publishv0.1.0

4 files changed, 1146 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..1f0432c
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,425 @@
+#![doc = include_str!("../README.md")]
+use std::collections::HashMap;
+
+use proc_macro::TokenStream;
+use proc_macro2::Group;
+use proc_macro2::Ident;
+use quote::format_ident;
+use quote::quote;
+use quote::quote_spanned;
+use quote::ToTokens;
+use syn::parse_macro_input;
+use syn::punctuated::Punctuated;
+use syn::token::Brace;
+use syn::token::Gt;
+use syn::token::Lt;
+use syn::token::Trait;
+use syn::AngleBracketedGenericArguments;
+use syn::Block;
+use syn::Expr;
+use syn::GenericArgument;
+use syn::GenericParam;
+use syn::ImplItemMethod;
+use syn::ItemTrait;
+use syn::Path;
+use syn::PathArguments;
+use syn::PathSegment;
+use syn::Signature;
+use syn::Stmt;
+use syn::TraitBound;
+use syn::TraitItem;
+use syn::TraitItemMethod;
+use syn::Type;
+use syn::TypeParam;
+use syn::TypeParamBound;
+use syn::TypePath;
+use syn::Visibility;
+use syn_utils::TypeMatcher;
+
+use crate::parse_assoc_type::parse_assoc_type;
+use crate::parse_assoc_type::AssocTypeParseError;
+use crate::parse_trait_sig::parse_trait_signature;
+use crate::parse_trait_sig::MethodParseError;
+use crate::parse_trait_sig::SignatureChanges;
+use crate::parse_trait_sig::TypeTransform;
+mod parse_assoc_type;
+mod parse_trait_sig;
+mod syn_utils;
+
+macro_rules! abort {
+    ($span:expr, $message:literal) => {
+        quote_spanned! {$span => compile_error!($message);}.into()
+    };
+}
+
+#[proc_macro_attribute]
+pub fn dynamize(_attr: TokenStream, input: TokenStream) -> TokenStream {
+    let mut original_trait = parse_macro_input!(input as ItemTrait);
+    assert!(original_trait.auto_token.is_none());
+
+    let original_trait_name = original_trait.ident.clone();
+
+    let mut objectifiable_methods: Vec<(Signature, SignatureChanges)> = Vec::new();
+
+    let mut assoc_type_conversions: HashMap<&Ident, &Type> = HashMap::new();
+
+    for item in &original_trait.items {
+        if let TraitItem::Type(assoc_type) = item {
+            match parse_assoc_type(assoc_type) {
+                Err((_, AssocTypeParseError::NoIntoBound)) => continue,
+                Err((span, AssocTypeParseError::AssocTypeInBound)) => {
+                    return abort!(span, "dynamize does not support associated types here")
+                }
+                Err((span, AssocTypeParseError::GenericAssociatedType)) => {
+                    return abort!(
+                        span,
+                        "dynamize does not (yet?) support generic associated types"
+                    )
+                }
+                Ok((ident, type_)) => {
+                    assoc_type_conversions.insert(ident, type_);
+                }
+            }
+        }
+    }
+
+    for item in &original_trait.items {
+        if let TraitItem::Method(method) = item {
+            let mut signature = method.sig.clone();
+            match parse_trait_signature(&mut signature, &assoc_type_conversions) {
+                Err((_, MethodParseError::NonDispatchableMethod)) => continue,
+                Err((span, MethodParseError::UnconvertibleAssocType)) => {
+                    return abort!(
+                        span,
+                        "associated type is either undefined or doesn't have an Into bound"
+                    )
+                }
+                Err((span, MethodParseError::AssocTypeInInputs)) => {
+                    return abort!(
+                        span,
+                        "dynamize does not support associated types in parameter types"
+                    )
+                }
+                Err((
+                    span,
+                    MethodParseError::AssocTypeInUnsupportedReturnType
+                    | MethodParseError::UnconvertibleAssocTypeInFnInput,
+                )) => return abort!(span, "dynamize does not know how to convert this type"),
+                Err((span, MethodParseError::UnconvertibleAssocTypeInTraitBound)) => {
+                    return abort!(span, "dynamize does not support associated types here")
+                }
+                Err((span, MethodParseError::ImplTraitInInputs)) => {
+                    return abort!(
+                        span,
+                        "dynamize does not support impl here, change it to a method generic"
+                    )
+                }
+                Ok(parsed_method) => objectifiable_methods.push((signature, parsed_method)),
+            };
+        }
+    }
+
+    let mut method_impls: Vec<ImplItemMethod> = Vec::new();
+
+    let mut blanket_impl_attrs = Vec::new();
+    let mut dyn_trait_attrs = Vec::new();
+
+    // FUTURE: use Vec::drain_filter once it's stable
+    let mut i = 0;
+    while i < original_trait.attrs.len() {
+        if original_trait.attrs[i].path.is_ident("blanket_impl_attr") {
+            let attr = original_trait.attrs.remove(i);
+            let group: Group = match syn::parse2(attr.tokens) {
+                Ok(g) => g,
+                Err(err) => {
+                    return abort!(
+                        err.span(),
+                        "expected parenthesis: #[blanket_impl_attr(...)]"
