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use proc_macro::TokenStream;
use syn::{parse_macro_input, DeriveInput};
use quote::{quote, format_ident};
fn infer_is_error(variant: &syn::Variant) -> bool {
if let syn::Fields::Named(_) = variant.fields {
return false;
}
if variant.fields.len() != 1 {
return false;
}
let field = variant.fields.iter().next().unwrap();
if let syn::Type::Path(path) = &field.ty {
if path.path.segments.len() == 0 {
return false;
}
if path.path.segments.last().unwrap().ident == "Error" {
return true;
}
}
false
}
fn infer_format_str(variant: &syn::Variant) -> String {
let mut result = "".to_string();
let mut first = true;
for _var in variant.fields.iter() {
if first {
result += "{}";
first = false;
} else {
result += " {}";
}
}
result
}
struct ErrorVariant<'a> {
err: bool,
make_from: bool,
format_str: String,
variant: &'a syn::Variant,
}
fn parse_variant(variant: &syn::Variant) -> Result<ErrorVariant, TokenStream> {
match variant.fields {
syn::Fields::Named(_) => { return Err(TokenStream::from(syn::Error::new_spanned(variant, "Named fields not supported").to_compile_error())); }
syn::Fields::Unnamed(_) => {}
syn::Fields::Unit => { }
}
let mut attr: Option<_> = None;
for attr_cand in variant.attrs.iter() {
if attr_cand.path.is_ident("auto_error") {
if attr != None {
return Err(TokenStream::from(syn::Error::new_spanned(&attr_cand, "Duplicate occurence of auto_error attribute").to_compile_error()));
}
attr = Some(attr_cand);
}
}
let mut result = ErrorVariant {
err: infer_is_error(variant),
make_from: infer_is_error(variant),
format_str: infer_format_str(variant),
variant,
};
if let Some(attr) = attr {
let meta = attr.parse_meta().map_err(|e| e.to_compile_error())?;
let meta = match meta {
syn::Meta::List(list) => list,
_ => { return Err(TokenStream::from(syn::Error::new_spanned(&meta, "Incorrect auto_error arguments").to_compile_error())); },
};
for arg in meta.nested.iter() {
let arg = match arg {
syn::NestedMeta::Meta(arg) => arg,
_ => { return Err(TokenStream::from(syn::Error::new_spanned(arg, "Incorrect auto_error arguments").to_compile_error())); },
};
let arg = match arg {
syn::Meta::NameValue(arg) => arg,
_ => { return Err(TokenStream::from(syn::Error::new_spanned(arg, "Incorrect auto_error arguments").to_compile_error())); },
};
if arg.path.is_ident("err") {
result.err = match &arg.lit {
syn::Lit::Bool(v) => v.value,
_ => { return Err(TokenStream::from(syn::Error::new_spanned(&arg.lit, "Incorrect value for err, expected bool").to_compile_error())); },
};
} else if arg.path.is_ident("format_str") {
result.format_str = match &arg.lit {
syn::Lit::Str(v) => v.value(),
_ => { return Err(TokenStream::from(syn::Error::new_spanned(&arg.lit, "Incorrect value for format_str, expected string").to_compile_error())); },
};
} else if arg.path.is_ident("make_from") {
result.make_from = match &arg.lit {
syn::Lit::Bool(v) => v.value,
_ => { return Err(TokenStream::from(syn::Error::new_spanned(&arg.lit, "Incorrect value for make_from, expected bool").to_compile_error())); },
};
} else {
return Err(TokenStream::from(syn::Error::new_spanned(variant, "Unknown parameter").to_compile_error()));
}
}
}
if result.err && result.variant.fields.len() != 1 {
return Err(TokenStream::from(syn::Error::new_spanned(variant, "Wrapped errors should have exactly 1 argument").to_compile_error()));
}
if result.make_from && result.variant.fields.len() != 1 {
return Err(TokenStream::from(syn::Error::new_spanned(variant, "Can only derive from for variants with 1 field").to_compile_error()));
}
Ok(result)
}
#[proc_macro_derive(AutoError, attributes(auto_error))]
pub fn derive(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
let enumdecl = if let syn::Data::Enum(e) = input.data {
e
} else {
return TokenStream::from(syn::Error::new_spanned(&input.ident, "AutoError only supports enums").to_compile_error());
};
let error_ident = input.ident;
let error_variants: Result<Vec<_>, TokenStream> = enumdecl.variants.iter().map(|v| parse_variant(v)).collect();
let error_variants = match error_variants {
Ok(v) => v,
Err(e) => {return e}
};
let from_impls = error_variants.iter().map(|var| {
if !var.make_from {
return None;
}
let sourcetype = &var.variant.fields.iter().next().unwrap().ty;
let curvar = &var.variant.ident;
Some(quote!{
impl ::std::convert::From<#sourcetype> for #error_ident {
fn from (e: #sourcetype) -> Self {
Self::#curvar(e)
}
}
})
});
let display_branches = error_variants.iter().map(|var| {
let format_str = &var.format_str;
let curvar = &var.variant.ident;
let params: Vec<_> = var.variant.fields.iter().enumerate().map(|(i, _field)| {
format_ident!("f{}", i)
}).collect();
match var.variant.fields {
syn::Fields::Unnamed(_) => quote!{
Self::#curvar(#(#params),*) => f.write_fmt(format_args!(#format_str #(,#params)*)),
},
syn::Fields::Unit => quote!{
Self::#curvar => f.write_fmt(format_args!(#format_str)),
},
_ => panic!("Internal error (AutoError)")
}
});
let source_branches = error_variants.iter().map(|var| {
if !var.err {
return None;
}
let curvar = &var.variant.ident;
Some(quote!{
Self::#curvar(e) => Some(e),
})
});
TokenStream::from(quote! {
#(#from_impls)*
impl ::std::fmt::Display for #error_ident {
fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
match self {
#(#display_branches)*
}
}
}
impl ::std::error::Error for #error_ident {
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
#(#source_branches)*
_ => None,
}
}
}
})
}