feat(derive): Specify defaults by native expressions

Right now
- `default_value="something"` is a raw method
- `default_value` uses native types

This commit splits the meanings
- `default_value="something"` is a raw method
- `default_value_t` uses `T::default()`
- `default_value_t=expr` uses an expression that evaluates to `T`

This is meant to mirror the `value_of` / `value_of_t` API.

At the moment, this is limited to `T: Display` to work with clap's
default system.  Something we can look at in the future is a way to
loosen that restriction.  One quick win is to specialize when `arg_enum`
is set.  The main downside is complicating the processing of attributes
because it then means we need some processed before others.

Since this builds on `clap`s existing default system, this also means
users do not get any performance gains out of using `default_value_t`,
since we still need to parse it but we also need to convert it to a
string.

Fixes clap-rs#1694

clap_derive/src/attrs.rs

@@ -340,34 +340,41 @@ impl Attrs {
                 VerbatimDocComment(ident) => self.verbatim_doc_comment = Some(ident),
 
                 DefaultValue(ident, lit) => {
-                    let val = if let Some(lit) = lit {
-                        quote!(#lit)
+                    let val = quote!(#lit);
+                    self.methods.push(Method::new(ident, val));
+                }
+
+                DefaultValueT(ident, expr) => {
+                    let val = if let Some(expr) = expr {
+                        quote!(#expr)
                     } else {
                         let ty = if let Some(ty) = self.ty.as_ref() {
                             ty
                         } else {
                             abort!(
                                 ident,
-                                "#[clap(default_value)] (without an argument) can be used \
+                                "#[clap(default_value_t)] (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
-                        })
+                        quote!(<#ty as ::std::default::Default>::default())
                     };
 
-                    self.methods.push(Method::new(ident, val));
+                    let val = quote_spanned!(ident.span()=> {
+                        clap::lazy_static::lazy_static! {
+                            static ref DEFAULT_VALUE: &'static str = {
+                                let val = #val;
+                                let s = ::std::string::ToString::to_string(&val);
+                                ::std::boxed::Box::leak(s.into_boxed_str())
+                            };
+                        }
+                        *DEFAULT_VALUE
+                    });
+
+                    let raw_ident = Ident::new("default_value", ident.span());
+                    self.methods.push(Method::new(raw_ident, val));
                 }
 
                 About(ident, about) => {

clap_derive/src/parse.rs

@@ -26,7 +26,7 @@ pub enum ClapAttr {
     About(Ident, Option<LitStr>),
     Author(Ident, Option<LitStr>),
     Version(Ident, Option<LitStr>),
-    DefaultValue(Ident, Option<LitStr>),
+    DefaultValue(Ident, LitStr),
 
     // ident = "string literal"
     RenameAllEnv(Ident, LitStr),
@@ -41,6 +41,7 @@ pub enum ClapAttr {
 
     // ident = arbitrary_expr
     NameExpr(Ident, Expr),
+    DefaultValueT(Ident, Option<Expr>),
 
     // ident(arbitrary_expr,*)
     MethodCall(Ident, Vec<Expr>),
@@ -75,7 +76,7 @@ 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))),
+                    "default_value" => Ok(DefaultValue(name, lit)),
 
                     "version" => {
                         check_empty_lit("version");
@@ -105,13 +106,11 @@ impl Parse for ClapAttr {
                 }
             } else {
                 match input.parse::<Expr>() {
-                    Ok(expr) => {
-                        if name_str == "skip" {
-                            Ok(Skip(name, Some(expr)))
-                        } else {
-                            Ok(NameExpr(name, expr))
-                        }
-                    }
+                    Ok(expr) => match &*name_str {
+                        "skip" => Ok(Skip(name, Some(expr))),
+                        "default_value_t" => Ok(DefaultValueT(name, Some(expr))),
+                        _ => Ok(NameExpr(name, expr)),
+                    },
 
                     Err(_) => abort! {
                         assign_token,
@@ -176,7 +175,7 @@ impl Parse for ClapAttr {
                 "external_subcommand" => Ok(ExternalSubcommand(name)),
                 "verbatim_doc_comment" => Ok(VerbatimDocComment(name)),
 
-                "default_value" => Ok(DefaultValue(name, None)),
+                "default_value_t" => Ok(DefaultValueT(name, None)),
                 "about" => (Ok(About(name, None))),
                 "author" => (Ok(Author(name, None))),
                 "version" => Ok(Version(name, None)),

clap_derive/tests/default_value.rs

@@ -5,10 +5,10 @@ mod utils;
 use utils::*;
 
 #[test]
-fn auto_default_value() {
+fn default_value() {
     #[derive(Clap, PartialEq, Debug)]
     struct Opt {
-        #[clap(default_value)]
+        #[clap(default_value = "0")]
         arg: i32,
     }
     assert_eq!(Opt { arg: 0 }, Opt::parse_from(&["test"]));
@@ -17,3 +17,31 @@ fn auto_default_value() {
     let help = get_long_help::<Opt>();
     assert!(help.contains("[default: 0]"));
 }
+
+#[test]
+fn auto_default_value_t() {
+    #[derive(Clap, PartialEq, Debug)]
+    struct Opt {
+        #[clap(default_value_t)]
+        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]"));
+}
+
+#[test]
+fn simple_default_value_t() {
+    #[derive(Clap, PartialEq, Debug)]
+    struct Opt {
+        #[clap(default_value_t = 3)]
+        arg: i32,
+    }
+    assert_eq!(Opt { arg: 3 }, Opt::parse_from(&["test"]));
+    assert_eq!(Opt { arg: 1 }, Opt::parse_from(&["test", "1"]));
+
+    let help = get_long_help::<Opt>();
+    assert!(help.contains("[default: 3]"));
+}