Skip to content
This repository was archived by the owner on Aug 16, 2021. It is now read-only.

Improving procmacro error reporting #274

Closed
idubrov wants to merge 2 commits into from
Closed

Improving procmacro error reporting #274

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
75e15ff
Improving procmacro error reporting
idubrov Nov 15, 2018
b784a69
Merge branch 'master' into better-error-messages
davidbarsky Dec 28, 2018
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
125 changes: 88 additions & 37 deletions failure_derive/src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -6,11 +6,34 @@ extern crate synstructure;
#[macro_use]
extern crate quote;

use proc_macro2::TokenStream;
use proc_macro2::{TokenStream, Span};
use syn::spanned::Spanned;

#[derive(Debug)]
struct Error(TokenStream);

impl Error {
fn new(span: Span, message: &str) -> Error {
Error(quote_spanned! { span =>
compile_error!(#message);
})
}

fn into_tokens(self) -> TokenStream {
self.0
}
}

decl_derive!([Fail, attributes(fail, cause)] => fail_derive);

fn fail_derive(s: synstructure::Structure) -> TokenStream {
match fail_derive_impl(s) {
Err(err) => err.into_tokens(),
Ok(tokens) => tokens,
}
}

fn fail_derive_impl(s: synstructure::Structure) -> Result<TokenStream, Error> {
let make_dyn = if cfg!(has_dyn_trait) {
quote! { &dyn }
} else {
Expand Down Expand Up @@ -49,7 +72,7 @@ fn fail_derive(s: synstructure::Structure) -> TokenStream {
}
},
);
let display = display_body(&s).map(|display_body| {
let display = display_body(&s)?.map(|display_body| {
s.unbound_impl(
quote!(::failure::_core::fmt::Display),
quote! {
Expand All @@ -62,38 +85,48 @@ fn fail_derive(s: synstructure::Structure) -> TokenStream {
)
});

(quote! {
Ok(quote! {
#fail
#display
})
.into()
}

fn display_body(s: &synstructure::Structure) -> Option<quote::__rt::TokenStream> {
fn display_body(s: &synstructure::Structure) -> Result<Option<quote::__rt::TokenStream>, Error> {
let mut msgs = s.variants().iter().map(|v| find_error_msg(&v.ast().attrs));
if msgs.all(|msg| msg.is_none()) {
return None;
if msgs.all(|msg| msg.map(|m| m.is_none()).unwrap_or(true)) {
return Ok(None);
}

Some(s.each_variant(|v| {
let mut tokens = TokenStream::new();
for v in s.variants() {
let msg =
find_error_msg(&v.ast().attrs).expect("All variants must have display attribute.");
find_error_msg(&v.ast().attrs)?
.ok_or_else(|| Error::new(
v.ast().ident.span(),
"All variants must have display attribute."
))?;
if msg.nested.is_empty() {
panic!("Expected at least one argument to fail attribute");
return Err(Error::new(
msg.span(),
"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.")
return Err(Error::new(
msg.span(),
"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)
Ok(quote!(#bi))
}
syn::NestedMeta::Meta(syn::Meta::Word(ref id)) => {
let id_s = id.to_string();
Expand All @@ -102,59 +135,77 @@ fn display_body(s: &synstructure::Structure) -> Option<quote::__rt::TokenStream>
let bi = match v.bindings().get(idx) {
Some(bi) => bi,
None => {
panic!(
"display attempted to access field `{}` in `{}::{}` which \
return Err(Error::new(
arg.span(),
&format!(
"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 { "" }
);
idx,
s.ast().ident,
v.ast().ident,
v.bindings().len(),
if v.bindings().len() != 1 { "s" } else { "" }
)
));
}
};
return quote!(#bi);
return Ok(quote!(#bi));
}
}
for bi in v.bindings() {
if bi.ast().ident.as_ref() == Some(id) {
return quote!(#bi);
return Ok(quote!(#bi));
}
}
panic!(
"Couldn't find field `{}` in `{}::{}`",
id,
s.ast().ident,
v.ast().ident
);
return Err(Error::new(
arg.span(),
&format!(
"Couldn't find field `{}` in `{}::{}`",
id,
s.ast().ident,
v.ast().ident
)
));
}
_ => panic!("Invalid argument to fail attribute!"),
ref arg => {
return Err(Error::new(
arg.span(),
"Invalid argument to fail attribute!"
));
},
});
let args = args.collect::<Result<Vec<_>, _>>()?;

quote! {
return write!(f, #format_string #(, #args)*)
}
}))
let pat = v.pat();
tokens.extend(quote!(#pat => { return write!(f, #format_string #(, #args)*) }));
}
Ok(Some(tokens))
}

fn find_error_msg(attrs: &[syn::Attribute]) -> Option<syn::MetaList> {
fn find_error_msg(attrs: &[syn::Attribute]) -> Result<Option<syn::MetaList>, Error> {
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")
return Err(Error::new(
meta.span(),
"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")
return Err(Error::new(
meta.span(),
"fail attribute must take a list in parentheses"
));
}
}
}
}
}
error_msg
Ok(error_msg)
}

fn is_backtrace(bi: &&synstructure::BindingInfo) -> bool {
Expand Down