Default value for default_value (yeah, sounds awkward)

Cargo.toml

@@ -53,11 +53,12 @@ exitcode              = "1.1.2"
 unicode-width         = "0.1.4"
 textwrap              = "0.11"
 indexmap              = "1.0.1"
-strsim    = { version = "0.9.0",  optional = true }
-yaml-rust = { version = "0.4.1",  optional = true }
-atty      = { version = "0.2.2",  optional = true }
-vec_map   = { version = "0.8", optional = true }
-term_size = { version = "1.0.0-beta1", optional = true }
+strsim      = { version = "0.9.0",  optional = true }
+yaml-rust   = { version = "0.4.1",  optional = true }
+atty        = { version = "0.2.2",  optional = true }
+vec_map     = { version = "0.8", optional = true }
+term_size   = { version = "1.0.0-beta1", optional = true }
+lazy_static = { version = "1", optional = true }
 clap_derive = { path = "./clap_derive", version = "3.0.0-beta.1", optional = true }
 
 [target.'cfg(not(windows))'.dependencies]
@@ -74,7 +75,7 @@ std         = [] # support for no_std in a backwards-compatible way
 suggestions = ["strsim"]
 color       = ["ansi_term", "atty"]
 wrap_help   = ["term_size", "textwrap/term_size"]
-derive      = ["clap_derive"]
+derive      = ["clap_derive", "lazy_static"]
 yaml        = ["yaml-rust"]
 unstable    = [] # for building with unstable clap features (doesn't require nightly Rust) (currently none)
 nightly     = [] # for building with unstable Rust features (currently none)

clap_derive/src/derives/attrs.rs

@@ -20,7 +20,7 @@ use heck::{CamelCase, KebabCase, MixedCase, ShoutySnakeCase, SnakeCase};
 use proc_macro2::{self, Span, TokenStream};
 use proc_macro_error::abort;
 use quote::{quote, quote_spanned, ToTokens};
-use syn::{self, ext::IdentExt, spanned::Spanned, Ident, LitStr, MetaNameValue};
+use syn::{self, ext::IdentExt, spanned::Spanned, Ident, LitStr, MetaNameValue, Type};
 
 /// Default casing style for generated arguments.
 pub const DEFAULT_CASING: CasingStyle = CasingStyle::Kebab;
@@ -87,6 +87,7 @@ 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>,
@@ -244,11 +245,13 @@ 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![],
@@ -314,6 +317,37 @@ 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()=> {
+                            ::clap::lazy_static::lazy_static! {
+                                static ref DEFAULT_VALUE: &'static str = {
+                                    let val = <#ty as ::std::default::Default>::default();
+                                    let s = ::std::string::ToString::to_string(&val);
+                                    ::std::boxed::Box::leak(s.into_boxed_str())
+                                };
+                            }
+                            *DEFAULT_VALUE
+                        })
+                    };
+
+                    self.methods.push(Method::new(ident, val));
+                }
+
                 About(ident, about) => {
                     self.about = Method::from_lit_or_env(ident, about, "CARGO_PKG_DESCRIPTION");
                 }
@@ -379,7 +413,7 @@ 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");
 
@@ -406,6 +440,7 @@ impl Attrs {
         let mut res = Self::new(
             field.span(),
             Name::Derived(name.clone()),
+            Some(field.ty.clone()),
             struct_casing,
             env_casing,
         );

clap_derive/src/derives/parse.rs

@@ -40,6 +40,7 @@ pub enum ClapAttr {
     // ident [= "string literal"]
     About(Ident, Option<LitStr>),
     Author(Ident, Option<LitStr>),
+    DefaultValue(Ident, Option<LitStr>),
 
     // ident = "string literal"
     Version(Ident, LitStr),
@@ -89,6 +90,7 @@ impl Parse for ClapAttr {
                 match &*name_str {
                     "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");
@@ -187,6 +189,7 @@ impl Parse for ClapAttr {
                 "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))),
 

clap_derive/tests/default_value.rs

@@ -0,0 +1,19 @@
+use clap::Clap;
+
+mod utils;
+
+use utils::*;
+
+#[test]
+fn auto_default_value() {
+    #[derive(Clap, PartialEq, Debug)]
+    struct Opt {
+        #[clap(default_value)]
+        arg: i32,
+    }
+    assert_eq!(Opt { arg: 0 }, Opt::parse_from(&["test"]));
+    assert_eq!(Opt { arg: 1 }, Opt::parse_from(&["test", "1"]));
+
+    let help = get_long_help::<Opt>();
+    assert!(help.contains("[default: 0]"));
+}

clap_derive/tests/ui/raw.stderr

@@ -1,7 +1,7 @@
 error: `#[clap(raw(...))` attributes are removed, they are replaced with raw methods
 
   = help: if you meant to call `clap::Arg::raw()` method you should use bool literal, like `raw(true)` or `raw(false)`
-  = note: if you need to call `clap::Arg/App::case_insensitive` method you can do it like this: #[structopt(case_insensitive = true)]
+  = note: if you need to call `clap::Arg/App::case_insensitive` method you can do it like this: #[clap(case_insensitive = true)]
 
   --> $DIR/raw.rs:13:12
    |
@@ -11,7 +11,7 @@ error: `#[clap(raw(...))` attributes are removed, they are replaced with raw met
 error: `#[clap(raw(...))` attributes are removed, they are replaced with raw methods
 
   = help: if you meant to call `clap::Arg::raw()` method you should use bool literal, like `raw(true)` or `raw(false)`
-  = note: if you need to call `clap::Arg/App::requires_if` method you can do it like this: #[structopt(requires_if("one", "two"))]
+  = note: if you need to call `clap::Arg/App::requires_if` method you can do it like this: #[clap(requires_if("one", "two"))]
 
   --> $DIR/raw.rs:19:12
    |

src/lib.rs

@@ -457,7 +457,11 @@ pub use yaml_rust::YamlLoader;
 
 #[cfg(feature = "derive")]
 #[cfg_attr(feature = "derive", doc(hidden))]
-pub use clap_derive::*;
+pub use clap_derive::{self, *};
+
+#[cfg(feature = "derive")]
+#[cfg_attr(feature = "derive", doc(hidden))]
+pub use lazy_static;
 
 use std::result::Result as StdResult;