Repository: dtolnay/thiserror (rust)

Issue:
Generic type parameter on error
I ran into an error when having a generic parameter in my error type. So the setup is that I want to have a variant in my error enum such that it delegates to a different error, like below:

```rust
use thiserror::Error;

#[derive(Debug, Clone, Error)]
enum Error<E> {
    #[error("variant error")]
    Variant,
    #[error(transparent)]
    Delegate(#[from] E),
}
```

Unfortunately, I get an error about `E` not having a `Debug` instance:
```
error[E0277]: `E` doesn't implement `std::fmt::Debug`
 --> src/main.rs:4:1
  |
4 | enum Error<E> {
  | ^^^^^^^^^^ - help: consider restricting this bound: `E: std::fmt::Debug`
  | |
  | `E` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
  |
  = help: the trait `std::fmt::Debug` is not implemented for `E`
  = note: required because of the requirements on the impl of `std::fmt::Debug` for `Error<E>`
```

Could this be possible to do or is there something that is restricting this use case? :thinking: 

For now, I'm going to write out the implementations by hand, but it would be cool to delegate that work to `thiserror`.

Thanks for any help :pray: 

Pull request description:
Implied bounds for Display and Error impl
Closes #79. This PR enables an error type with generic parameter(s):

```rust
#[derive(Error, Debug)]
enum MyError<E, F> {
    #[error("thing {0}")]
    Variant(E),
    #[error("some error")]
    Delegate(#[source] F),
}
```

to emit appropriate impls, without further manual intervention to provide handwritten trait bounds. In the above case namely:

```rust
impl<E, F> Error for MyError<E, F>
where
    F: Error + 'static,
    Self: Debug + Display;

impl<E, F> Display for MyError<E, F>
where
    E: Display;
```

Implied bounds for `error(transparent)` are being done in an upcoming PR.

Patch:
diff --git a/impl/src/ast.rs b/impl/src/ast.rs
index 8698ecf..2aa7246 100644
--- a/impl/src/ast.rs
+++ b/impl/src/ast.rs
@@ -1,4 +1,5 @@
 use crate::attr::{self, Attrs};
+use crate::generics::ParamsInScope;
 use proc_macro2::Span;
 use syn::{
     Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Generics, Ident, Index, Member, Result,
@@ -38,6 +39,7 @@ pub struct Field<'a> {
     pub attrs: Attrs<'a>,
     pub member: Member,
     pub ty: &'a Type,
+    pub contains_generic: bool,
 }
 
 impl<'a> Input<'a> {
@@ -56,8 +58,9 @@ impl<'a> Input<'a> {
 impl<'a> Struct<'a> {
     fn from_syn(node: &'a DeriveInput, data: &'a DataStruct) -> Result<Self> {
         let mut attrs = attr::get(&node.attrs)?;
+        let scope = ParamsInScope::new(&node.generics);
         let span = attrs.span().unwrap_or_else(Span::call_site);
-        let fields = Field::multiple_from_syn(&data.fields, span)?;
+        let fields = Field::multiple_from_syn(&data.fields, &scope, span)?;
         if let Some(display) = &mut attrs.display {
             display.expand_shorthand(&fields);
         }
@@ -74,12 +77,13 @@ impl<'a> Struct<'a> {
 impl<'a> Enum<'a> {
     fn from_syn(node: &'a DeriveInput, data: &'a DataEnum) -> Result<Self> {
         let attrs = attr::get(&node.attrs)?;
+        let scope = ParamsInScope::new(&node.generics);
         let span = attrs.span().unwrap_or_else(Span::call_site);
         let variants = data
             .variants
             .iter()
             .map(|node| {
-                let mut variant = Variant::from_syn(node, span)?;
+                let mut variant = Variant::from_syn(node, &scope, span)?;
                 if let display @ None = &mut variant.attrs.display {
                     *display = attrs.display.clone();
                 }
@@ -102,28 +106,37 @@ impl<'a> Enum<'a> {
 }
 
 impl<'a> Variant<'a> {
-    fn from_syn(node: &'a syn::Variant, span: Span) -> Result<Self> {
+    fn from_syn(node: &'a syn::Variant, scope: &ParamsInScope<'a>, span: Span) -> Result<Self> {
         let attrs = attr::get(&node.attrs)?;
         let span = attrs.span().unwrap_or(span);
         Ok(Variant {
             original: node,
             attrs,
             ident: node.ident.clone(),
-            fields: Field::multiple_from_syn(&node.fields, span)?,
+            fields: Field::multiple_from_syn(&node.fields, scope, span)?,
         })
     }
 }
 
 impl<'a> Field<'a> {
-    fn multiple_from_syn(fields: &'a Fields, span: Span) -> Result<Vec<Self>> {
+    fn multiple_from_syn(
+        fields: &'a Fields,
+        scope: &ParamsInScope<'a>,
+        span: Span,
+    ) -> Result<Vec<Self>> {
         fields
             .iter()
             .enumerate()
-            .map(|(i, field)| Field::from_syn(i, field, span))
+            .map(|(i, field)| Field::from_syn(i, field, scope, span))
             .collect()
     }
 
