Improve error handling by adding custom Error type

src/command.rs

@@ -3,9 +3,10 @@ use crate::{
     command_enum::CommandEnum,
     fields_parse::ParserType,
     rename_rules::rename_by_rule,
+    Result,
 };
 
-pub struct Command {
+pub(crate) struct Command {
     pub prefix: Option<String>,
     pub description: Option<String>,
     pub parser: Option<ParserType>,
@@ -14,7 +15,7 @@ pub struct Command {
 }
 
 impl Command {
-    pub fn try_from(attrs: &[Attr], name: &str) -> Result<Self, String> {
+    pub fn try_from(attrs: &[Attr], name: &str) -> Result<Self> {
         let attrs = parse_attrs(attrs)?;
         let mut new_name = name.to_string();
         let mut renamed = false;
@@ -64,7 +65,7 @@ impl Command {
     }
 }
 
-pub struct CommandAttrs {
+pub(crate) struct CommandAttrs {
     pub(crate) prefix: Option<String>,
     pub(crate) description: Option<String>,
     pub(crate) rename: Option<String>,
@@ -72,7 +73,7 @@ pub struct CommandAttrs {
     pub(crate) separator: Option<String>,
 }
 
-pub fn parse_attrs(attrs: &[Attr]) -> Result<CommandAttrs, String> {
+pub(crate) fn parse_attrs(attrs: &[Attr]) -> Result<CommandAttrs> {
     let mut prefix = None;
     let mut description = None;
     let mut rename_rule = None;

src/command_enum.rs

@@ -1,7 +1,10 @@
-use crate::{attr::Attr, command::parse_attrs, fields_parse::ParserType};
+use crate::{
+    attr::Attr, command::parse_attrs, error::compile_error,
+    fields_parse::ParserType, Result,
+};
 
 #[derive(Debug)]
-pub struct CommandEnum {
+pub(crate) struct CommandEnum {
     pub prefix: Option<String>,
     pub description: Option<String>,
     pub rename_rule: Option<String>,
@@ -9,7 +12,7 @@ pub struct CommandEnum {
 }
 
 impl CommandEnum {
-    pub fn try_from(attrs: &[Attr]) -> Result<Self, String> {
+    pub fn try_from(attrs: &[Attr]) -> Result<Self> {
         let attrs = parse_attrs(attrs)?;
 
         let prefix = attrs.prefix;
@@ -32,7 +35,7 @@ impl CommandEnum {
                 | "SCREAMING_SNAKE_CASE"
                 | "kebab-case"
                 | "SCREAMING-KEBAB-CASE" => {}
-                _ => return Err("disallowed value".to_owned()),
+                _ => return Err(compile_error("disallowed value")),
             }
         }
         Ok(Self {

src/error.rs

@@ -0,0 +1,25 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+
+pub(crate) type Result<T, E = Error> = std::result::Result<T, E>;
+
+pub(crate) struct Error(TokenStream);
+
+pub(crate) fn compile_error<T>(data: T) -> Error
+where
+    T: ToTokens,
+{
+    Error(TokenStream::from(quote! { compile_error!(#data) }))
+}
+
+impl From<Error> for proc_macro::TokenStream {
+    fn from(Error(e): Error) -> Self {
+        e.into()
+    }
+}
+
+impl From<syn::Error> for Error {
+    fn from(e: syn::Error) -> Self {
+        Self(e.to_compile_error())
+    }
+}

src/lib.rs

@@ -3,6 +3,7 @@
 mod attr;
 mod command;
 mod command_enum;
+mod error;
 mod fields_parse;
 mod rename_rules;
 
@@ -16,22 +17,22 @@ use crate::{
     fields_parse::{impl_parse_args_named, impl_parse_args_unnamed},
 };
 use proc_macro::TokenStream;
-use quote::{quote, ToTokens};
+use quote::quote;
 use syn::{DeriveInput, Fields};
 
+pub(crate) use error::{compile_error, Error, Result};
+
 #[proc_macro_derive(BotCommands, attributes(command))]
 pub fn bot_commands_derive(tokens: TokenStream) -> TokenStream {
-    bot_commands_impl(tokens).unwrap_or_else(|err| err)
+    bot_commands_impl(tokens).unwrap_or_else(Error::into)
 }
 
-fn bot_commands_impl(tokens: TokenStream) -> Result<TokenStream, TokenStream> {
+fn bot_commands_impl(tokens: TokenStream) -> Result<TokenStream, Error> {
-    let input = syn::parse_macro_input::parse::<DeriveInput>(tokens)
+    let input = syn::parse_macro_input::parse::<DeriveInput>(tokens)?;
-        .map_err(|e| compile_error(e.to_compile_error()))?;
 
     let data_enum: &syn::DataEnum = get_enum_data(&input)?;
     let enum_attrs: Vec<Attr> = parse_attributes(&input.attrs)?;
-    let command_enum =
+    let command_enum = CommandEnum::try_from(enum_attrs.as_slice())?;
-        CommandEnum::try_from(enum_attrs.as_slice()).map_err(compile_error)?;
 
     let variants: Vec<&syn::Variant> = data_enum.variants.iter().collect();
 
@@ -45,8 +46,7 @@ fn bot_commands_impl(tokens: TokenStream) -> Result<TokenStream, TokenStream> {
             attrs.append(attrs_.data.as_mut());
         }
         let command =
-            Command::try_from(attrs.as_slice(), &variant.ident.to_string())
+            Command::try_from(attrs.as_slice(), &variant.ident.to_string())?;
-                .map_err(compile_error)?;
 
         variant_infos.push(command);
     }
@@ -176,16 +176,14 @@ fn impl_parse(
     }
 }
 
-fn get_enum_data(input: &DeriveInput) -> Result<&syn::DataEnum, TokenStream> {
+fn get_enum_data(input: &DeriveInput) -> Result<&syn::DataEnum> {
     match &input.data {
         syn::Data::Enum(data) => Ok(data),
         _ => Err(compile_error("TelegramBotCommand allowed only for enums")),
     }
 }
 
-fn parse_attributes(
+fn parse_attributes(input: &[syn::Attribute]) -> Result<Vec<Attr>> {
-    input: &[syn::Attribute],
-) -> Result<Vec<Attr>, TokenStream> {
     let mut enum_attrs = Vec::new();
     for attr in input.iter() {
         match attr.parse_args::<VecAttrs>() {
@@ -199,10 +197,3 @@ fn parse_attributes(
     }
     Ok(enum_attrs)
 }
-
-fn compile_error<T>(data: T) -> TokenStream
-where
-    T: ToTokens,
-{
-    TokenStream::from(quote! { compile_error!(#data) })
-}