Initial attempt- messy but passes tests

impl/src/attr.rs

@@ -2,9 +2,10 @@ use proc_macro2::{Delimiter, Group, Span, TokenStream, TokenTree};
 use quote::{format_ident, quote, ToTokens};
 use std::iter::FromIterator;
 use syn::parse::{Nothing, ParseStream};
+use syn::punctuated::Punctuated;
 use syn::{
     braced, bracketed, parenthesized, token, Attribute, Error, Ident, Index, LitInt, LitStr,
-    Result, Token,
+    Result, Token, TypeParamBound,
 };
 
 pub struct Attrs<'a> {
@@ -13,6 +14,7 @@ pub struct Attrs<'a> {
     pub backtrace: Option<&'a Attribute>,
     pub from: Option<&'a Attribute>,
     pub transparent: Option<Transparent<'a>>,
+    pub bound: Option<Bound<'a>>,
 }
 
 #[derive(Clone)]
@@ -29,13 +31,20 @@ pub struct Transparent<'a> {
     pub span: Span,
 }
 
+#[derive(Clone)]
+pub struct Bound<'a> {
+    pub original: &'a Attribute,
+    pub bounds: Punctuated<TypeParamBound, token::Add>,
+}
+
 pub fn get(input: &[Attribute]) -> Result<Attrs> {
     let mut attrs = Attrs {
         display: None,
         source: None,
         backtrace: None,
         from: None,
         transparent: None,
+        bound: None,
     };
 
     for attr in input {
@@ -70,6 +79,7 @@ pub fn get(input: &[Attribute]) -> Result<Attrs> {
 
 fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Result<()> {
     syn::custom_keyword!(transparent);
+    syn::custom_keyword!(bound);
 
     attr.parse_args_with(|input: ParseStream| {
         if let Some(kw) = input.parse::<Option<transparent>>()? {
@@ -84,6 +94,22 @@ fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Resu
                 span: kw.span,
             });
             return Ok(());
+        } else if input.parse::<Option<bound>>()?.is_some() {
+            if attrs.bound.is_some() {
+                return Err(Error::new_spanned(
+                    attr,
+                    "duplicate #[error(bound)] attribute",
+                ));
+            }
+            input.parse::<token::Eq>().map_err(|_| {
+                Error::new_spanned(attr, "\"bound\" keyword must be followed by '='")
+            })?;
+            let bound = Bound {
+                original: attr,
+                bounds: Punctuated::<TypeParamBound, token::Add>::parse_separated_nonempty(input)?,
+            };
+            attrs.bound = Some(bound);
+            return Ok(());
         }
 
         let display = Display {

impl/src/expand.rs

@@ -2,7 +2,9 @@ use crate::ast::{Enum, Field, Input, Struct};
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use syn::spanned::Spanned;
-use syn::{Data, DeriveInput, Member, PathArguments, Result, Type, Visibility};
+use syn::{
+    token, Data, DeriveInput, Member, PathArguments, Result, Type, Visibility, WherePredicate,
+};
 
 pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
     let input = Input::from_syn(node)?;
@@ -112,9 +114,27 @@ fn impl_struct(input: Struct) -> TokenStream {
         None
     };
     let display_impl = display_body.map(|body| {
+        let display_impl_generics = {
+            let mut lifetime_params = input.generics.lifetimes().peekable();
+            let mut type_params = input.generics.type_params().peekable();
+            if let Some(bounds) = &input.attrs.bound {
+                let bounds = std::iter::repeat(bounds).map(|x| &x.bounds);
+                quote! {
+                    <#(#type_params: #bounds),*>
+                }
+            } else if lifetime_params.peek().is_none() || type_params.peek().is_none() {
+                quote! {
+                    <#(#lifetime_params),* #(#type_params),*>
+                }
+            } else {
+                quote! {
+                    <#(#lifetime_params),* , #(#type_params),*>
+                }
+            }
+        };
         quote! {
             #[allow(unused_qualifications)]
-            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+            impl #display_impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
                 #[allow(
                     // Clippy bug: https://github.com/rust-lang/rust-clippy/issues/7422
                     clippy::nonstandard_macro_braces,
@@ -143,6 +163,17 @@ fn impl_struct(input: Struct) -> TokenStream {
     });
 
     let error_trait = spanned_error_trait(input.original);
+    let bounded_where_predicates: Vec<syn::WherePredicate> = input
+        .attrs
+        .bound
+        .as_ref()
+        .map(|bound| apply_type_bounds(input.generics.type_params(), bound))
+        .unwrap_or_default();
+    let where_clause = extend_where_clause(
+        where_clause.cloned(),
+        input.original.span(),
+        bounded_where_predicates.into_iter(),
+    );
 
