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Use field span in rustc_macros when emitting decode call #92011

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Jan 3, 2022
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Use field span in rustc_macros when emitting decode call #92011

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Use field span in rustc_macros when emitting decode call 
This will cause backtraces to point to the location of
the field in the struct/enum, rather than the derive macro.

This makes it clear which field was being decoded when the
backtrace was captured (which is especially useful if
there are multiple fields with the same type).
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@Aaron1011
Aaron1011 committed Dec 16, 2021
commit 44fdb98764e13faecbe2c307798bbe97348bddde
26 changes: 15 additions & 11 deletions compiler/rustc_macros/src/serialize.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
use proc_macro2::TokenStream;
use quote::quote;
use quote::{quote, quote_spanned};
use syn::parse_quote;
use syn::spanned::Spanned;

pub fn type_decodable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
let decoder_ty = quote! { __D };
Expand Down Expand Up @@ -104,27 +105,30 @@ fn decodable_body(
}

fn decode_field(field: &syn::Field, index: usize, is_struct: bool) -> proc_macro2::TokenStream {
let field_span = field.ident.as_ref().map_or(field.ty.span(), |ident| ident.span());

let decode_inner_method = if let syn::Type::Reference(_) = field.ty {
quote! { ::rustc_middle::ty::codec::RefDecodable::decode }
} else {
quote! { ::rustc_serialize::Decodable::decode }
};
let (decode_method, opt_field_name) = if is_struct {
let field_name = field.ident.as_ref().map_or_else(|| index.to_string(), |i| i.to_string());
(
proc_macro2::Ident::new("read_struct_field", proc_macro2::Span::call_site()),
quote! { #field_name, },
)
(proc_macro2::Ident::new("read_struct_field", field_span), quote! { #field_name, })
} else {
(
proc_macro2::Ident::new("read_enum_variant_arg", proc_macro2::Span::call_site()),
quote! {},
)
(proc_macro2::Ident::new("read_enum_variant_arg", field_span), quote! {})
};

let __decoder = quote! { __decoder };
// Use the span of the field for the method call, so
// that backtraces will point to the field.
let decode_call = quote_spanned! {field_span=>
::rustc_serialize::Decoder::#decode_method(
#__decoder, #opt_field_name #decode_inner_method)
};

quote! {
match ::rustc_serialize::Decoder::#decode_method(
__decoder, #opt_field_name #decode_inner_method) {
match #decode_call {
::std::result::Result::Ok(__res) => __res,
::std::result::Result::Err(__err) => return ::std::result::Result::Err(__err),
}
Expand Down