+                    )
+                }
+            };
+            let tokens = group.stream();
+            blanket_impl_attrs.push(quote! {#[#tokens]});
+        } else if original_trait.attrs[i].path.is_ident("dyn_trait_attr") {
+            let attr = original_trait.attrs.remove(i);
+            let group: Group = match syn::parse2(attr.tokens) {
+                Ok(g) => g,
+                Err(err) => {
+                    return abort!(err.span(), "expected parenthesis: #[dyn_trait_attr(...)]")
+                }
+            };
+            let tokens = group.stream();
+            dyn_trait_attrs.push(quote! {#[#tokens]});
+        } else {
+            i += 1;
+        }
+    }
+
+    let mut dyn_trait = ItemTrait {
+        ident: format_ident!("Dyn{}", original_trait.ident),
+
+        attrs: Vec::new(),
+        vis: original_trait.vis.clone(),
+        unsafety: original_trait.unsafety,
+        auto_token: None,
+        trait_token: Trait::default(),
+        generics: original_trait.generics.clone(),
+        colon_token: None,
+        supertraits: Punctuated::new(),
+        brace_token: Brace::default(),
+        items: Vec::new(),
+    };
+
+    for (signature, parsed_method) in objectifiable_methods {
+        let mut new_method = TraitItemMethod {
+            attrs: Vec::new(),
+            sig: signature,
+            default: None,
+            semi_token: None,
+        };
+
+        let fun_name = &new_method.sig.ident;
+
+        let args = new_method
+            .sig
+            .inputs
+            .iter()
+            .enumerate()
+            .map(|(idx, arg)| match arg {
+                syn::FnArg::Receiver(_) => quote! {self},
+                syn::FnArg::Typed(pat_type) => match pat_type.pat.as_ref() {
+                    syn::Pat::Ident(ident) => match &parsed_method.inputs[idx] {
+                        None => ident.ident.to_token_stream(),
+                        Some(transforms) => {
+                            let args = (0..transforms.len()).map(|i| format_ident!("a{}", i));
+                            let mut calls: Vec<_> =
+                                args.clone().map(|i| i.into_token_stream()).collect();
+                            for i in 0..calls.len() {
+                                transforms[i].append_conversion(&mut calls[i]);
+                            }
+                            let move_opt = new_method.sig.asyncness.map(|_| quote! {move});
+                            quote!(#move_opt |#(#args),*| #ident(#(#calls),*))
+                        }
+                    },
+                    _other => {
+                        panic!("unexpected");
+                    }
+                },
+            });
+
+        // in order for a trait to be object-safe its methods may not have
+        // generics so we convert method generics into trait generics
+        if new_method
+            .sig
+            .generics
+            .params
+            .iter()
+            .any(|p| matches!(p, GenericParam::Type(_)))
+        {
+            // syn::punctuated::Punctuated doesn't have a remove(index)
+            // method so we firstly move all elements to a vector
+            let mut params = Vec::new();
+            while let Some(generic_param) = new_method.sig.generics.params.pop() {
+                params.push(generic_param.into_value());
+            }
+
+            // FUTURE: use Vec::drain_filter once it's stable
+            let mut i = 0;
+            while i < params.len() {
+                if matches!(params[i], GenericParam::Type(_)) {
+                    dyn_trait.generics.params.push(params.remove(i));
+                } else {
+                    i += 1;
+                }
+            }
+
+            new_method.sig.generics.params.extend(params);
+
+            if dyn_trait.generics.lt_token.is_none() {
+                dyn_trait.generics.lt_token = Some(Lt::default());