-    fn from_syn(i: usize, node: &'a syn::Field, span: Span) -> Result<Self> {
+    fn from_syn(
+        i: usize,
+        node: &'a syn::Field,
+        scope: &ParamsInScope<'a>,
+        span: Span,
+    ) -> Result<Self> {
         Ok(Field {
             original: node,
             attrs: attr::get(&node.attrs)?,
@@ -134,6 +147,7 @@ impl<'a> Field<'a> {
                 })
             }),
             ty: &node.ty,
+            contains_generic: scope.intersects(&node.ty),
         })
     }
 }
diff --git a/impl/src/attr.rs b/impl/src/attr.rs
index 1ab1e28..9793586 100644
--- a/impl/src/attr.rs
+++ b/impl/src/attr.rs
@@ -1,5 +1,6 @@
 use proc_macro2::{Delimiter, Group, Span, TokenStream, TokenTree};
 use quote::{format_ident, quote, ToTokens};
+use std::collections::BTreeSet as Set;
 use std::iter::FromIterator;
 use syn::parse::{Nothing, ParseStream};
 use syn::{
@@ -21,6 +22,7 @@ pub struct Display<'a> {
     pub fmt: LitStr,
     pub args: TokenStream,
     pub has_bonus_display: bool,
+    pub implied_bounds: Set<(usize, Trait)>,
 }
 
 #[derive(Copy, Clone)]
@@ -29,6 +31,12 @@ pub struct Transparent<'a> {
     pub span: Span,
 }
 
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub enum Trait {
+    Debug,
+    Display,
+}
+
 pub fn get(input: &[Attribute]) -> Result<Attrs> {
     let mut attrs = Attrs {
         display: None,
@@ -91,6 +99,7 @@ fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Resu
             fmt: input.parse()?,
             args: parse_token_expr(input, false)?,
             has_bonus_display: false,
+            implied_bounds: Set::new(),
         };
         if attrs.display.is_some() {
             return Err(Error::new_spanned(
@@ -188,3 +197,12 @@ impl ToTokens for Display<'_> {
         });
     }
 }
+
+impl ToTokens for Trait {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.extend(match self {
+            Trait::Debug => quote!(std::fmt::Debug),
+            Trait::Display => quote!(std::fmt::Display),
+        });
+    }
+}
diff --git a/impl/src/expand.rs b/impl/src/expand.rs
index 19ce42a..3976ddd 100644
--- a/impl/src/expand.rs
+++ b/impl/src/expand.rs
@@ -1,8 +1,12 @@
 use crate::ast::{Enum, Field, Input, Struct};
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned, ToTokens};
+use std::collections::BTreeSet as Set;
 use syn::spanned::Spanned;
-use syn::{Data, DeriveInput, GenericArgument, Member, PathArguments, Result, Type, Visibility};
+use syn::{
+    parse_quote, Data, DeriveInput, GenericArgument, Member, PathArguments, Result, Type,
+    Visibility,
+};
 
 pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
     let input = Input::from_syn(node)?;
@@ -16,6 +20,8 @@ pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
 fn impl_struct(input: Struct) -> TokenStream {
     let ty = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut error_generics = input.generics.clone();
+    let error_where_clause = error_generics.make_where_clause();
 
     let source_body = if input.attrs.transparent.is_some() {
         let only_field = &input.fields[0].member;
@@ -24,6 +30,12 @@ fn impl_struct(input: Struct) -> TokenStream {
         })
     } else if let Some(source_field) = input.source_field() {
         let source = &source_field.member;
+        if source_field.contains_generic {
+            let ty = unoptional_type(source_field.ty);
+            error_where_clause
+                .predicates
+                .push(parse_quote!(#ty: std::error::Error + 'static));
+        }
         let asref = if type_is_option(source_field.ty) {
             Some(quote_spanned!(source.span()=> .as_ref()?))
         } else {
@@ -89,12 +101,14 @@ fn impl_struct(input: Struct) -> TokenStream {
         }
     });
 
