase\", \"camelCase\", \"snake_case\", \"SCREAMING_SNAKE_CASE\", \"kebab-case\", or \"SCREAMING-KEBAB-CASE\""))
+        }))
+    }
+}
+
 /// Representation of a [`fmt`]-like attribute.
 ///
 /// ```rust,ignore
@@ -516,19 +575,81 @@ struct ContainerAttributes {
 
     /// Addition trait bounds.
     bounds: BoundsAttribute,
+
+    /// Rename unit enum variants following a similar behavior as [`serde`](https://serde.rs/container-attrs.html#rename_all).
+    rename_all: Option<RenameAllAttribute>,
+}
+
+impl Spanning<ContainerAttributes> {
+    fn validate_for_struct(&self, attr_name: impl Display) -> syn::Result<()> {
+        if self.rename_all.is_some() {
+            Err(syn::Error::new(
+                self.span,
+                format_args!("`#[{attr_name}(rename_all=\"...\")]` can not be specified on structs or variants"),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+}
+
+mod kw {
+    use syn::custom_keyword;
+
+    custom_keyword!(rename_all);
+    custom_keyword!(bounds);
+    custom_keyword!(bound);
 }
 
 impl Parse for ContainerAttributes {
     fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
         // We do check `FmtAttribute::check_legacy_fmt` eagerly here, because `Either` will swallow
         // any error of the `Either::Left` if the `Either::Right` succeeds.
         FmtAttribute::check_legacy_fmt(input)?;
-        <Either<FmtAttribute, BoundsAttribute>>::parse(input).map(|v| match v {
-            Either::Left(fmt) => Self {
+        let lookahead = input.lookahead1();
+        Ok(if lookahead.peek(LitStr) {
+            Self {
+                fmt: Some(input.parse()?),
                 bounds: BoundsAttribute::default(),
-                fmt: Some(fmt),
-            },
-            Either::Right(bounds) => Self { bounds, fmt: None },
+                rename_all: None,
+            }
+        } else if lookahead.peek(kw::rename_all)
+            || lookahead.peek(kw::bounds)
+            || lookahead.peek(kw::bound)
+            || lookahead.peek(Token![where])
+        {
+            let mut bounds = BoundsAttribute::default();
+            let mut rename_all = None;
+
+            while !input.is_empty() {
+                let lookahead = input.lookahead1();
+                if lookahead.peek(kw::rename_all) {
+                    if rename_all.is_some() {
+                        return Err(
+                            input.error("`rename_all` can only be specified once")
+                        );
+                    } else {
+                        rename_all = Some(input.parse()?);
+                    }
+                } else if lookahead.peek(kw::bounds)
+                    || lookahead.peek(kw::bound)
+                    || lookahead.peek(Token![where])
+                {
+                    bounds.0.extend(input.parse::<BoundsAttribute>()?.0)
+                } else {
+                    return Err(lookahead.error());
+                }
+                if !input.is_empty() {
+                    input.parse::<Token![,]>()?;
+                }
+            }
+            Self {
+                fmt: None,
+                bounds,
+                rename_all,
+            }
+        } else {
+            return Err(lookahead.error());
         })
     }
 }
@@ -554,6 +675,16 @@ impl attr::ParseMultiple for ContainerAttributes {
                 format!("multiple `#[{name}(\"...\", ...)]` 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"),
+            ));
+        }
         prev.bounds.0.extend(new.bounds.0);
 
         Ok(Spanning::new(
@@ -582,7 +713,7 @@ where
     }
 }
 
-/// Extension of a [`syn::Type`] and a [`syn::Path`] allowing to travers its type parameters.
+/// Extension of a [`syn::Type`] and a [`syn::Path`] allowing to traverse its type parameters.
 trait ContainsGenericsExt {
     /// Checks whether this definition contains any of the provided `type_params`.
     fn contains_generics(&self, type_params: &[&syn::Ident]) -> bool;

tests/compile_fail/debug/unknown_attribute.stderr

@@ -1,4 +1,4 @@
-error: unknown attribute argument, expected `bound(...)`
+error: expected one of: string literal, `rename_all`, `bounds`, `bound`, `where`
  --> tests/compile_fail/debug/unknown_attribute.rs:2:9
   |
 2 | #[debug(unknown = "unknown")]

tests/compile_fail/display/invalid_casing.rs

@@ -0,0 +1,7 @@
+#[derive(derive_more::Display)]
+#[display(rename_all = "Whatever")]
+enum Enum {
+    UnitVariant,
+}
+
+fn main() {}

tests/compile_fail/display/invalid_casing.stderr

@@ -0,0 +1,5 @@
+error: unexpected casing expected one of: "lowercase", "UPPERCASE", "PascalCase", "camelCase", "snake_case", "SCREAMING_SNAKE_CASE", "kebab-case", or "SCREAMING-KEBAB-CASE"
+ --> tests/compile_fail/display/invalid_casing.rs:2:24
+  |
+2 | #[display(rename_all = "Whatever")]
+  |                        ^^^^^^^^^^

tests/compile_fail/display/rename_all_on_struct.rs

@@ -0,0 +1,11 @@
+#[derive(derive_more::Display)]
+enum Enum {
+    #[display(rename_all = "lowercase")]
+    RenameAllOnVariant,
+}
+
+#[derive(derive_more::Display)]
+#[display(rename_all = "lowercase")]
+struct Struct;
+
+fn main() {}

tests/compile_fail/display/rename_all_on_struct.stderr

@@ -0,0 +1,11 @@
+error: `#[display(rename_all="...")]` can not be specified on structs or variants
+ --> tests/compile_fail/display/rename_all_on_struct.rs:3:5
+  |
+3 |     #[display(rename_all = "lowercase")]
+  |     ^
+
+error: `#[display(rename_all="...")]` can not be specified on structs or variants
+ --> tests/compile_fail/display/rename_all_on_struct.rs:8:1
+  |
+8 | #[display(rename_all = "lowercase")]
+  | ^

tests/compile_fail/display/unknown_attribute.stderr

@@ -1,4 +1,4 @@
-error: unknown attribute argument, expected `bound(...)`
+error: expected one of: string literal, `rename_all`, `bounds`, `bound`, `where`
  --> tests/compile_fail/display/unknown_attribute.rs:3:11
   |
 3 | #[display(unknown = "unknown")]