Return Box<dyn Backend> in default_backend()

This patch refactors the default_backend function to return a Box<dyn
Backend> instead of impl Backend.  This will allow us to dynamically
choose the backend implementation in a future patch.  To keep the
current interface, we change show_with to accept both a reference to a
backend instance as well as a reference to a boxed backend instance.
This also means we have to implement AsRef<Self> for the backend
structs.

4 files changed, 42 insertions, 13 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2feb87..58a4919 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,6 @@
+# Unreleased
+- Refactor `default_backend()` to return a `Box<dyn Backend>`.
+
 # v0.1.1 (2019-01-11)
 - Add the `Password` dialog box.
 - Add the `Zenity` backend.
diff --git a/src/backends/dialog.rs b/src/backends/dialog.rs
index 939b8f3..6f078e6 100644
--- a/src/backends/dialog.rs
+++ b/src/backends/dialog.rs
@@ -77,6 +77,12 @@ impl Dialog {
     }
 }
 
+impl AsRef<Dialog> for Dialog {
+    fn as_ref(&self) -> &Self {
+        self
+    }
+}
+
 fn require_success(status: process::ExitStatus) -> Result<()> {
     if status.success() {
         Ok(())
diff --git a/src/backends/zenity.rs b/src/backends/zenity.rs
index a90c3da..4bf862b 100644
--- a/src/backends/zenity.rs
+++ b/src/backends/zenity.rs
@@ -88,6 +88,12 @@ impl Zenity {
     }
 }
 
+impl AsRef<Zenity> for Zenity {
+    fn as_ref(&self) -> &Self {
+        self
+    }
+}
+
 fn require_success(status: process::ExitStatus) -> Result<()> {
     if status.success() {
         Ok(())
diff --git a/src/lib.rs b/src/lib.rs
index 7e9c53d..87234bd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -96,13 +96,15 @@ pub trait DialogBox {
 
     /// Shows this dialog box using the default backend and returns the output.
     ///
-    /// `box.show()` is a shorthand for `box.show_with(&default_backend())`.
+    /// `box.show()` is a shorthand for `box.show_with(default_backend())`.
     fn show(&self) -> Result<Self::Output> {
-        self.show_with(&default_backend())
+        self.show_with(default_backend())
     }
 
     /// Shows this dialog box using the given backend and returns the output.
-    fn show_with(&self, backend: &impl backends::Backend) -> Result<Self::Output>;
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized;
 }
 
 /// A message box.
@@ -146,8 +148,11 @@ impl Message {
 impl DialogBox for Message {
     type Output = ();
 
-    fn show_with(&self, backend: &impl backends::Backend) -> Result<Self::Output> {
-        backend.show_message(self)
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized,
+    {
+        backend.as_ref().show_message(self)
     }
 }
 
@@ -206,8 +211,11 @@ impl Input {
 impl DialogBox for Input {
     type Output = Option<String>;
 
-    fn show_with(&self, backend: &impl backends::Backend) -> Result<Self::Output> {
-        backend.show_input(self)
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized,
+    {
+        backend.as_ref().show_input(self)
     }
 }
 
@@ -256,8 +264,11 @@ impl Password {
 impl DialogBox for Password {
     type Output = Option<String>;
 
-    fn show_with(&self, backend: &impl backends::Backend) -> Result<Self::Output> {
-        backend.show_password(self)
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized,
+    {
+        backend.as_ref().show_password(self)
     }
 }
 
@@ -314,8 +325,11 @@ impl Question {
 impl DialogBox for Question {
     type Output = Choice;
 
-    fn show_with(&self, backend: &impl backends::Backend) -> Result<Self::Output> {
-        backend.show_question(self)
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized,
+    {
+        backend.as_ref().show_question(self)
     }
 }
 
@@ -324,6 +338,6 @@ impl DialogBox for Question {
 /// The current implementation always returns a [`Dialog`][] instance.
 ///
 /// [`Dialog`]: backends/struct.Dialog.html
-pub fn default_backend() -> impl backends::Backend {
-    backends::Dialog::new()
+pub fn default_backend() -> Box<dyn backends::Backend> {
+    Box::new(backends::Dialog::new())
 }