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use proc_macro2::TokenStream; use quote::quote; use syn::{Attribute, LifetimeDef, TypeParam}; enum Variance { Covariant, Contravariant, Invariant, } pub trait HasVarianceAttribute { fn attrs(&mut self) -> &mut Vec<Attribute>; } impl HasVarianceAttribute for TypeParam { fn attrs(&mut self) -> &mut Vec<Attribute> { &mut self.attrs } } impl HasVarianceAttribute for LifetimeDef { fn attrs(&mut self) -> &mut Vec<Attribute> { &mut self.attrs } } pub fn apply(param: &mut dyn HasVarianceAttribute, base: TokenStream) -> TokenStream { let mut variance = Variance::Covariant; let attrs = param.attrs(); *attrs = attrs .drain(..) .filter(|attr| { if attr.path.is_ident("contra") && attr.tokens.is_empty() { variance = Variance::Contravariant; false } else if attr.path.is_ident("invariant") && attr.tokens.is_empty() { variance = Variance::Invariant; false } else { true } }) .collect(); let phantom = quote!(core::marker::PhantomData<#base>); match variance { Variance::Covariant => base, Variance::Contravariant => quote!(fn(#phantom)), Variance::Invariant => quote!(fn(#phantom) -> #phantom), } }