Ident sanitization causes name conflicts

[Daniel Schoepe]

Deduplicating consecutive underscores in sanitize_ident causes automatic derives to fail if one attempts to derive an implementation for a type Foo and _Foo. Here's a small example using abomonation_derive:

#[derive(Abomonation)]
pub struct Foo {
    x: String,
}

#[derive(Abomonation)]
pub struct _Foo {
    x: String,
}

Trying to compile this fails with the following error:

error[E0428]: the name `_DERIVE_abomonation_Abomonation_FOR_Foo` is defined multiple times
   --> dusty_example/src/main.rs:155:10
    |
149 | #[derive(Abomonation)]
    |          ----------- previous definition of the value `_DERIVE_abomonation_Abomonation_FOR_Foo` here
...
155 | #[derive(Abomonation)]
    |          ^^^^^^^^^^^ `_DERIVE_abomonation_Abomonation_FOR_Foo` redefined here
    |
    = note: `_DERIVE_abomonation_Abomonation_FOR_Foo` must be defined only once in the value namespace of this module

[Nika Layzell]

This would also be fixed by #41 allowing avoiding generating a custom _DERIVE_ name.

[Nika Layzell]

#43 has been merged, and published, which should allow avoiding this on rust >= 1.37 by using the new underscore_const method.

synstructure/src/lib.rs

Lines 1616 to 1660 in 864690b

 /// Configure whether to use `const _` instead of a generated const name in 
 /// code generated by `gen_impl` and `bound_impl`. 
 /// 
 /// This syntax is only supported by rust 1.37, and later versions. 
 /// 
 /// Defaults to `false` for backwards compatibility reasons. 
 /// 
 /// # Example 
 /// 
 /// ``` 
 /// # use synstructure::*; 
 /// let di: syn::DeriveInput = syn::parse_quote! { 
 ///     struct MyStruct; 
 /// }; 
 /// let mut s = Structure::new(&di); 
 /// 
 /// assert_eq!( 
 ///     s.underscore_const(true) 
 ///         .gen_impl(quote! { gen impl Trait for @Self { } }) 
 ///         .to_string(), 
 ///     quote! { 
 ///         const _: () = { 
 ///             impl Trait for MyStruct { } 
 ///         }; 
 ///     } 
 ///     .to_string() 
 /// ); 
 /// 
 /// assert_eq!( 
 ///     s.underscore_const(false) 
 ///         .gen_impl(quote! { gen impl Trait for @Self { } }) 
 ///         .to_string(), 
 ///     quote! { 
 ///         #[allow(non_upper_case_globals)] 
 ///         const _DERIVE_Trait_FOR_MyStruct: () = { 
 ///             impl Trait for MyStruct { } 
 ///         }; 
 ///     } 
 ///     .to_string() 
 /// ); 
 /// ``` 
 pub fn underscore_const(&mut self, enabled: bool) -> &mut Self { 
     self.underscore_const = enabled; 
     self 
 } 

This will become the default behaviour in 0.13, if we end up making a breaking change.