+                dyn_trait.generics.gt_token = Some(Gt::default());
+            }
+        }
+
+        let mut expr = quote!(#original_trait_name::#fun_name(#(#args),*));
+        if new_method.sig.asyncness.is_some() {
+            expr.extend(quote! {.await})
+        }
+        parsed_method.return_type.append_conversion(&mut expr);
+
+        method_impls.push(ImplItemMethod {
+            attrs: Vec::new(),
+            vis: Visibility::Inherited,
+            defaultness: None,
+            sig: new_method.sig.clone(),
+            block: Block {
+                brace_token: Brace::default(),
+                stmts: vec![Stmt::Expr(Expr::Verbatim(expr))],
+            },
+        });
+        dyn_trait.items.push(new_method.into());
+    }
+
+    let blanket_impl = generate_blanket_impl(&dyn_trait, &original_trait, &method_impls);
+
+    let expanded = quote! {
+        #original_trait
+
+        #(#dyn_trait_attrs)*
+        #dyn_trait
+
+        #(#blanket_impl_attrs)*
+        #blanket_impl
+    };
+    TokenStream::from(expanded)
+}
+
+fn generate_blanket_impl(
+    dyn_trait: &ItemTrait,
+    original_trait: &ItemTrait,
+    method_impls: &[ImplItemMethod],
+) -> proc_macro2::TokenStream {
+    let mut blanket_generics = dyn_trait.generics.clone();
+    let some_ident = format_ident!("__to_be_dynamized");
+    blanket_generics.params.push(GenericParam::Type(TypeParam {
+        attrs: Vec::new(),
+        ident: some_ident.clone(),
+        colon_token: None,
+        bounds: std::iter::once(TypeParamBound::Trait(TraitBound {
+            paren_token: None,
+            modifier: syn::TraitBoundModifier::None,
+            lifetimes: None,
+            path: Path {
+                leading_colon: None,
+                segments: std::iter::once(path_segment_for_trait(original_trait)).collect(),
+            },
+        }))
+        .collect(),
+        eq_token: None,
+        default: None,
+    }));
+    let (_, type_gen, _where) = dyn_trait.generics.split_for_impl();
+    let dyn_trait_name = &dyn_trait.ident;
+    quote! {
+        impl #blanket_generics #dyn_trait_name #type_gen for #some_ident {
+            #(#method_impls)*
+        }
+    }
+}
+
+struct AssocTypeMatcher;
+impl TypeMatcher<Path> for AssocTypeMatcher {
+    fn match_path<'a>(&self, path: &'a Path) -> Option<&'a Path> {
+        if path.segments.first().unwrap().ident == "Self" {
+            return Some(path);
+        }
+        None
+    }
+}
+
+impl<TypeMatch, PathMatch, T> TypeMatcher<T> for (TypeMatch, PathMatch)
+where
+    TypeMatch: Fn(&Type) -> Option<&T>,
+    PathMatch: Fn(&Path) -> Option<&T>,
+{
+    fn match_type<'a>(&self, t: &'a Type) -> Option<&'a T> {
+        self.0(t)
+    }
+
+    fn match_path<'a>(&self, path: &'a Path) -> Option<&'a T> {
+        self.1(path)
+    }
+}
+
+impl TypeTransform {
+    fn append_conversion(&self, stream: &mut proc_macro2::TokenStream) {
+        match self {
+            TypeTransform::Into => stream.extend(quote! {.into()}),
+            TypeTransform::Map(opt) => {
+                let mut inner = quote!(x);
+                opt.append_conversion(&mut inner);
+                stream.extend(quote! {.map(|x| #inner)})
+            }
+            TypeTransform::Result(ok, err) => {
+                if !matches!(ok.as_ref(), TypeTransform::NoOp) {
+                    let mut inner = quote!(x);
+                    ok.append_conversion(&mut inner);
+                    stream.extend(quote! {.map(|x| #inner)})
+                }
+                if !matches!(err.as_ref(), TypeTransform::NoOp) {
+                    let mut inner = quote!(x);
+                    err.append_conversion(&mut inner);
+                    stream.extend(quote! {.map_err(|x| #inner)})
+                }
+            }
+            _other => {}
+        }
+    }
+}
+
+/// Just a convenience trait for us to avoid match/if-let blocks everywhere.
