Add subcommands to "rustup show" for active-toolchain and it's version

src/rustup-cli/help.rs

@@ -17,6 +17,15 @@ pub static SHOW_HELP: &'static str = r"DISCUSSION:
     If there are multiple toolchains installed then all installed
     toolchains are listed as well.";
 
+pub static SHOW_ACTIVE_TOOLCHAIN_HELP: &'static str = r"DISCUSSION:
+    Shows the name of the active toolchain.
+
+    This is useful for figuring out the active tool chain from
+    scripts.
+
+    You should use `rustc --print sysroot` to get the sysroot, or
+    `rustc --version` to get the toolchain version.";
+
 pub static UPDATE_HELP: &'static str = r"DISCUSSION:
     With no toolchain specified, the `update` command updates each of
     the installed toolchains from the official release channels, then

src/rustup-cli/rustup_mode.rs

@@ -29,7 +29,10 @@ pub fn main() -> Result<()> {
     cfg.check_metadata_version()?;
 
     match matches.subcommand() {
-        ("show", Some(_)) => show(cfg)?,
+        ("show", Some(c)) => match c.subcommand() {
+            ("active-toolchain", Some(_)) => show_active_toolchain(cfg)?,
+            (_, _) => show(cfg)?
+        },
         ("install", Some(m)) => update(cfg, m)?,
         ("update", Some(m)) => update(cfg, m)?,
         ("uninstall", Some(m)) => toolchain_remove(cfg, m)?,
@@ -110,7 +113,13 @@ pub fn cli() -> App<'static, 'static> {
         .subcommand(
             SubCommand::with_name("show")
                 .about("Show the active and installed toolchains")
-                .after_help(SHOW_HELP),
+                .after_help(SHOW_HELP)
+                .setting(AppSettings::VersionlessSubcommands)
+                .setting(AppSettings::DeriveDisplayOrder)
+                .subcommand(SubCommand::with_name("active-toolchain")
+                    .about("Show the active toolchain")
+                    .after_help(SHOW_ACTIVE_TOOLCHAIN_HELP)
+                ),
         )
         .subcommand(
             SubCommand::with_name("install")
@@ -733,6 +742,19 @@ fn show(cfg: &Cfg) -> Result<()> {
     Ok(())
 }
 
+fn show_active_toolchain(cfg: &Cfg) -> Result<()> {
+    let ref cwd = utils::current_dir()?;
+    match cfg.find_override_toolchain_or_default(cwd)? {
+        Some((ref toolchain, _)) => {
+            println!("{}", toolchain.name());
+        },
+        None => {
+            // Print nothing
+        }
+    }
+    Ok(())
+}
+
 fn target_list(cfg: &Cfg, m: &ArgMatches) -> Result<()> {
     let toolchain = explicit_or_dir_toolchain(cfg, m)?;
 

tests/cli-rustup.rs

@@ -845,6 +845,34 @@ fn show_toolchain_env_not_installed() {
     });
 }
 
+#[test]
+fn show_active_toolchain() {
+    setup(&|config| {
+        expect_ok(config, &["rustup", "default", "nightly"]);
+        expect_ok_ex(
+            config,
+            &["rustup", "show", "active-toolchain"],
+            for_host!(
+                r"nightly-{0}
+"
+            ),
+            r"",
+        );
+    });
+}
+
+#[test]
+fn show_active_toolchain_none() {
+    setup(&|config| {
+        expect_ok_ex(
+            config,
+            &["rustup", "show", "active-toolchain"],
+            r"",
+            r"",
+        );
+    });
+}
+
 // #846
 #[test]
 fn set_default_host() {