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extern crate proc_macro2;
extern crate syn;
#[macro_use]
extern crate synstructure;
#[macro_use]
extern crate quote;
use proc_macro2::TokenStream;
decl_derive!([Fail, attributes(fail, cause)] => fail_derive);
fn fail_derive(s: synstructure::Structure) -> TokenStream {
let make_dyn = if cfg!(has_dyn_trait) {
quote! { &dyn }
} else {
quote! { & }
};
let cause_body = s.each_variant(|v| {
if let Some(cause) = v.bindings().iter().find(is_cause) {
quote!(return Some(::failure::AsFail::as_fail(#cause)))
} else {
quote!(return None)
}
});
let bt_body = s.each_variant(|v| {
if let Some(bi) = v.bindings().iter().find(is_backtrace) {
quote!(return Some(#bi))
} else {
quote!(return None)
}
});
let fail = s.unbound_impl(
quote!(::failure::Fail),
quote! {
#[allow(unreachable_code)]
fn cause(&self) -> ::failure::_core::option::Option<#make_dyn(::failure::Fail)> {
match *self { #cause_body }
None
}
#[allow(unreachable_code)]
fn backtrace(&self) -> ::failure::_core::option::Option<&::failure::Backtrace> {
match *self { #bt_body }
None
}
},
);
let display = display_body(&s).map(|display_body| {
s.unbound_impl(
quote!(::failure::_core::fmt::Display),
quote! {
#[allow(unreachable_code)]
fn fmt(&self, f: &mut ::failure::_core::fmt::Formatter) -> ::failure::_core::fmt::Result {
match *self { #display_body }
write!(f, "An error has occurred.")
}
},
)
});
(quote! {
#fail
#display
}).into()
}
fn display_body(s: &synstructure::Structure) -> Option<quote::__rt::TokenStream> {
let mut msgs = s.variants().iter().map(|v| find_error_msg(&v.ast().attrs));
if msgs.all(|msg| msg.is_none()) {
return None;
}
Some(s.each_variant(|v| {
let msg =
find_error_msg(&v.ast().attrs).expect("All variants must have display attribute.");
if msg.nested.is_empty() {
panic!("Expected at least one argument to fail attribute");
}
let format_string = match msg.nested[0] {
syn::NestedMeta::Meta(syn::Meta::NameValue(ref nv)) if nv.ident == "display" => {
nv.lit.clone()
}
_ => {
panic!("Fail attribute must begin `display = \"\"` to control the Display message.")
}
};
let args = msg.nested.iter().skip(1).map(|arg| match *arg {
syn::NestedMeta::Literal(syn::Lit::Int(ref i)) => {
let bi = &v.bindings()[i.value() as usize];
quote!(#bi)
}
syn::NestedMeta::Meta(syn::Meta::Word(ref id)) => {
let id_s = id.to_string();
if id_s.starts_with("_") {
if let Ok(idx) = id_s[1..].parse::<usize>() {
let bi = match v.bindings().get(idx) {
Some(bi) => bi,
None => {
panic!(
"display attempted to access field `{}` in `{}::{}` which \
does not exist (there are {} field{})",
idx,
s.ast().ident,
v.ast().ident,
v.bindings().len(),
if v.bindings().len() != 1 { "s" } else { "" }
);
}
};
return quote!(#bi);
}
}
for bi in v.bindings() {
if bi.ast().ident.as_ref() == Some(id) {
return quote!(#bi);
}
}
panic!(
"Couldn't find field `{}` in `{}::{}`",
id,
s.ast().ident,
v.ast().ident
);
}
_ => panic!("Invalid argument to fail attribute!"),
});
quote! {
return write!(f, #format_string #(, #args)*)
}
}))
}
fn find_error_msg(attrs: &[syn::Attribute]) -> Option<syn::MetaList> {
let mut error_msg = None;
for attr in attrs {
if let Some(meta) = attr.interpret_meta() {
if meta.name() == "fail" {
if error_msg.is_some() {
panic!("Cannot have two display attributes")
} else {
if let syn::Meta::List(list) = meta {
error_msg = Some(list);
} else {
panic!("fail attribute must take a list in parentheses")
}
}
}
}
}
error_msg
}
fn is_backtrace(bi: &&synstructure::BindingInfo) -> bool {
match bi.ast().ty {
syn::Type::Path(syn::TypePath {
qself: None,
path: syn::Path {
segments: ref path, ..
},
}) => path.last().map_or(false, |s| {
s.value().ident == "Backtrace" && s.value().arguments.is_empty()
}),
_ => false,
}
}
fn is_cause(bi: &&synstructure::BindingInfo) -> bool {
let mut found_cause = false;
for attr in &bi.ast().attrs {
if let Some(meta) = attr.interpret_meta() {
if meta.name() == "cause" {
if found_cause {
panic!("Cannot have two `cause` attributes");
}
found_cause = true;
}
if meta.name() == "fail" {
if let syn::Meta::List(ref list) = meta {
if let Some(ref pair) = list.nested.first() {
if let &&syn::NestedMeta::Meta(syn::Meta::Word(ref word)) = pair.value() {
if word == "cause" {
if found_cause {
panic!("Cannot have two `cause` attributes");
}
found_cause = true;
}
}
}
}
}
}
}
found_cause
}