+trait As<T> {
+    fn get_as(&self) -> Option<&T>;
+    fn get_as_mut(&mut self) -> Option<&mut T>;
+}
+
+impl As<AngleBracketedGenericArguments> for PathArguments {
+    fn get_as(&self) -> Option<&AngleBracketedGenericArguments> {
+        match self {
+            PathArguments::AngleBracketed(args) => Some(args),
+            _other => None,
+        }
+    }
+    fn get_as_mut(&mut self) -> Option<&mut AngleBracketedGenericArguments> {
+        match self {
+            PathArguments::AngleBracketed(args) => Some(args),
+            _other => None,
+        }
+    }
+}
+
+impl As<Type> for GenericArgument {
+    fn get_as(&self) -> Option<&Type> {
+        match self {
+            GenericArgument::Type(typearg) => Some(typearg),
+            _other => None,
+        }
+    }
+    fn get_as_mut(&mut self) -> Option<&mut Type> {
+        match self {
+            GenericArgument::Type(typearg) => Some(typearg),
+            _other => None,
+        }
+    }
+}
+
+fn path_segment_for_trait(sometrait: &ItemTrait) -> PathSegment {
+    PathSegment {
+        ident: sometrait.ident.clone(),
+        arguments: match sometrait.generics.params.is_empty() {
+            true => PathArguments::None,
+            false => PathArguments::AngleBracketed(AngleBracketedGenericArguments {
+                colon2_token: None,
+                lt_token: Lt::default(),
+                args: sometrait
+                    .generics
+                    .params
+                    .iter()
+                    .map(|param| match param {
+                        GenericParam::Type(type_param) => {
+                            GenericArgument::Type(Type::Path(TypePath {
+                                path: type_param.ident.clone().into(),
+                                qself: None,
+                            }))
+                        }
+                        GenericParam::Lifetime(lifetime_def) => {
+                            GenericArgument::Lifetime(lifetime_def.lifetime.clone())
+                        }
+                        GenericParam::Const(_) => todo!("const generic param not supported"),
+                    })
+                    .collect(),
+                gt_token: Gt::default(),
+            }),
+        },
+    }
+}
diff --git a/src/parse_assoc_type.rs b/src/parse_assoc_type.rs
new file mode 100644
index 0000000..37fd78c
--- /dev/null
+++ b/src/parse_assoc_type.rs
@@ -0,0 +1,119 @@
+use proc_macro2::Span;
+use syn::spanned::Spanned;
+use syn::{GenericArgument, Ident, PathArguments, PathSegment, TraitItemType, Type};
+
+use crate::syn_utils::{find_in_type, trait_bounds};
+use crate::AssocTypeMatcher;
+
+#[derive(Debug)]
+pub enum AssocTypeParseError {
+    AssocTypeInBound,
+    GenericAssociatedType,
+    NoIntoBound,
+}
+
+pub fn parse_assoc_type(
+    assoc_type: &TraitItemType,
+) -> Result<(&Ident, &Type), (Span, AssocTypeParseError)> {
+    for bound in trait_bounds(&assoc_type.bounds) {
+        if let PathSegment {
+            ident,
+            arguments: PathArguments::AngleBracketed(args),
+        } = bound.path.segments.first().unwrap()
+        {
+            if ident == "Into" && args.args.len() == 1 {
+                if let GenericArgument::Type(into_type) = args.args.first().unwrap() {
+                    // provide a better error message for type A: Into<Self::B>
+                    if find_in_type(into_type, &AssocTypeMatcher).is_some() {
+                        return Err((into_type.span(), AssocTypeParseError::AssocTypeInBound));
+                    }
+
+                    // TODO: support lifetime GATs (see the currently failing tests/gats.rs)
+                    if !assoc_type.generics.params.is_empty() {
+                        return Err((
+                            assoc_type.generics.params.span(),
+                            AssocTypeParseError::GenericAssociatedType,
+                        ));
+                    }
+
+                    return Ok((&assoc_type.ident, into_type));
+                }
+            }
+        }
+    }
+    Err((assoc_type.span(), AssocTypeParseError::NoIntoBound))
+}
+
+#[cfg(test)]
+mod tests {
+    use quote::quote;
+    use syn::{TraitItemType, Type};
+
+    use crate::parse_assoc_type::{parse_assoc_type, AssocTypeParseError};
+
+    #[test]
+    fn ok() {
