chore: move to 2018 edition

Cargo.toml

@@ -38,6 +38,7 @@ categories = ["command-line-interface"]
 description = """
 A simple to use, efficient, and full featured Command Line Argument Parser
 """
+edition = "2018"
 
 [badges]
 travis-ci = { repository = "clap-rs/clap" }

src/build/arg/key/position.rs

@@ -0,0 +1,13 @@
+pub struct Position(u64);
+
+impl Default for Position {
+    fn default() -> Self {
+        Position(1)
+    }
+}
+
+impl Position {
+    fn new() -> Self {
+        Position(1)
+    }
+}

src/build/arg/position.rs

@@ -1,3 +0,0 @@
-pub struct Position {
-    index: u64
-}

src/build/arg/value/default_values.rs

@@ -1,13 +1,14 @@
-pub struct DefaultValue<'help> {
-    value: &'help str,
+pub struct EnvDefaultValue<'help> {
+    key: &'help OsStr,
+    value: Option<OsString>
 }
-
 pub struct ConditionalDefault<'help> {
-    value: DefaultValue<'help>,
+    value: &'help str,
     other_arg: Option<u64>,
     other_value: Option<&'help str>,
 }
 
 pub struct DefaultValues<'help> {
     defaults: Vec<ConditionalDefault<'help>>,
+    env: Option<EnvDefaultValue<'help>>,
 }

src/build/arg/value/mod.rs

@@ -1,6 +1,5 @@
 pub struct Value<'help> {
     defaults: Option<DefaultValues>,
-    name: Option<&'help str>,
     filter: Filter,
     occurrence: Occurrence,
     requires_equals: bool,

src/build/arg/value/value_name.rs

@@ -0,0 +1,15 @@
+use std::io;
+
+pub struct ValueName<'help>(&'help str);
+
+impl<'help> ValueName<'help> {
+    fn write_as_required<W: io::Write>(&self, w: W) -> io::Result {
+        write!(w, "<{}>", self.0)
+    }
+    fn write_as_optional<W: io::Write>(&self, w: W) -> io::Result {
+        write!(w, "[{}]", self.0)
+    }
+    fn write<W: io::Write>(&self, w: W) -> io::Result {
+        write!(w, "{}", self.0)
+    }
+}

src/build/arg/value_name.rs

@@ -1,3 +0,0 @@
-pub struct ValueName<'help> {
-    name: &'help str,
-}