Add --toolchain-version argument

This allows the user to specify which toolchain to use to build their
script. This allows for the use of nightly features, features only
stabilized in beta, and even the use of custom-built toolchains if
the need ever arises.

src/main.rs

@@ -56,6 +56,11 @@ struct Args {
     build_kind: BuildKind,
     template: Option<String>,
     list_templates: bool,
+    // This is a String instead of an
+    // enum since one can have custom
+    // toolchains (ex. a rustc developer
+    // will probably have `stage1`).
+    toolchain_version: Option<String>,
 
     #[cfg(windows)]
     install_file_association: bool,
@@ -235,6 +240,15 @@ fn parse_args() -> Args {
                 .takes_value(true)
                 .requires("expr")
             )
+            .arg(Arg::new("toolchain-version")
+                .about("Build the script using the given toolchain version.")
+                .long("toolchain-version")
+                // "channel"
+                .short('c')
+                .takes_value(true)
+                // FIXME: remove if benchmarking is stabilized
+                .conflicts_with("bench")
+            )
         .arg(Arg::new("list-templates")
             .about("List the available templates.")
             .long("list-templates")
@@ -289,6 +303,7 @@ fn parse_args() -> Args {
         build_kind: BuildKind::from_flags(m.is_present("test"), m.is_present("bench")),
         template: m.value_of("template").map(Into::into),
         list_templates: m.is_present("list-templates"),
+        toolchain_version: m.value_of("toolchain-version").map(Into::into),
         #[cfg(windows)]
         install_file_association: m.is_present("install-file-association"),
         #[cfg(windows)]
@@ -658,7 +673,12 @@ struct InputAction {
     */
     using_cache: bool,
 
-    use_nightly: bool,
+    /**
+    Which toolchain the script should be built with.
+
+    `None` indicates that the script should be built with a stable toolchain.
+    */
+    toolchain_version: Option<String>,
 
     /// The package metadata structure for the current invocation.
     metadata: PackageMetadata,
@@ -685,7 +705,7 @@ impl InputAction {
         cargo(
             cmd,
             &*self.manifest_path().to_string_lossy(),
-            self.use_nightly,
+            self.toolchain_version.as_ref().map(|s| s.as_str()),
             &self.metadata,
             script_args,
             run_quietly,
@@ -788,14 +808,22 @@ fn decide_action_for(
     };
     info!("input_meta: {:?}", input_meta);
 
+    let toolchain_version = args.toolchain_version.clone().or_else(|| {
+        if matches!(args.build_kind, BuildKind::Bench) {
+            Some("nightly".into())
+        } else {
+            None
+        }
+    });
+
     let mut action = InputAction {
         cargo_output: args.cargo_output,
         force_compile: force,
         emit_metadata: true,
         execute: true,
         pkg_path,
         using_cache,
-        use_nightly: matches!(args.build_kind, BuildKind::Bench),
+        toolchain_version,
         metadata: input_meta,
         old_metadata: None,
         manifest: mani_str,
@@ -1123,14 +1151,14 @@ Constructs a Cargo command that runs on the script package.
 fn cargo(
     cmd_name: &str,
     manifest: &str,
-    nightly: bool,
+    maybe_toolchain_version: Option<&str>,
     meta: &PackageMetadata,
     script_args: &[String],
     run_quietly: bool,
 ) -> MainResult<Command> {
     let mut cmd = Command::new("cargo");
-    if nightly {
-        cmd.arg("+nightly");
+    if let Some(toolchain_version) = maybe_toolchain_version {
+        cmd.arg(format!("+{}", toolchain_version));
     }
     cmd.arg(cmd_name);