feat: FromStr derive could support setting the error type

strum_macros/src/helpers/metadata.rs

@@ -18,6 +18,8 @@ pub mod kw {
     custom_keyword!(serialize_all);
     custom_keyword!(use_phf);
     custom_keyword!(prefix);
+    custom_keyword!(parse_err_ty);
+    custom_keyword!(parse_err_fn);
 
     // enum discriminant metadata
     custom_keyword!(derive);
@@ -51,6 +53,14 @@ pub enum EnumMeta {
         kw: kw::prefix,
         prefix: LitStr,
     },
+    ParseErrTy {
+        kw: kw::parse_err_ty,
+        path: Path,
+    },
+    ParseErrFn {
+        kw: kw::parse_err_fn,
+        path: Path,
+    },
 }
 
 impl Parse for EnumMeta {
@@ -80,6 +90,20 @@ impl Parse for EnumMeta {
             input.parse::<Token![=]>()?;
             let prefix = input.parse()?;
             Ok(EnumMeta::Prefix { kw, prefix })
+        } else if lookahead.peek(kw::parse_err_ty) {
+            let kw = input.parse::<kw::parse_err_ty>()?;
+            input.parse::<Token![=]>()?;
+            let path_str: LitStr = input.parse()?;
+            let path_tokens = parse_str(&path_str.value())?;
+            let path = parse2(path_tokens)?;
+            Ok(EnumMeta::ParseErrTy { kw, path })
+        } else if lookahead.peek(kw::parse_err_fn) {
+            let kw = input.parse::<kw::parse_err_fn>()?;
+            input.parse::<Token![=]>()?;
+            let path_str: LitStr = input.parse()?;
+            let path_tokens = parse_str(&path_str.value())?;
+            let path = parse2(path_tokens)?;
+            Ok(EnumMeta::ParseErrFn { kw, path })
         } else {
             Err(lookahead.error())
         }

strum_macros/src/helpers/mod.rs

@@ -13,6 +13,13 @@ use proc_macro2::Span;
 use quote::ToTokens;
 use syn::spanned::Spanned;
 
+pub fn missing_parse_err_attr_error() -> syn::Error {
+    syn::Error::new(
+        Span::call_site(),
+        "`parse_err_ty` and `parse_err_fn` attribute is both required.",
+    )
+}
+
 pub fn non_enum_error() -> syn::Error {
     syn::Error::new(Span::call_site(), "This macro only supports enums.")
 }

strum_macros/src/helpers/type_props.rs

@@ -13,6 +13,8 @@ pub trait HasTypeProperties {
 
 #[derive(Clone, Default)]
 pub struct StrumTypeProperties {
+    pub parse_err_ty: Option<Path>,
+    pub parse_err_fn: Option<Path>,
     pub case_style: Option<CaseStyle>,
     pub ascii_case_insensitive: bool,
     pub crate_module_path: Option<Path>,
@@ -32,6 +34,8 @@ impl HasTypeProperties for DeriveInput {
         let strum_meta = self.get_metadata()?;
         let discriminants_meta = self.get_discriminants_metadata()?;
 
+        let mut parse_err_ty_kw = None;
+        let mut parse_err_fn_kw = None;
         let mut serialize_all_kw = None;
         let mut ascii_case_insensitive_kw = None;
         let mut use_phf_kw = None;
@@ -82,6 +86,22 @@ impl HasTypeProperties for DeriveInput {
                     prefix_kw = Some(kw);
                     output.prefix = Some(prefix);
                 }
+                EnumMeta::ParseErrTy { path, kw } => {
+                    if let Some(fst_kw) = parse_err_ty_kw {
+                        return Err(occurrence_error(fst_kw, kw, "parse_err_ty"));
+                    }
+
+                    parse_err_ty_kw = Some(kw);
+                    output.parse_err_ty = Some(path);
+                }
+                EnumMeta::ParseErrFn { path, kw } => {
+                    if let Some(fst_kw) = parse_err_fn_kw {
+                        return Err(occurrence_error(fst_kw, kw, "parse_err_fn"));
+                    }
+
+                    parse_err_fn_kw = Some(kw);
+                    output.parse_err_fn = Some(path);
+                }
             }
         }
 

strum_macros/src/macros/strings/from_string.rs

@@ -1,10 +1,10 @@
 use proc_macro2::TokenStream;
 use quote::quote;
-use syn::{Data, DeriveInput, Fields};
+use syn::{parse_quote, Data, DeriveInput, Fields, Path};
 
