add basic support for generic structs and enums (#483)

Author: njeffords

CHANGELOG.md

@@ -1,3 +1,7 @@
+# unreleased
+
+* Add support for [generics in derive](https://github.com/TeXitoi/structopt/issues/128)
+
 # v0.3.21 (2020-11-30)
 
 * Fixed [another breakage](https://github.com/TeXitoi/structopt/issues/447)

src/lib.rs

@@ -52,6 +52,7 @@
 //!     - [Flattening subcommands](#flattening-subcommands)
 //! - [Flattening](#flattening)
 //! - [Custom string parsers](#custom-string-parsers)
+//! - [Generics](#generics)
 //!
 //!
 //!
@@ -1053,6 +1054,42 @@
 //! In the `try_from_*` variants, the function will run twice on valid input:
 //! once to validate, and once to parse. Hence, make sure the function is
 //! side-effect-free.
+//!
+//! ## Generics
+//!
+//! Generic structs and enums can be used. They require explicit trait bounds
+//! on any generic types that will be used by the `StructOpt` derive macro. In
+//! some cases, associated types will require additional bounds. See the usage
+//! of `FromStr` below for an example of this.
+//!
+//! ```
+//! # use structopt::StructOpt;
+//! use std::{fmt, str::FromStr};
+//!
+//! // a struct with single custom argument
+//! #[derive(StructOpt)]
+//! struct GenericArgs<T:FromStr> where <T as FromStr>::Err: fmt::Display + fmt::Debug {
+//!     generic_arg_1: String,
+//!     generic_arg_2: String,
+//!     custom_arg_1: T
+//! }
+//! ```
+//!
+//! or
+//!
+//! ```
+//! # use structopt::StructOpt;
+//! // a struct with multiple custom arguments in a substructure
+//! #[derive(StructOpt)]
+//! struct GenericArgs<T:StructOpt> {
+//!     generic_arg_1: String,
+//!     generic_arg_2: String,
+//!     #[structopt(flatten)]
+//!     custom_args: T
+//! }
+//! ```
+
+
 
 // those mains are for a reason
 #![allow(clippy::needless_doctest_main)]

structopt-derive/src/lib.rs

@@ -559,10 +559,10 @@ fn gen_augment_clap_enum(
     }
 }
 
-fn gen_from_clap_enum(name: &Ident) -> TokenStream {
+fn gen_from_clap_enum() -> TokenStream {
     quote! {
         fn from_clap(matches: &::structopt::clap::ArgMatches) -> Self {
-            <#name as ::structopt::StructOptInternal>::from_subcommand(matches.subcommand())
+            <Self as ::structopt::StructOptInternal>::from_subcommand(matches.subcommand())
                 .expect("structopt misuse: You likely tried to #[flatten] a struct \
                          that contains #[subcommand]. This is forbidden.")
         }
@@ -736,9 +736,9 @@ fn gen_from_subcommand(
 }
 
 #[cfg(feature = "paw")]
-fn gen_paw_impl(name: &Ident) -> TokenStream {
+fn gen_paw_impl(impl_generics: &ImplGenerics, name: &Ident, ty_generics: &TypeGenerics, where_clause: &TokenStream) -> TokenStream {
     quote! {
-        impl ::structopt::paw::ParseArgs for #name {
+        impl #impl_generics ::structopt::paw::ParseArgs for #name #ty_generics #where_clause {
             type Error = std::io::Error;
 
             fn parse_args() -> std::result::Result<Self, Self::Error> {
@@ -748,19 +748,109 @@ fn gen_paw_impl(name: &Ident) -> TokenStream {
     }
 }
 #[cfg(not(feature = "paw"))]
-fn gen_paw_impl(_: &Ident) -> TokenStream {
+fn gen_paw_impl(_: &ImplGenerics, _: &Ident, _: &TypeGenerics, _: &TokenStream) -> TokenStream {
     TokenStream::new()
 }
 