+        let type1: TraitItemType = syn::parse2(quote! {
+            type A: Into<String>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_assoc_type(&type1),
+            Ok((id, Type::Path(path)))
+            if id == "A" && path.path.is_ident("String")
+        ));
+    }
+
+    #[test]
+    fn err_no_bound() {
+        let type1: TraitItemType = syn::parse2(quote! {
+            type A;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_assoc_type(&type1),
+            Err((_, AssocTypeParseError::NoIntoBound))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_bound() {
+        let type1: TraitItemType = syn::parse2(quote! {
+            type A: Into<Self::B>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_assoc_type(&type1),
+            Err((_, AssocTypeParseError::AssocTypeInBound))
+        ));
+    }
+
+    #[test]
+    fn err_gat_type() {
+        let type1: TraitItemType = syn::parse2(quote! {
+            type A<X>: Into<Foobar<X>>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_assoc_type(&type1),
+            Err((_, AssocTypeParseError::GenericAssociatedType))
+        ));
+    }
+
+    #[test]
+    fn err_gat_lifetime() {
+        let type1: TraitItemType = syn::parse2(quote! {
+            type A<'a>: Into<Foobar<'a>>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_assoc_type(&type1),
+            Err((_, AssocTypeParseError::GenericAssociatedType))
+        ));
+    }
+}
diff --git a/src/parse_trait_sig.rs b/src/parse_trait_sig.rs
new file mode 100644
index 0000000..55a3214
--- /dev/null
+++ b/src/parse_trait_sig.rs
@@ -0,0 +1,505 @@
+use std::collections::HashMap;
+
+use proc_macro2::Span;
+use syn::{
+    spanned::Spanned, FnArg, Ident, PathArguments, PredicateType, Receiver, ReturnType, Type,
+    TypePath, WherePredicate,
+};
+use syn::{GenericParam, Signature, TypeImplTrait, TypeParamBound};
+
+use crate::syn_utils::{find_in_path, find_in_type, trait_bounds, TypeMatcher};
+use crate::{As, AssocTypeMatcher};
+
+#[derive(Debug, Clone)]
+pub enum TypeTransform {
+    NoOp,
+    Into,
+    Map(Box<TypeTransform>),
+    Result(Box<TypeTransform>, Box<TypeTransform>),
+}
+
+#[derive(Debug)]
+pub enum MethodParseError {
+    NonDispatchableMethod,
+    AssocTypeInInputs,
+    ImplTraitInInputs,
+    AssocTypeInUnsupportedReturnType,
+    UnconvertibleAssocTypeInFnInput,
+    UnconvertibleAssocTypeInTraitBound,
+    UnconvertibleAssocType,
+}
+
+struct ImplTraitMatcher;
+impl TypeMatcher<TypeImplTrait> for ImplTraitMatcher {
+    fn match_type<'a>(&self, t: &'a Type) -> Option<&'a TypeImplTrait> {
+        if let Type::ImplTrait(impltrait) = t {
+            Some(impltrait)
+        } else {
+            None
+        }
+    }
+}
+
+pub struct SignatureChanges {
+    pub return_type: TypeTransform,
+    pub inputs: Vec<Option<Vec<TypeTransform>>>,
+}
+
+pub fn parse_trait_signature(
+    signature: &mut Signature,
+    assoc_type_conversions: &HashMap<&Ident, &Type>,
+) -> Result<SignatureChanges, (Span, MethodParseError)> {
+    let assoc_type_conversions = AssocTypeConversions(assoc_type_conversions);
+
+    if is_non_dispatchable(signature) {
+        return Err((signature.span(), MethodParseError::NonDispatchableMethod));
+    }
+
+    // provide better error messages for associated types in params
+    for input in &signature.inputs {
+        if let FnArg::Typed(pattype) = input {
+            if find_in_type(&pattype.ty, &AssocTypeMatcher).is_some() {
+                return Err((pattype.ty.span(), MethodParseError::AssocTypeInInputs));
+            }
+            if let Some(impl_trait) = find_in_type(&pattype.ty, &ImplTraitMatcher) {
+                return Err((impl_trait.span(), MethodParseError::ImplTraitInInputs));
+            }
+        }
+    }
+
+    let mut type_param_transforms = HashMap::new();
+    let mut input_transforms = Vec::new();
+
+    for generic_param in &mut signature.generics.params {
+        if let GenericParam::Type(type_param) = generic_param {
+            for bound in &mut type_param.bounds {
+                if let TypeParamBound::Trait(bound) = bound {
+                    if bound.path.segments.len() == 1 {
+                        let segment = bound.path.segments.first_mut().unwrap();
+
+                        if let PathArguments::Parenthesized(args) = &mut segment.arguments {