+    let mut display_implied_bounds = &Set::new();
     let display_body = if input.attrs.transparent.is_some() {
         let only_field = &input.fields[0].member;
         Some(quote! {
             std::fmt::Display::fmt(&self.#only_field, __formatter)
         })
     } else if let Some(display) = &input.attrs.display {
+        display_implied_bounds = &display.implied_bounds;
         let use_as_display = if display.has_bonus_display {
             Some(quote! {
                 #[allow(unused_imports)]
@@ -114,9 +128,20 @@ fn impl_struct(input: Struct) -> TokenStream {
         None
     };
     let display_impl = display_body.map(|body| {
+        let mut display_generics = input.generics.clone();
+        let display_where_clause = display_generics.make_where_clause();
+        for &(field, bound) in display_implied_bounds {
+            let field = &input.fields[field];
+            if field.contains_generic {
+                let field_ty = field.ty;
+                display_where_clause
+                    .predicates
+                    .push(parse_quote!(#field_ty: #bound));
+            }
+        }
         quote! {
             #[allow(unused_qualifications)]
-            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+            impl #impl_generics std::fmt::Display for #ty #ty_generics #display_where_clause {
                 #[allow(clippy::used_underscore_binding)]
                 fn fmt(&self, __formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
                     #body
@@ -141,10 +166,15 @@ fn impl_struct(input: Struct) -> TokenStream {
     });
 
     let error_trait = spanned_error_trait(input.original);
+    if input.generics.type_params().next().is_some() {
+        error_where_clause
+            .predicates
+            .push(parse_quote!(Self: std::fmt::Debug + std::fmt::Display));
+    }
 
     quote! {
         #[allow(unused_qualifications)]
-        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
+        impl #impl_generics #error_trait for #ty #ty_generics #error_where_clause {
             #source_method
             #backtrace_method
         }
@@ -156,6 +186,8 @@ fn impl_struct(input: Struct) -> TokenStream {
 fn impl_enum(input: Enum) -> TokenStream {
     let ty = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut error_generics = input.generics.clone();
+    let error_where_clause = error_generics.make_where_clause();
 
     let source_method = if input.has_source() {
         let arms = input.variants.iter().map(|variant| {
@@ -168,6 +200,12 @@ fn impl_enum(input: Enum) -> TokenStream {
                 }
             } else if let Some(source_field) = variant.source_field() {
                 let source = &source_field.member;
+                if source_field.contains_generic {
+                    let ty = unoptional_type(source_field.ty);
+                    error_where_clause
+                        .predicates
+                        .push(parse_quote!(#ty: std::error::Error + 'static));
+                }
                 let asref = if type_is_option(source_field.ty) {
                     Some(quote_spanned!(source.span()=> .as_ref()?))
                 } else {
@@ -286,6 +324,8 @@ fn impl_enum(input: Enum) -> TokenStream {
     };
 
     let display_impl = if input.has_display() {
+        let mut display_generics = input.generics.clone();
+        let display_where_clause = display_generics.make_where_clause();
         let use_as_display = if input.variants.iter().any(|v| {
             v.attrs
                 .display
@@ -305,8 +345,12 @@ fn impl_enum(input: Enum) -> TokenStream {
             None
         };
         let arms = input.variants.iter().map(|variant| {
+            let mut display_implied_bounds = &Set::new();
             let display = match &variant.attrs.display {
-                Some(display) => display.to_token_stream(),
+                Some(display) => {
+                    display_implied_bounds = &display.implied_bounds;
+                    display.to_token_stream()
+                }
                 None => {
                     let only_field = match &variant.fields[0].member {
                         Member::Named(ident) => ident.clone(),
@@ -315,15 +359,25 @@ fn impl_enum(input: Enum) -> TokenStream {
                     quote!(std::fmt::Display::fmt(#only_field, __formatter))
                 }
             };
+            for &(field, bound) in display_implied_bounds {
+                let field = &variant.fields[field];
+                if field.contains_generic {
+                    let field_ty = field.ty;
+                    display_where_clause
+                        .predicates
+                        .push(parse_quote!(#field_ty: #bound));
+                }
+            }
             let ident = &variant.ident;
             let pat = fields_pat(&variant.fields);
             quote! {
                 #ty::#ident #pat => #display
             }
         });
+        let arms = arms.collect::<Vec<_>>();
         Some(quote! {
             #[allow(unused_qualifications)]
-            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+            impl #impl_generics std::fmt::Display for #ty #ty_generics #display_where_clause {
                 fn fmt(&self, __formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
                     #use_as_display
                     #[allow(unused_variables, deprecated, clippy::used_underscore_binding)]
@@ -355,10 +409,15 @@ fn impl_enum(input: Enum) -> TokenStream {
     });
 
