Preserve input body when attribute macro expansion fails (#1244)

[tyranron]

Resolves #1244

Synopsis

See #1244 (comment):

Describe the bug

#[graphql_object] attribute macro prevents syntax errors from being shown:

juniper/juniper_codegen/src/graphql_object/attr.rs

Line 35 in 8a69e14

"#[graphql_object] attribute is applicable to non-trait `impl` blocks only",

To Reproduce Steps to reproduce the behavior:

1. Add a valid `struct`;

2. Impl fields resolvers with `#[graphql_object]`;

3. Broke syntax inside method impl.

struct MyObject {
  my_field: i32,
}

#[graphql_object] // ERROR: #[graphql_object] attribute is applicable to non-trait `impl` blocks only
impl MyObject {
    fn my_field(&self) -> i32 {
        self.self.my_field // actual syntax error is not reported.
    }
}

Expected behavior

A syntax error will be shown.

Additional context

The problem is happening because attribute macro ignores errors of impl block parsing:

juniper/juniper_codegen/src/graphql_object/attr.rs

Line 22 in 8a69e14

if let Ok(mut ast) = syn::parse2::<syn::ItemImpl>(body) {

Solution

Preserve input body when proc_macro_attribute expansion fails.

Checklist

• Tests are added
• CHANGELOG entry is added
• Documentation is updated (not required)