Error Report on Unused Generic Parameters

[realquantumcookie]

Given the following code: code

pub trait SomeGenericTrait<Struct>{
    fn do_something(&self,data: &Struct) -> String;
}

pub struct SomeOtherStruct<T: SomeGenericTrait<S>, S>{
    placeholder: PhantomData<S>, //a placeholder variable is required here for the compiler to remain calm
    pub abc: T
}

If we were to remove the PhantomData<S> typed member placeholder in SomeOtherStruct, the compiler would spit out the following error

 |
7 | pub struct SomeOtherStruct<T: SomeGenericTrait<S>, S>{
  |                                                    ^ unused parameter
  |
  = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `S` to be a const parameter, use `const S: usize` instead

But if we follow the direction of the compiler and further remove the S type parameter in SomeOtherStruct, we get

error[[E0412]](https://doc.rust-lang.org/nightly/error-index.html#E0412): cannot find type `S` in this scope
 --> src/main.rs:7:48
  |
7 | pub struct SomeOtherStruct<T: SomeGenericTrait<S>>{
  |                                                ^ not found in this scope

For more information about this error, try `rustc --explain E0412`.

So I was wondering if there's a better way for the compiler to either:

1. not output an error since S was used as a type parameter for T
2. add support to place holder type parameters to enable semantics like pub struct SomeOtherStruct<T: SomeGenericTrait<_>>{...}

[realquantumcookie]

Would like to add a certain use case other than my small examples,
See the second proposal of Axum's issue 1116, tokio-rs/axum#1116