Function gen_trait_fn_body 

pub(crate) fn gen_trait_fn_body(
    make: &SyntaxFactory,
    func: &Fn,
    trait_path: &Path,
    adt: &Adt,
    trait_ref: Option<TraitRef<'_>>,
) -> Option<BlockExpr>

Generate custom trait bodies without default implementation where possible.

If func is defined within an existing impl block, pass TraitRef. Otherwise pass None.

Returns Option so that we can use ? rather than if let Some. Returning None means that generating a custom trait body failed, and the body will remain as todo! instead.