Default value

CHANGELOG.md

@@ -3,6 +3,7 @@
 * You don't have to apply `#[no_version]` to every `enum` variant anymore.
   Just annotate the `enum` and the setting will be propagated down
   ([#242](https://github.com/TeXitoi/structopt/issues/242)).
+* [Auto-default](https://docs.rs/structopt/0.3/structopt/#default-values).
 
 # v0.3.7 (2019-12-28)
 

Cargo.toml

@@ -29,6 +29,7 @@ travis-ci = { repository = "TeXitoi/structopt" }
 [dependencies]
 clap = { version = "2.33", default-features = false }
 structopt-derive = { path = "structopt-derive", version = "=0.4.0" }
+lazy_static = "1.4.0"
 
 [dev-dependencies]
 trybuild = "1.0.5"

src/lib.rs

@@ -41,6 +41,7 @@
 //! - Arguments
 //!     - [Type magic](#type-magic)
 //!     - [Specifying argument types](#specifying-argument-types)
+//!     - [Default values](#default-values)
 //!     - [Help messages](#help-messages)
 //!     - [Environment variable fallback](#environment-variable-fallback)
 //! - [Skipping fields](#skipping-fields)
@@ -259,7 +260,12 @@
 //!
 //!     Usable only on field-level.
 //!
-//! - [`rename_all`](#specifying-argument-types): [`rename_all = "kebab"/"snake"/"screaming-snake"/"camel"/"pascal"/"verbatim"]`
+//! - [`defautl_value`](#default-values): `default_value [= "default value"]`
+//!
+//!     Usable only on field-level.
+//!
+//! - [`rename_all`](#specifying-argument-types):
+//!     [`rename_all = "kebab"/"snake"/"screaming-snake"/"camel"/"pascal"/"verbatim"]`
 //!
 //!     Usable both on top level and field level.
 //!
@@ -283,7 +289,8 @@
 //!
 //!     Usable only on field-level.
 //!
-//! - [`rename_all_env`](##auto-deriving-environment-variables): [`rename_all_env = "kebab"/"snake"/"screaming-snake"/"camel"/"pascal"/"verbatim"]`
+//! - [`rename_all_env`](##auto-deriving-environment-variables):
+//!     [`rename_all_env = "kebab"/"snake"/"screaming-snake"/"camel"/"pascal"/"verbatim"]`
 //!
 //!     Usable both on top level and field level.
 //!
@@ -315,7 +322,7 @@
 //! If you would like to use a custom string parser other than `FromStr`, see
 //! the [same titled section](#custom-string-parsers) below.
 //!
-//! **Note:**
+//! **Important:**
 //! _________________
 //! Pay attention that *only literal occurrence* of this types is special, for example
 //! `Option<T>` is special while `::std::option::Option<T>` is not.
@@ -419,6 +426,47 @@
 //! # }
 //! ```
 //!
+//! ## Default values
+//!
+//! In clap, default values for options can be specified via [`Arg::default_value`].
+//!
+//! Of course, you can use as a raw method:
+//! ```
+//! # use structopt::StructOpt;
+//! #[derive(StructOpt)]
+//! struct Opt {
+//!     #[structopt(default_value = "", long)]
+//!     prefix: String
+//! }
+//! ```
+//!
+//! This is quite mundane and error-prone to type the `"..."` default by yourself,
+//! especially when the Rust ecosystem uses the [`Default`] trait for that.
+//! It would be wonderful to have `structopt` to take the `Default_default` and fill it
+//! for you. And yes, `structopt` can do that.
+//!
+//! Unfortunately, `default_value` takes `&str` but `Default::default`
+//! gives us some `Self` value. We need to map `Self` to `&str` somehow.
+//!
+//! `structopt` solves this problem via [`ToString`] trait.
+//!
+//! To be able to use auto-default the type must implement *both* `Default` and `ToString`:
+//!
+//! ```
+//! # use structopt::StructOpt;
+//! #[derive(StructOpt)]
+//! struct Opt {
+//!     // just leave the `= "..."` part and structopt will figure it for you
+//!     #[structopt(default_value, long)]
+//!     prefix: String // `String` implements both `Default` and `ToString`
+//! }
+//! ```
+//!
+//! [`Default`]: https://doc.rust-lang.org/std/default/trait.Default.html
+//! [`ToString`]: https://doc.rust-lang.org/std/string/trait.ToString.html
+//! [`Arg::default_value`]: https://docs.rs/clap/2.33.0/clap/struct.Arg.html#method.default_value
+//!
+//!
 //! ## Help messages
 //!
 //! In clap, help messages for the whole binary can be specified
@@ -583,8 +631,6 @@
 //! ///
 //! /// Hello!
 //! ```
-//!
-//! Summary
 //! ______________
 //!
 //! [`App::about`]:      https://docs.rs/clap/2/clap/struct.App.html#method.about
@@ -921,8 +967,9 @@ pub use structopt_derive::*;
 