     let error_trait = spanned_error_trait(input.original);
+    if input.generics.type_params().next().is_some() {
+        error_where_clause
+            .predicates
+            .push(parse_quote!(Self: std::fmt::Debug + std::fmt::Display));
+    }
 
     quote! {
         #[allow(unused_qualifications)]
-        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
+        impl #impl_generics #error_trait for #ty #ty_generics #error_where_clause {
             #source_method
             #backtrace_method
         }
diff --git a/impl/src/fmt.rs b/impl/src/fmt.rs
index e12e94b..f214ec8 100644
--- a/impl/src/fmt.rs
+++ b/impl/src/fmt.rs
@@ -1,8 +1,8 @@
 use crate::ast::Field;
-use crate::attr::Display;
+use crate::attr::{Display, Trait};
 use proc_macro2::TokenTree;
 use quote::{format_ident, quote_spanned};
-use std::collections::HashSet as Set;
+use std::collections::{BTreeSet as Set, HashMap as Map};
 use syn::ext::IdentExt;
 use syn::parse::{ParseStream, Parser};
 use syn::{Ident, Index, LitStr, Member, Result, Token};
@@ -12,7 +12,10 @@ impl Display<'_> {
     pub fn expand_shorthand(&mut self, fields: &[Field]) {
         let raw_args = self.args.clone();
         let mut named_args = explicit_named_args.parse2(raw_args).unwrap();
-        let fields: Set<Member> = fields.iter().map(|f| f.member.clone()).collect();
+        let mut member_index = Map::new();
+        for (i, field) in fields.iter().enumerate() {
+            member_index.insert(field.member.clone(), i);
+        }
 
         let span = self.fmt.span();
         let fmt = self.fmt.value();
@@ -20,6 +23,7 @@ impl Display<'_> {
         let mut out = String::new();
         let mut args = self.args.clone();
         let mut has_bonus_display = false;
+        let mut implied_bounds = Set::new();
 
         let mut has_trailing_comma = false;
         if let Some(TokenTree::Punct(punct)) = args.clone().into_iter().last() {
@@ -47,7 +51,7 @@ impl Display<'_> {
                         Ok(index) => Member::Unnamed(Index { index, span }),
                         Err(_) => return,
                     };
-                    if !fields.contains(&member) {
+                    if !member_index.contains_key(&member) {
                         out += &int;
                         continue;
                     }
@@ -82,9 +86,21 @@ impl Display<'_> {
                 args.extend(quote_spanned!(span=> ,));
             }
             args.extend(quote_spanned!(span=> #formatvar = #local));
-            if read.starts_with('}') && fields.contains(&member) {
-                has_bonus_display = true;
-                args.extend(quote_spanned!(span=> .as_display()));
+            if let Some(&field) = member_index.get(&member) {
+                let end_spec = match read.find('}') {
+                    Some(end_spec) => end_spec,
+                    None => return,
+                };
+                let bound = match read[..end_spec].chars().next_back() {
+                    Some('?') => Trait::Debug,
+                    Some(_) => Trait::Display,
+                    None => {
+                        has_bonus_display = true;
+                        args.extend(quote_spanned!(span=> .as_display()));
+                        Trait::Display
+                    }
+                };
+                implied_bounds.insert((field, bound));
             }
             has_trailing_comma = false;
         }
@@ -93,6 +109,7 @@ impl Display<'_> {
         self.fmt = LitStr::new(&out, self.fmt.span());
         self.args = args;
         self.has_bonus_display = has_bonus_display;
+        self.implied_bounds = implied_bounds;
     }
 }
 