 use crate::helpers::{
-    non_enum_error, occurrence_error, HasInnerVariantProperties, HasStrumVariantProperties,
-    HasTypeProperties,
+    missing_parse_err_attr_error, non_enum_error, occurrence_error, HasInnerVariantProperties,
+    HasStrumVariantProperties, HasTypeProperties,
 };
 
 pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
@@ -19,9 +19,25 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
     let strum_module_path = type_properties.crate_module_path();
 
     let mut default_kw = None;
-    let mut default =
-        quote! { ::core::result::Result::Err(#strum_module_path::ParseError::VariantNotFound) };
-
+    let (mut default_err_ty, mut default) = match (
+        type_properties.parse_err_ty,
+        type_properties.parse_err_fn,
+    ) {
+        (None, None) => (
+            quote! { #strum_module_path::ParseError },
+            quote! { ::core::result::Result::Err(#strum_module_path::ParseError::VariantNotFound) },
+        ),
+        (Some(ty), Some(f)) => {
+            let ty_path: Path = parse_quote!(#ty);
+            let fn_path: Path = parse_quote!(#f);
+
+            (
+                quote! { #ty_path },
+                quote! { ::core::result::Result::Err(#fn_path(s)) },
+            )
+        }
+        _ => return Err(missing_parse_err_attr_error()),
+    };
     let mut phf_exact_match_arms = Vec::new();
     let mut standard_match_arms = Vec::new();
     for variant in variants {
@@ -47,6 +63,7 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
                 }
             }
             default_kw = Some(kw);
+            default_err_ty = quote! { #strum_module_path::ParseError };
             default = quote! {
                 ::core::result::Result::Ok(#name::#ident(s.into()))
             };
@@ -146,7 +163,7 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
     let from_str = quote! {
         #[allow(clippy::use_self)]
         impl #impl_generics ::core::str::FromStr for #name #ty_generics #where_clause {
-            type Err = #strum_module_path::ParseError;
+            type Err = #default_err_ty;
 
             #[inline]
             fn from_str(s: &str) -> ::core::result::Result< #name #ty_generics , <Self as ::core::str::FromStr>::Err> {
@@ -160,7 +177,7 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
         &impl_generics,
         &ty_generics,
         where_clause,
-        &strum_module_path,
+        &default_err_ty,
     );
 
     Ok(quote! {
@@ -186,12 +203,12 @@ fn try_from_str(
     impl_generics: &syn::ImplGenerics,
     ty_generics: &syn::TypeGenerics,
     where_clause: Option<&syn::WhereClause>,
-    strum_module_path: &syn::Path,
+    default_err_ty: &TokenStream,
 ) -> TokenStream {
     quote! {
         #[allow(clippy::use_self)]
         impl #impl_generics ::core::convert::TryFrom<&str> for #name #ty_generics #where_clause {
-            type Error = #strum_module_path::ParseError;
+            type Error = #default_err_ty;
 
             #[inline]
             fn try_from(s: &str) -> ::core::result::Result< #name #ty_generics , <Self as ::core::convert::TryFrom<&str>>::Error> {

strum_tests/tests/from_str.rs

@@ -229,3 +229,36 @@ fn color_default_with_white() {
         }
     }
 }
+
+#[derive(Debug, EnumString)]
+#[strum(
+    parse_err_fn = "some_enum_not_found_err",
+    parse_err_ty = "CaseCustomParseErrorNotFoundError"
+)]
+enum CaseCustomParseErrorEnum {
+    #[strum(serialize = "red")]
+    Red,
+    #[strum(serialize = "blue")]
+    Blue,
+}
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+struct CaseCustomParseErrorNotFoundError(String);
+impl std::fmt::Display for CaseCustomParseErrorNotFoundError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "not found `{}`", self.0)
+    }
+}
+impl std::error::Error for CaseCustomParseErrorNotFoundError {}
+fn some_enum_not_found_err(s: &str) -> CaseCustomParseErrorNotFoundError {
+    CaseCustomParseErrorNotFoundError(s.to_string())
+}
+
+#[test]
+fn case_custom_parse_error() {
+    let r = "yellow".parse::<CaseCustomParseErrorEnum>();
+    assert!(r.is_err());
+    assert_eq!(
+        CaseCustomParseErrorNotFoundError("yellow".to_string()),
+        r.unwrap_err()
+    );
+}