     quote! {
         #[allow(unused_qualifications)]
@@ -302,9 +333,27 @@ fn impl_enum(input: Enum) -> TokenStream {
                 #ty::#ident #pat => #display
             }
         });
+        let display_impl_generics = {
+            let mut lifetime_params = input.generics.lifetimes().peekable();
+            let mut type_params = input.generics.type_params().peekable();
+            if let Some(bounds) = &input.attrs.bound {
+                let bounds = std::iter::repeat(bounds).map(|x| &x.bounds);
+                quote! {
+                    <#(#type_params: #bounds),*>
+                }
+            } else if lifetime_params.peek().is_none() || type_params.peek().is_none() {
+                quote! {
+                    <#(#lifetime_params),* #(#type_params),*>
+                }
+            } else {
+                quote! {
+                    <#(#lifetime_params),* , #(#type_params),*>
+                }
+            }
+        };
         Some(quote! {
             #[allow(unused_qualifications)]
-            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+            impl #display_impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
                 fn fmt(&self, __formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
                     #use_as_display
                     #[allow(
@@ -342,7 +391,17 @@ fn impl_enum(input: Enum) -> TokenStream {
     });
 
     let error_trait = spanned_error_trait(input.original);
-
+    let bounded_where_predicates: Vec<syn::WherePredicate> = input
+        .attrs
+        .bound
+        .as_ref()
+        .map(|bound| apply_type_bounds(input.generics.type_params(), bound))
+        .unwrap_or_default();
+    let where_clause = extend_where_clause(
+        where_clause.cloned(),
+        input.original.span(),
+        bounded_where_predicates.into_iter(),
+    );
     quote! {
         #[allow(unused_qualifications)]
         impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
@@ -424,3 +483,48 @@ fn spanned_error_trait(input: &DeriveInput) -> TokenStream {
     let error = quote_spanned!(last_span=> Error);
     quote!(#path #error)
 }
+
+/// Enhance a where clause with the given predicates, or create one with them if needed.
+/// When no new predicates are provided, return without alteration.
+fn extend_where_clause<TPredicates: std::iter::ExactSizeIterator<Item = syn::WherePredicate>>(
+    // Clause to be enhanced; created if absent when predicates are provided
+    where_clause: Option<syn::WhereClause>,
+    // Used to create span for new where clause if populating
+    where_span: proc_macro2::Span,
+    predicates: TPredicates,
+) -> Option<syn::WhereClause> {
+    // If we don't have any predicates to add, it doesn't matter if we
+    // have a where clause to extend or not; return whatever was given
+    if predicates.len() == 0 {
+        return where_clause;
+    }
+    Some(match where_clause {
+        // Extend the existing clause with the new predicates
+        Some(mut where_clause) => {
+            where_clause.predicates.extend(predicates);
+            where_clause
+        }
+        // No where clause provided; create a new one with the provided span
+        None => syn::WhereClause {
+            where_token: token::Where(where_span),
+            predicates: predicates.collect(),
+        },
+    })
+}
+
+fn apply_type_bounds<'a, TTypeParams: std::iter::Iterator<Item = &'a syn::TypeParam>>(
+    type_params: TTypeParams,
+    bound_attr: &'a crate::attr::Bound<'_>,
+) -> Vec<WherePredicate> {
+    let bounds = &bound_attr.bounds;
+    if bounds.is_empty() {
+        return Vec::new();
+    }
+    type_params
+        .map(move |p| {
+            let predicate = quote! { #p: #bounds };
+            syn::parse2::<syn::WherePredicate>(predicate)
+                .expect("quasiquote must create predicate bounds")
+        })
+        .collect()
+}

tests/test_error.rs

@@ -49,6 +49,56 @@ enum EnumError {
     Unit,
 }
 
+// TODO: This scenario should fail because no generics are present to be bounded; add ui test for it
+// #[derive(Error, Debug)]
+// #[error(bound = std::error::Error + 'static)]
+// enum WithoutBoundNoGeneric {
+//     #[error(transparent)]
+//     Variant(u32),
+// }
+
+#[derive(Error, Debug)]
+#[error(bound = std::fmt::Display + std::error::Error + 'static)]
+enum WithGeneric<T> {
+    Variant,
+    Generic(T),
+}
+
+#[derive(Error, Debug)]
+#[error(bound = std::fmt::Debug + std::error::Error + 'static)]
+enum WithGenericFrom<T> {
+    Variant,
+    Generic(#[from] T),
+}
+
+#[derive(Error, Debug)]
+#[error(bound = std::fmt::Display + std::fmt::Debug + std::error::Error + 'static)]
+enum WithGenericTransparent<T> {
+    #[error("variant")]
+    Variant,
+    #[error(transparent)]
+    Generic(#[from] T),
+}
+
+#[derive(Error, Debug)]
+#[error(bound = std::fmt::Display + std::error::Error + 'static)]
+#[error(transparent)]
+struct WithGenericStruct<T> {
+    #[from] inner: T,
+}
+
+impl<T: Display> Display for WithGeneric<T> {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+impl<T: Display> Display for WithGenericFrom<T> {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
 unimplemented_display!(BracedError);
 unimplemented_display!(TupleError);
 unimplemented_display!(UnitError);