 use std::ffi::OsString;
 
-/// Re-export of clap
+/// Re-exports
 pub use clap;
+pub use lazy_static;
 
 /// A struct that is converted from command line arguments.
 pub trait StructOpt {

structopt-derive/src/attrs.rs

@@ -14,8 +14,10 @@ use std::env;
 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 quote::{format_ident, quote, quote_spanned, ToTokens};
+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>,
@@ -216,6 +219,7 @@ impl Attrs {
         default_span: Span,
         name: Name,
         parent_attrs: Option<&Attrs>,
+        ty: Option<Type>,
         casing: Sp<CasingStyle>,
         env_casing: Sp<CasingStyle>,
     ) -> Self {
@@ -226,6 +230,7 @@ impl Attrs {
 
         Self {
             name,
+            ty,
             casing,
             env_casing,
             doc_comment: vec![],
@@ -291,6 +296,39 @@ 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")
+                        };
+
+                        let static_name = format_ident!("__STRUCTOPT_DEFAULT_VALUE_{}", fresh_id());
+
+                        quote_spanned!(ident.span()=> {
+                            ::structopt::lazy_static::lazy_static! {
+                                static ref #static_name: &'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())
+                                };
+                            }
+                            *#static_name
+                        })
+                    };
+
+                    self.methods.push(Method::new(ident, val));
+                }
+
                 About(ident, about) => {
                     self.about = Method::from_lit_or_env(ident, about, "CARGO_PKG_DESCRIPTION");
                 }
@@ -357,7 +395,7 @@ impl Attrs {
         argument_casing: Sp<CasingStyle>,
         env_casing: Sp<CasingStyle>,
     ) -> Self {
-        let mut res = Self::new(span, name, parent_attrs, argument_casing, env_casing);
+        let mut res = Self::new(span, name, parent_attrs, None, argument_casing, env_casing);
         res.push_attrs(attrs);
         res.push_doc_comment(attrs, "about");
 
@@ -386,6 +424,7 @@ impl Attrs {
             field.span(),
             Name::Derived(name.clone()),
             parent_attrs,
+            Some(field.ty.clone()),
             struct_casing,
             env_casing,
         );
@@ -603,6 +642,20 @@ impl Attrs {
     }
 }
 
+fn fresh_id() -> usize {
+    use std::cell::Cell;
+
+    thread_local! {
+        static NEXT_ID: Cell<usize> = Cell::new(0);
+    }
+
+    NEXT_ID.with(|next_id| {
+        let id = next_id.get();
+        next_id.set(id + 1);
+        id
+    })
+}
+
 /// replace all `:` with `, ` when not inside the `<>`
 ///
 /// `"author1:author2:author3" => "author1, author2, author3"`

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 +1,19 @@
+use structopt::StructOpt;
+
+mod utils;
+
+use utils::*;
+
+#[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"]));
+
+    let help = get_long_help::<Opt>();
+    assert!(help.contains("[default: 0]"));
+}