Support --toolchain option to rustup which

Update code format by cargo fmt for cli-misc.rs

Test both windows and non-windows which() in cli-misc.rs

src/cli/rustup_mode.rs

@@ -129,6 +129,12 @@ pub fn cli() -> App<'static, 'static> {
                 .short("q")
                 .long("quiet"),
         )
+        .arg(
+            Arg::with_name("toolchain")
+                .help(TOOLCHAIN_ARG_HELP)
+                .long("toolchain")
+                .takes_value(true),
+        )
         .subcommand(
             SubCommand::with_name("dump-testament")
                 .about("Dump information about the build")
@@ -451,7 +457,13 @@ pub fn cli() -> App<'static, 'static> {
         .subcommand(
             SubCommand::with_name("which")
                 .about("Display which binary will be run for a given command")
-                .arg(Arg::with_name("command").required(true)),
+                .arg(Arg::with_name("command").required(true))
+                .arg(
+                    Arg::with_name("toolchain")
+                        .help(TOOLCHAIN_ARG_HELP)
+                        .long("toolchain")
+                        .takes_value(true),
+                ),
         )
         .subcommand(
             SubCommand::with_name("doc")
@@ -783,10 +795,15 @@ fn run(cfg: &Cfg, m: &ArgMatches<'_>) -> Result<()> {
 
 fn which(cfg: &Cfg, m: &ArgMatches<'_>) -> Result<()> {
     let binary = m.value_of("command").expect("");
-
-    let binary_path = cfg
-        .which_binary(&utils::current_dir()?, binary)?
-        .expect("binary not found");
+    let toolchain_provided = m.is_present("toolchain");
+    let binary_path = if toolchain_provided {
+        let toolchain = m.value_of("toolchain").expect("");
+        cfg.which_binary_by_toolchain(toolchain, binary)?
+            .expect("binary not found")
+    } else {
+        cfg.which_binary(&utils::current_dir()?, binary)?
+            .expect("binary not found")
+    };
 
     utils::assert_is_file(&binary_path)?;
 

src/config.rs

@@ -183,6 +183,19 @@ impl Cfg {
         Ok(self.update_hash_dir.join(toolchain))
     }
 
+    pub fn which_binary_by_toolchain(
+        &self,
+        toolchain: &str,
+        binary: &str,
+    ) -> Result<Option<PathBuf>> {
+        let toolchain = self.get_toolchain(toolchain, false)?;
+        if toolchain.exists() {
+            Ok(Some(toolchain.binary_file(binary)))
+        } else {
+            Ok(None)
+        }
+    }
+
     pub fn which_binary(&self, path: &Path, binary: &str) -> Result<Option<PathBuf>> {
         if let Some((toolchain, _)) = self.find_override_toolchain_or_default(path)? {
             Ok(Some(toolchain.binary_file(binary)))

tests/cli-misc.rs

@@ -862,3 +862,46 @@ fn add_remove_component() {
         expect_component_executable(config, "rustc");
     });
 }
+
+#[test]
+fn which() {
+    setup(&|config| {
+        let path_1 = config.customdir.join("custom-1");
+        let path_1 = path_1.to_string_lossy();
+        expect_ok(
+            config,
+            &["rustup", "toolchain", "link", "custom-1", &path_1],
+        );
+        expect_ok(config, &["rustup", "default", "custom-1"]);
+        #[cfg(windows)]
+        expect_stdout_ok(
+            config,
+            &["rustup", "which", "rustc"],
+            "\\toolchains\\custom-1\\bin\\rustc",
+        );
+        #[cfg(not(windows))]
+        expect_stdout_ok(
+            config,
+            &["rustup", "which", "rustc"],
+            "/toolchains/custom-1/bin/rustc",
+        );
+        let path_2 = config.customdir.join("custom-2");
+        let path_2 = path_2.to_string_lossy();
+        expect_ok(
+            config,
+            &["rustup", "toolchain", "link", "custom-2", &path_2],
+        );
+        #[cfg(windows)]
+        expect_stdout_ok(
+            config,
+            &["rustup", "which", "--toolchain=custom-2", "rustc"],
+            "\\toolchains\\custom-2\\bin\\rustc",
+        );
+        #[cfg(not(windows))]
+        expect_stdout_ok(
+            config,
+            &["rustup", "which", "--toolchain=custom-2", "rustc"],
+            "/toolchains/custom-2/bin/rustc",
+        );
+    });
+}