Select default viewer based on TTY

Previosuly, the plain text viewer was the default option.  But for
interactive use, rich text output would be a more sensible default.
With this patch, we check whether we are called from a TTY to choose the
default viewer:  rich for interactive use, text for non-interactive use.

src/main.rs

@@ -24,8 +24,8 @@ struct Opt {
     source_paths: Vec<String>,
 
     /// The viewer for the rustdoc documentation
-    #[structopt(long, default_value = "text", parse(try_from_str = viewer::get_viewer))]
-    viewer: Box<dyn viewer::Viewer>,
+    #[structopt(long, parse(try_from_str = viewer::get_viewer))]
+    viewer: Option<Box<dyn viewer::Viewer>>,
 
     /// Do not search the default documentation sources
     ///
@@ -40,7 +40,8 @@ fn main() -> anyhow::Result<()> {
 
     let sources = load_sources(&opt.source_paths, !opt.no_default_sources)?;
     let doc = find_doc(&sources, &opt.keyword)?;
-    opt.viewer.open(&doc)
+    let viewer = opt.viewer.unwrap_or_else(viewer::get_default);
+    viewer.open(&doc)
 }
 
 const DEFAULT_SOURCES: &[&str] = &[

src/viewer/mod.rs

@@ -5,6 +5,7 @@ mod rich;
 mod text;
 
 use std::fmt;
+use std::io;
 
 use crate::doc;
 
@@ -19,3 +20,11 @@ pub fn get_viewer(s: &str) -> anyhow::Result<Box<dyn Viewer>> {
         _ => Err(anyhow::anyhow!("The viewer {} is not supported", s)),
     }
 }
+
+pub fn get_default() -> Box<dyn Viewer> {
+    if termion::is_tty(&io::stdout()) {
+        Box::new(rich::RichViewer::new())
+    } else {
+        Box::new(text::TextViewer::new())
+    }
+}