+                            if segment.ident == "Fn"
+                                || segment.ident == "FnOnce"
+                                || segment.ident == "FnMut"
+                            {
+                                let mut transforms = Vec::new();
+                                for input_type in &mut args.inputs {
+                                    match assoc_type_conversions.parse_type_path(input_type) {
+                                        Ok(ret_type) => {
+                                            transforms.push(ret_type);
+                                        }
+                                        Err(TransformError::UnconvertibleAssocType(span)) => {
+                                            return Err((
+                                                span,
+                                                MethodParseError::UnconvertibleAssocType,
+                                            ));
+                                        }
+                                        Err(TransformError::AssocTypeInUnsupportedType(span)) => {
+                                            return Err((
+                                                span,
+                                                MethodParseError::UnconvertibleAssocTypeInFnInput,
+                                            ));
+                                        }
+                                    }
+                                }
+                                if transforms.iter().any(|t| !matches!(t, TypeTransform::NoOp)) {
+                                    type_param_transforms.insert(&type_param.ident, transforms);
+                                }
+                            }
+                        }
+                    }
+                    if let Some(path) = find_in_path(&bound.path, &AssocTypeMatcher) {
+                        return Err((
+                            path.span(),
+                            MethodParseError::UnconvertibleAssocTypeInTraitBound,
+                        ));
+                    }
+                }
+            }
+        }
+    }
+
+    for input in &signature.inputs {
+        if let FnArg::Typed(pattype) = input {
+            if let Type::Path(path) = &*pattype.ty {
+                if let Some(ident) = path.path.get_ident() {
+                    input_transforms.push(type_param_transforms.get(ident).map(|x| (*x).clone()));
+                    continue;
+                }
+            }
+        }
+        input_transforms.push(None);
+    }
+
+    let return_type = match &mut signature.output {
+        ReturnType::Type(_, og_type) => match assoc_type_conversions.parse_type_path(og_type) {
+            Ok(ret_type) => ret_type,
+            Err(TransformError::UnconvertibleAssocType(span)) => {
+                return Err((span, MethodParseError::UnconvertibleAssocType));
+            }
+            Err(TransformError::AssocTypeInUnsupportedType(span)) => {
+                return Err((span, MethodParseError::AssocTypeInUnsupportedReturnType));
+            }
+        },
+        ReturnType::Default => TypeTransform::NoOp,
+    };
+    Ok(SignatureChanges {
+        return_type,
+        inputs: input_transforms,
+    })
+}
+
+struct AssocTypeConversions<'a>(&'a HashMap<&'a Ident, &'a Type>);
+
+enum TransformError {
+    UnconvertibleAssocType(Span),
+    AssocTypeInUnsupportedType(Span),
+}
+
+impl AssocTypeConversions<'_> {
+    fn parse_type_path(&self, type_: &mut Type) -> Result<TypeTransform, TransformError> {
+        let assoc_span = match find_in_type(type_, &AssocTypeMatcher) {
+            Some(path) => path.span(),
+            None => return Ok(TypeTransform::NoOp),
+        };
+
+        if let Type::Path(TypePath { path, qself: None }) = type_ {
+            let ident = &path.segments.first().unwrap().ident;
+
+            // TODO: support &mut dyn Iterator<Item = Self::A>
+            // conversion to  Box<dyn Iterator<Item = Whatever>> via .map(Into::into)
+
+            if ident == "Self" && path.segments.len() == 2 {
+                let ident = &path.segments.last().unwrap().ident;
+                *type_ = (*self
+                    .0
+                    .get(&ident)
+                    .ok_or_else(|| TransformError::UnconvertibleAssocType(ident.span()))?)
