Ignore clippy warnings in generated code

[7sDream]

Just meet clippy::unused_unit warning with a tiny test wit file:

because of generated code has redundant unit type/value in function signature/body, example:

    pub fn send(evt: EventParam<'_>) -> () {
        unsafe {
            // ... omit ...
            wit_import(result3_0, result3_1, result3_2, result3_3, result3_4);
            ()
        }
    }

This PR attempts to remove them, which in turn eliminates thoose warnings.

PS：Didn't find a good way to determine the return type during instruction translation, currently using string matching, if there is a better way please let me know.

[alexcrichton]

Personally I would prefer to keep the code generator clean so I would rather not be appeasing clippy in the generated code. Is there a way to get clippy to ignore all generated code?

[Liamolucko]

From rust-lang/rust-clippy#702, it looks like you can blanket #[allow(clippy)] #[allow(clippy::all)] (looks like it was changed since the question was first answered).

[7sDream]

I would prefer to keep the code generator clean

it makes sense.

Add a allow attr on the top of each generated module seems a better idea, will try to change to use this method today.

[7sDream]

#[allow(clippy::all)] attribute at module level just finished.

But I notice that there is a standalone option (in gen-rust-wasm), when user enable it, generated items will not be wrapped in a module, so they will continue face lint warnings.

Since I never use this option, It's pretty OK for me, but I'm not sure if this inconsistency is acceptable...