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.

2 files changed, 12 insertions, 0 deletions

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(())