+                .clone();
+                return Ok(TypeTransform::Into);
+            } else if ident == "Option" && path.segments.len() == 1 {
+                let first_seg = path.segments.first_mut().unwrap();
+
+                if let Some(args) = first_seg.arguments.get_as_mut() {
+                    if args.args.len() == 1 {
+                        if let Some(generic_type) = args.args.first_mut().unwrap().get_as_mut() {
+                            if find_in_type(generic_type, &AssocTypeMatcher).is_some() {
+                                return Ok(TypeTransform::Map(
+                                    self.parse_type_path(generic_type)?.into(),
+                                ));
+                            }
+                        }
+                    }
+                }
+            } else if ident == "Result" && path.segments.len() == 1 {
+                let first_seg = path.segments.first_mut().unwrap();
+                if let Some(args) = first_seg.arguments.get_as_mut() {
+                    if args.args.len() == 2 {
+                        let mut args_iter = args.args.iter_mut();
+                        if let (Some(ok_type), Some(err_type)) = (
+                            args_iter.next().unwrap().get_as_mut(),
+                            args_iter.next().unwrap().get_as_mut(),
+                        ) {
+                            if find_in_type(ok_type, &AssocTypeMatcher).is_some()
+                                || find_in_type(err_type, &AssocTypeMatcher).is_some()
+                            {
+                                return Ok(TypeTransform::Result(
+                                    self.parse_type_path(ok_type)?.into(),
+                                    self.parse_type_path(err_type)?.into(),
+                                ));
+                            }
+                        }
+                    }
+                }
+            } else {
+                let last_seg = &path.segments.last().unwrap();
+                if last_seg.ident == "Result" {
+                    let last_seg = path.segments.last_mut().unwrap();
+                    if let Some(args) = last_seg.arguments.get_as_mut() {
+                        if args.args.len() == 1 {
+                            if let Some(generic_type) = args.args.first_mut().unwrap().get_as_mut()
+                            {
+                                if find_in_type(generic_type, &AssocTypeMatcher).is_some() {
+                                    return Ok(TypeTransform::Map(
+                                        self.parse_type_path(generic_type)?.into(),
+                                    ));
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        // the type contains an associated type but we
+        // don't know how to deal with it so we abort
+        Err(TransformError::AssocTypeInUnsupportedType(assoc_span))
+    }
+}
+
+fn is_non_dispatchable(signature: &Signature) -> bool {
+    // non-dispatchable: fn example(&self) where Self: Sized;
+    if let Some(where_clause) = &signature.generics.where_clause {
+        if where_clause
+            .predicates
+            .iter()
+            .any(bounds_self_and_has_bound_sized)
+        {
+            return true;
+        }
+    }
+
+    // non-dispatchable: fn example();
+    if signature.inputs.is_empty() {
+        return true;
+    }
+
+    // non-dispatchable: fn example(arg: Type);
+    if matches!(signature.inputs.first(), Some(FnArg::Typed(_))) {
+        return true;
+    }
+
+    // non-dispatchable: fn example(self);
+    if matches!(
+        signature.inputs.first(),
+        Some(FnArg::Receiver(Receiver {
+            reference: None,
+            ..
+        }))
+    ) {
+        return true;
+    }
+    false
+}
+
+/// Returns true if the bounded type is `Self` and the bounds contain `Sized`.
+fn bounds_self_and_has_bound_sized(predicate: &WherePredicate) -> bool {
+    matches!(
+        predicate,
+        WherePredicate::Type(PredicateType {
+            bounded_ty: Type::Path(TypePath { path, .. }),
+            bounds,
+            ..
+        })
+        if path.is_ident("Self")
+        && trait_bounds(bounds).any(|b| b.path.is_ident("Sized"))
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashMap;
+
+    use quote::{format_ident, quote};
+    use syn::{TraitItemMethod, Type};
+
+    use crate::parse_trait_sig::{
+        parse_trait_signature, MethodParseError, SignatureChanges, TypeTransform,
+    };
+
+    #[test]
+    fn ok_void() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Ok(SignatureChanges {
+                return_type: TypeTransform::NoOp,
+                ..
+            })
+        ));
+    }
+
+    #[test]
+    fn ok_assoc_type() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Self::A;
+        })
+        .unwrap();
+
+        let mut assoc_type_map = HashMap::new();
+        let ident = format_ident!("A");
+        let dest = Type::Verbatim(quote! {Example});
+        assoc_type_map.insert(&ident, &dest);
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &assoc_type_map),
+            Ok(SignatureChanges {
+                return_type: TypeTransform::Into,
+                ..