+fn split_structopt_generics_for_impl(generics: &Generics) -> (ImplGenerics, TypeGenerics, TokenStream) {
+    use syn::{ token::Add, TypeParamBound::Trait };
+
+    fn path_ends_with(path: &Path, ident: &str) -> bool {
+        path.segments.last().unwrap().ident == ident
+    }
+
+    fn type_param_bounds_contains(bounds: &Punctuated<TypeParamBound, Add>, ident: &str) -> bool {
+        for bound in bounds {
+            if let Trait(bound) = bound {
+                if path_ends_with(&bound.path, ident) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    struct TraitBoundAmendments{
+        tokens: TokenStream,
+        need_where: bool,
+        need_comma: bool,
+    }
+
+    impl TraitBoundAmendments {
+        fn new(where_clause: Option<&WhereClause>) -> Self {
+            let tokens = TokenStream::new();
+            let (need_where,need_comma) = if let Some(where_clause) = where_clause {
+                if where_clause.predicates.trailing_punct() {
+                    (false, false)
+                } else {
+                    (false, true)
+                }
+            } else {
+                (true, false)
+            };
+            Self{tokens, need_where, need_comma}
+        }
+
+        fn add(&mut self, amendment: TokenStream) {
+            if self.need_where {
+                self.tokens.extend(quote!{ where });
+                self.need_where = false;
+            }
+            if self.need_comma {
+                self.tokens.extend(quote!{ , });
+            }
+            self.tokens.extend(amendment);
+            self.need_comma = true;
+        }
+
+        fn into_tokens(self) -> TokenStream {
+            self.tokens
+        }
+    }
+
+    let mut trait_bound_amendments = TraitBoundAmendments::new(generics.where_clause.as_ref());
+
+    for param in &generics.params {
+        if let GenericParam::Type(param) = param {
+            let param_ident = &param.ident;
+            if type_param_bounds_contains(&param.bounds, "StructOpt") {
+                trait_bound_amendments.add(quote!{ #param_ident : ::structopt::StructOptInternal });
+            }
+        }
+    }
+
+    if let Some(where_clause) = &generics.where_clause {
+        for predicate in &where_clause.predicates {
+            if let WherePredicate::Type(predicate) = predicate {
+                let predicate_bounded_ty = &predicate.bounded_ty;
+                if type_param_bounds_contains(&predicate.bounds, "StructOpt") {
+                    trait_bound_amendments.add(quote!{ #predicate_bounded_ty : ::structopt::StructOptInternal });
+                }
+            }
+        }
+    }
+
+    let trait_bound_amendments = trait_bound_amendments.into_tokens();
+
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let where_clause = quote!{ #where_clause #trait_bound_amendments };
+
+    (impl_generics, ty_generics, where_clause)
+}
+
 fn impl_structopt_for_struct(
     name: &Ident,
     fields: &Punctuated<Field, Comma>,
     attrs: &[Attribute],
+    generics: &Generics,
 ) -> TokenStream {
+    let (impl_generics, ty_generics, where_clause) = split_structopt_generics_for_impl(&generics);
+
     let basic_clap_app_gen = gen_clap_struct(attrs);
     let augment_clap = gen_augment_clap(fields, &basic_clap_app_gen.attrs);
     let from_clap = gen_from_clap(name, fields, &basic_clap_app_gen.attrs);
-    let paw_impl = gen_paw_impl(name);
+    let paw_impl = gen_paw_impl(&impl_generics, name, &ty_generics, &where_clause);
 
     let clap_tokens = basic_clap_app_gen.tokens;
     quote! {
@@ -778,7 +868,7 @@ fn impl_structopt_for_struct(
         )]
         #[deny(clippy::correctness)]
         #[allow(dead_code, unreachable_code)]
-        impl ::structopt::StructOpt for #name {
+        impl #impl_generics ::structopt::StructOpt for #name #ty_generics #where_clause {
             #clap_tokens
             #from_clap
         }
@@ -797,7 +887,7 @@ fn impl_structopt_for_struct(
         )]
         #[deny(clippy::correctness)]
         #[allow(dead_code, unreachable_code)]
-        impl ::structopt::StructOptInternal for #name {
+        impl #impl_generics ::structopt::StructOptInternal for #name #ty_generics #where_clause {
             #augment_clap
             fn is_subcommand() -> bool { false }
         }
@@ -810,15 +900,19 @@ fn impl_structopt_for_enum(
     name: &Ident,
     variants: &Punctuated<Variant, Comma>,
     attrs: &[Attribute],
+    generics: &Generics,
 ) -> TokenStream {
+
+    let (impl_generics, ty_generics, where_clause) = split_structopt_generics_for_impl(&generics);
+
     let basic_clap_app_gen = gen_clap_enum(attrs);
     let clap_tokens = basic_clap_app_gen.tokens;
     let attrs = basic_clap_app_gen.attrs;
 
     let augment_clap = gen_augment_clap_enum(variants, &attrs);
-    let from_clap = gen_from_clap_enum(name);
+    let from_clap = gen_from_clap_enum();
     let from_subcommand = gen_from_subcommand(name, variants, &attrs);
-    let paw_impl = gen_paw_impl(name);
+    let paw_impl = gen_paw_impl(&impl_generics, name, &ty_generics, &where_clause);
 
     quote! {
         #[allow(unknown_lints)]
@@ -834,7 +928,7 @@ fn impl_structopt_for_enum(
             clippy::cargo
         )]
         #[deny(clippy::correctness)]
-        impl ::structopt::StructOpt for #name {
+        impl #impl_generics ::structopt::StructOpt for #name #ty_generics #where_clause {
             #clap_tokens
             #from_clap
         }
@@ -853,7 +947,7 @@ fn impl_structopt_for_enum(
         )]
         #[deny(clippy::correctness)]
         #[allow(dead_code, unreachable_code)]
-        impl ::structopt::StructOptInternal for #name {
+        impl #impl_generics ::structopt::StructOptInternal for #name #ty_generics #where_clause {
             #augment_clap
             #from_subcommand
             fn is_subcommand() -> bool { true }
@@ -885,8 +979,8 @@ fn impl_structopt(input: &DeriveInput) -> TokenStream {
         Struct(DataStruct {
             fields: syn::Fields::Named(ref fields),
             ..
-        }) => impl_structopt_for_struct(struct_name, &fields.named, &input.attrs),
-        Enum(ref e) => impl_structopt_for_enum(struct_name, &e.variants, &input.attrs),
+        }) => impl_structopt_for_struct(struct_name, &fields.named, &input.attrs, &input.generics),
+        Enum(ref e) => impl_structopt_for_enum(struct_name, &e.variants, &input.attrs, &input.generics),
         _ => abort_call_site!("structopt only supports non-tuple structs and enums"),
     }
 }