diff --git a/impl/src/generics.rs b/impl/src/generics.rs
new file mode 100644
index 0000000..ff77e50
--- /dev/null
+++ b/impl/src/generics.rs
@@ -0,0 +1,41 @@
+use std::collections::BTreeSet as Set;
+use syn::{GenericArgument, Generics, Ident, PathArguments, Type};
+
+pub struct ParamsInScope<'a> {
+    names: Set<&'a Ident>,
+}
+
+impl<'a> ParamsInScope<'a> {
+    pub fn new(generics: &'a Generics) -> Self {
+        ParamsInScope {
+            names: generics.type_params().map(|param| &param.ident).collect(),
+        }
+    }
+
+    pub fn intersects(&self, ty: &Type) -> bool {
+        let mut found = false;
+        crawl(self, ty, &mut found);
+        found
+    }
+}
+
+fn crawl(in_scope: &ParamsInScope, ty: &Type, found: &mut bool) {
+    if let Type::Path(ty) = ty {
+        if ty.qself.is_none() {
+            if let Some(ident) = ty.path.get_ident() {
+                if in_scope.names.contains(ident) {
+                    *found = true;
+                }
+            }
+        }
+        for segment in &ty.path.segments {
+            if let PathArguments::AngleBracketed(arguments) = &segment.arguments {
+                for arg in &arguments.args {
+                    if let GenericArgument::Type(ty) = arg {
+                        crawl(in_scope, ty, found);
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/impl/src/lib.rs b/impl/src/lib.rs
index f0577d4..a4d5ae7 100644
--- a/impl/src/lib.rs
+++ b/impl/src/lib.rs
@@ -16,6 +16,7 @@ mod ast;
 mod attr;
 mod expand;
 mod fmt;
+mod generics;
 mod prop;
 mod valid;
 


Test patch:
diff --git a/tests/test_generics.rs b/tests/test_generics.rs
new file mode 100644
index 0000000..1297c76
--- /dev/null
+++ b/tests/test_generics.rs
@@ -0,0 +1,129 @@
+#![deny(clippy::all, clippy::pedantic)]
+
+use std::fmt::{self, Debug, Display};
+use thiserror::Error;
+
+pub struct NoFormat;
+
+#[derive(Debug)]
+pub struct DebugOnly;
+
+pub struct DisplayOnly;
+
+impl Display for DisplayOnly {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("display only")
+    }
+}
+
+#[derive(Debug)]
+pub struct DebugAndDisplay;
+
+impl Display for DebugAndDisplay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("debug and display")
+    }
+}
+
+// Should expand to:
+//
+//     impl<E> Display for EnumDebugField<E>
+//     where
+//         E: Debug;
+//
+//     impl<E> Error for EnumDebugField<E>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum EnumDebugGeneric<E> {
+    #[error("{0:?}")]
+    FatalError(E),
+}
+
+// Should expand to:
+//
+//     impl<E> Display for EnumFromGeneric<E>;
+//
+//     impl<E> Error for EnumFromGeneric<E>
+//     where
+//         EnumDebugGeneric<E>: Error + 'static,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum EnumFromGeneric<E> {
+    #[error("enum from generic")]
+    Source(#[from] EnumDebugGeneric<E>),
+}
+
+// Should expand to:
+//
+//     impl<HasDisplay, HasDebug, HasNeither> Display
+//         for EnumCompound<HasDisplay, HasDebug, HasNeither>
+//     where
+//         HasDisplay: Display,
+//         HasDebug: Debug;
+//
+//     impl<HasDisplay, HasDebug, HasNeither> Error
+//         for EnumCompound<HasDisplay, HasDebug, HasNeither>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error)]
+pub enum EnumCompound<HasDisplay, HasDebug, HasNeither> {
+    #[error("{0} {1:?}")]
+    DisplayDebug(HasDisplay, HasDebug),
+    #[error("{0}")]
+    Display(HasDisplay, HasNeither),
+    #[error("{1:?}")]
+    Debug(HasNeither, HasDebug),
+}
+
+impl<HasDisplay, HasDebug, HasNeither> Debug for EnumCompound<HasDisplay, HasDebug, HasNeither> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("EnumCompound")
+    }
+}
+
+#[test]
+fn test_display_enum_compound() {
+    let mut instance: EnumCompound<DisplayOnly, DebugOnly, NoFormat>;
+
+    instance = EnumCompound::DisplayDebug(DisplayOnly, DebugOnly);
+    assert_eq!(format!("{}", instance), "display only DebugOnly");
+
+    instance = EnumCompound::Display(DisplayOnly, NoFormat);
+    assert_eq!(format!("{}", instance), "display only");
+
+    instance = EnumCompound::Debug(NoFormat, DebugOnly);
+    assert_eq!(format!("{}", instance), "DebugOnly");
+}
+
+// Should expand to:
+//
+//     impl<E> Display for StructDebugGeneric<E>
+//     where
+//         E: Debug;
+//
+//     impl<E> Error for StructDebugGeneric<E>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+#[error("{underlying:?}")]
+pub struct StructDebugGeneric<E> {
+    pub underlying: E,
+}
+
+// Should expand to:
+//
+//     impl<E> Error for StructFromGeneric<E>
+//     where
+//         StructDebugGeneric<E>: Error + 'static,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub struct StructFromGeneric<E> {
+    #[from]
+    pub source: StructDebugGeneric<E>,
+}