+            })
+        ));
+    }
+
+    #[test]
+    fn err_unconvertible_assoc_type() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Self::A;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::UnconvertibleAssocType))
+        ));
+    }
+
+    #[test]
+    fn err_non_dispatchable_assoc_function_no_args() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test();
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::NonDispatchableMethod))
+        ));
+    }
+
+    #[test]
+    fn err_non_dispatchable_assoc_function_with_args() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(arg: Type);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::NonDispatchableMethod))
+        ));
+    }
+
+    #[test]
+    fn err_non_dispatchable_consume_self() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(self);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::NonDispatchableMethod))
+        ));
+    }
+
+    #[test]
+    fn err_non_dispatchable_where_self_sized() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) where Self: Sized;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::NonDispatchableMethod))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_unsupported_return() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Foo<Self::A>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInUnsupportedReturnType))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_unsupported_return_in_opt() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Option<Foo<Self::A>>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInUnsupportedReturnType))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_unsupported_return_in_ok() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Result<Foo<Self::A>, Error>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInUnsupportedReturnType))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_unsupported_return_in_err() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self) -> Result<Ok, Foo<Self::A>>;
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInUnsupportedReturnType))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_input() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self, x: Self::A);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInInputs))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_input_opt() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self, x: Option<Self::A>);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::AssocTypeInInputs))
+        ));
+    }
+
+    #[test]
+    fn err_impl_in_input() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test(&self, arg: Option<impl SomeTrait>);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::ImplTraitInInputs))
+        ));
+    }
+
+    #[test]
+    fn err_assoc_type_in_generic() {
+        let mut type1: TraitItemMethod = syn::parse2(quote! {
+            fn test<F: Fn(Foo<Self::A>)>(&self, fun: F);
+        })
+        .unwrap();
+
+        assert!(matches!(
+            parse_trait_signature(&mut type1.sig, &Default::default()),
+            Err((_, MethodParseError::UnconvertibleAssocTypeInFnInput))
+        ));
+    }
+}
diff --git a/src/syn_utils.rs b/src/syn_utils.rs
new file mode 100644
index 0000000..4588186
--- /dev/null
+++ b/src/syn_utils.rs
@@ -0,0 +1,97 @@
+use syn::{
+    punctuated::Punctuated, GenericArgument, Path, PathArguments, ReturnType, TraitBound, Type,
+    TypeParamBound,
+};
+
+pub trait TypeMatcher<T> {
+    fn match_type<'a>(&self, t: &'a Type) -> Option<&'a T> {
+        None
+    }
+    fn match_path<'a>(&self, path: &'a Path) -> Option<&'a T> {
+        None
+    }
+}
+
+pub fn trait_bounds<T>(
+    bounds: &Punctuated<TypeParamBound, T>,
+) -> impl Iterator<Item = &TraitBound> {
+    bounds.iter().filter_map(|b| match b {
+        TypeParamBound::Trait(t) => Some(t),
+        TypeParamBound::Lifetime(_) => None,
+    })
+}
+
+pub fn find_in_type<'a, T>(t: &'a Type, matcher: &dyn TypeMatcher<T>) -> Option<&'a T> {
+    if let Some(ret) = matcher.match_type(t) {
+        return Some(ret);
+    }
+    match t {
+        Type::Array(array) => find_in_type(&array.elem, matcher),
+        Type::BareFn(fun) => {
+            for input in &fun.inputs {
+                if let Some(ret) = find_in_type(&input.ty, matcher) {
+                    return Some(ret);
+                }
+            }
+            if let ReturnType::Type(_, t) = &fun.output {
+                return find_in_type(t, matcher);
+            }
+            None
+        }
+        Type::Group(group) => find_in_type(&group.elem, matcher),
+        Type::ImplTrait(impltrait) => {
+            for bound in &impltrait.bounds {
+                if let TypeParamBound::Trait(bound) = bound {
+                    if let Some(ret) = find_in_path(&bound.path, matcher) {
+                        return Some(ret);
+                    }
+                }
+            }
+            None
+        }
+        Type::Infer(_) => None,
+        Type::Macro(_) => None,
+        Type::Paren(paren) => find_in_type(&paren.elem, matcher),
+        Type::Path(path) => find_in_path(&path.path, matcher),
+        Type::Ptr(ptr) => find_in_type(&ptr.elem, matcher),
+        Type::Reference(reference) => find_in_type(&reference.elem, matcher),
+        Type::Slice(slice) => find_in_type(&slice.elem, matcher),
+        Type::TraitObject(traitobj) => {
+            for bound in &traitobj.bounds {
+                if let TypeParamBound::Trait(bound) = bound {
+                    if let Some(ret) = find_in_path(&bound.path, matcher) {
+                        return Some(ret);
+                    }
+                }
+            }
+            None
+        }
+        Type::Tuple(tuple) => {
+            for elem in &tuple.elems {
+                if let Some(ret) = find_in_type(elem, matcher) {
+                    return Some(ret);
+                }
+            }
+            None
+        }
+        _other => None,
+    }
+}
+
+pub fn find_in_path<'a, T>(path: &'a Path, matcher: &dyn TypeMatcher<T>) -> Option<&'a T> {
+    if let Some(ret) = matcher.match_path(path) {
+        return Some(ret);
+    }
+    for segment in &path.segments {
+        if let PathArguments::AngleBracketed(args) = &segment.arguments {
+            for arg in &args.args {
+                if let GenericArgument::Type(t) = arg {
+                    if let Some(ret) = find_in_type(t, matcher) {
+                        return Some(ret);
+                    }
+                }
+            }
+        }
+    }
+    None
+}