Add --path flag to 'rustup doc'

Author: joerivanruth

src/rustup-cli/rustup_mode.rs

@@ -360,6 +360,11 @@ pub fn cli() -> App<'static, 'static> {
                 .alias("docs")
                 .about("Open the documentation for the current toolchain")
                 .after_help(DOC_HELP)
+                .arg(
+                    Arg::with_name("path")
+                        .long("path")
+                        .help("Only print the path to the documentation"),
+                )
                 .arg(
                     Arg::with_name("book")
                         .long("book")
@@ -929,7 +934,14 @@ fn doc(cfg: &Cfg, m: &ArgMatches) -> Result<()> {
         "index.html"
     };
 
-    Ok(cfg.open_docs_for_dir(&utils::current_dir()?, doc_url)?)
+    let cwd = &utils::current_dir()?;
+    if m.is_present("path") {
+        let doc_path = try!(cfg.doc_path_for_dir(cwd, doc_url));
+        println!("{}", doc_path.display());
+        Ok(())
+    } else {
+        Ok(cfg.open_docs_for_dir(cwd, doc_url)?)
+    }
 }
 
 fn man(cfg: &Cfg, m: &ArgMatches) -> Result<()> {