Default value for default_value (yeah, sounds awkward)

Author: CreepySkeleton

structopt-derive/src/attrs.rs

@@ -15,7 +15,9 @@ use heck::{CamelCase, KebabCase, MixedCase, ShoutySnakeCase, SnakeCase};
 use proc_macro2::{Span, TokenStream};
 use proc_macro_error::abort;
 use quote::{quote, quote_spanned, ToTokens};
-use syn::{self, ext::IdentExt, spanned::Spanned, Attribute, Expr, Ident, LitStr, MetaNameValue};
+use syn::{
+    self, ext::IdentExt, spanned::Spanned, Attribute, Expr, Ident, LitStr, MetaNameValue, Type,
+};
 
 #[derive(Clone)]
 pub enum Kind {
@@ -75,6 +77,7 @@ pub struct Attrs {
     name: Name,
     casing: Sp<CasingStyle>,
     env_casing: Sp<CasingStyle>,
+    ty: Option<Type>,
     doc_comment: Vec<Method>,
     methods: Vec<Method>,
     parser: Sp<Parser>,
@@ -215,11 +218,13 @@ impl Attrs {
     fn new(
         default_span: Span,
         name: Name,
+        ty: Option<Type>,
         casing: Sp<CasingStyle>,
         env_casing: Sp<CasingStyle>,
     ) -> Self {
         Self {
             name,
+            ty,
             casing,
             env_casing,
             doc_comment: vec![],
@@ -285,6 +290,31 @@ impl Attrs {
 
                 VerbatimDocComment(ident) => self.verbatim_doc_comment = Some(ident),
 
+                DefaultValue(ident, lit) => {
+                    let val = if let Some(lit) = lit {
+                        quote!(#lit)
+                    } else {
+                        let ty = if let Some(ty) = self.ty.as_ref() {
+                            ty
+                        } else {
+                            abort!(
+                                ident.span(),
+                                "#[structopt(default_value)] (without an argument) can be used \
+                                only on field level";
+
+                                note = "see \
+                                    https://docs.rs/structopt/0.3.5/structopt/#magical-methods")
+                        };
+                        quote_spanned!(ident.span()=> {
+                            let val = <#ty as ::std::default::Default>::default();
+                            let s = ::std::string::ToString::to_string(&val);
+                            ::std::boxed::Box::leak(s.into_boxed_str())
+                        })
+                    };
+
+                    self.methods.push(Method::new(ident, val));
+                }
+
                 About(ident, about) => {
                     self.about = Method::from_lit_or_env(ident, about, "CARGO_PKG_DESCRIPTION");
                 }
@@ -350,7 +380,7 @@ impl Attrs {
         argument_casing: Sp<CasingStyle>,
         env_casing: Sp<CasingStyle>,
     ) -> Self {
-        let mut res = Self::new(span, name, argument_casing, env_casing);
+        let mut res = Self::new(span, name, None, argument_casing, env_casing);
         res.push_attrs(attrs);
         res.push_doc_comment(attrs, "about");
 
@@ -377,6 +407,7 @@ impl Attrs {
         let mut res = Self::new(
             field.span(),
             Name::Derived(name.clone()),
+            Some(field.ty.clone()),
             struct_casing,
             env_casing,
         );

structopt-derive/src/parse.rs

@@ -39,6 +39,7 @@ pub enum StructOptAttr {
     // ident [= "string literal"]
     About(Ident, Option<LitStr>),
     Author(Ident, Option<LitStr>),
+    DefaultValue(Ident, Option<LitStr>),
 
     // ident = "string literal"
     Version(Ident, LitStr),
@@ -88,6 +89,7 @@ impl Parse for StructOptAttr {
                 match &*name_str.to_string() {
                     "rename_all" => Ok(RenameAll(name, lit)),
                     "rename_all_env" => Ok(RenameAllEnv(name, lit)),
+                    "default_value" => Ok(DefaultValue(name, Some(lit))),
 
                     "version" => {
                         check_empty_lit("version");
@@ -186,6 +188,7 @@ impl Parse for StructOptAttr {
                 "no_version" => Ok(NoVersion(name)),
                 "verbatim_doc_comment" => Ok(VerbatimDocComment(name)),
 
+                "default_value" => Ok(DefaultValue(name, None)),
                 "about" => (Ok(About(name, None))),
                 "author" => (Ok(Author(name, None))),
 

tests/default_value.rs

@@ -0,0 +1,12 @@
+use structopt::StructOpt;
+
+#[test]
+fn auto_default_value() {
+    #[derive(StructOpt, PartialEq, Debug)]
+    struct Opt {
+        #[structopt(default_value)]
+        arg: i32,
+    }
+    assert_eq!(Opt { arg: 0 }, Opt::from_iter(&["test"]));
+    assert_eq!(Opt { arg: 1 }, Opt::from_iter(&["test", "1"]));
+}