Add "rustup default none" as a way to unset the default toolchain

This can be useful when you want to be extra certain you're not using the wrong
toolchain by accident.

This also changes the testsuite to make it move obvious what went wrong when
`expect_err_ex` fails.

src/cli/rustup_mode.rs

@@ -853,7 +853,7 @@ fn default_(cfg: &Cfg, m: &ArgMatches<'_>) -> Result<utils::ExitCode> {
         let status = if !toolchain.is_custom() {
             let distributable = DistributableToolchain::new(&toolchain)?;
             Some(distributable.install_from_dist_if_not_installed()?)
-        } else if !toolchain.exists() {
+        } else if !toolchain.exists() && toolchain.name() != "none" {
             return Err(RustupError::ToolchainNotInstalled(toolchain.name().to_string()).into());
         } else {
             None

src/config.rs

@@ -382,7 +382,11 @@ impl Cfg {
 
     pub fn set_default(&self, toolchain: &str) -> Result<()> {
         self.settings_file.with_mut(|s| {
-            s.default_toolchain = Some(toolchain.to_owned());
+            s.default_toolchain = if toolchain == "none" {
+                None
+            } else {
+                Some(toolchain.to_owned())
+            };
             Ok(())
         })?;
         (self.notify_handler)(Notification::SetDefaultToolchain(toolchain));

src/notifications.rs

@@ -96,6 +96,7 @@ impl<'a> Display for Notification<'a> {
             Install(n) => n.fmt(f),
             Utils(n) => n.fmt(f),
             Temp(n) => n.fmt(f),
+            SetDefaultToolchain("none") => write!(f, "default toolchain unset"),
             SetDefaultToolchain(name) => write!(f, "default toolchain set to '{}'", name),
             SetOverrideToolchain(path, name) => write!(
                 f,

tests/cli-exact.rs

@@ -4,8 +4,8 @@
 pub mod mock;
 
 use crate::mock::clitools::{
-    self, check_update_setup, expect_err_ex, expect_ok, expect_ok_ex, expect_stdout_ok,
-    self_update_setup, set_current_dist_date, Config, Scenario,
+    self, check_update_setup, expect_err_ex, expect_ok, expect_ok_ex, expect_stderr_ok,
+    expect_stdout_ok, self_update_setup, set_current_dist_date, Config, Scenario,
 };
 use rustup::for_host;
 use rustup::test::this_host_triple;
@@ -587,6 +587,23 @@ error: target '2016-03-1' not found in channel.  Perhaps check https://doc.rust-
     });
 }
 
+#[test]
+fn default_none() {
+    setup(&|config| {
+        expect_stderr_ok(
+            config,
+            &["rustup", "default", "none"],
+            "info: default toolchain unset",
+        );
+        expect_err_ex(
+            config,
+            &["rustc", "--version"],
+            "",
+            "error: no override and no default toolchain set\n",
+        );
+    })
+}
+
 #[test]
 fn list_targets() {
     setup(&|config| {

tests/cli-self-upd.rs

@@ -193,7 +193,9 @@ fn uninstall_works_if_some_bins_dont_exist() {
             .cargodir
             .join(&format!("bin/rust-lldb{}", EXE_SUFFIX));
         let rust_gdb = config.cargodir.join(&format!("bin/rust-gdb{}", EXE_SUFFIX));
-        let rust_gdbgui = config.cargodir.join(&format!("bin/rust-gdbgui{}", EXE_SUFFIX));
+        let rust_gdbgui = config
+            .cargodir
+            .join(&format!("bin/rust-gdbgui{}", EXE_SUFFIX));
 
         fs::remove_file(&rustc).unwrap();
         fs::remove_file(&cargo).unwrap();

tests/mock/clitools.rs

@@ -384,7 +384,15 @@ pub fn expect_err_ex(config: &Config, args: &[&str], stdout: &str, stderr: &str)
         println!("expected.ok: false");
         print_indented("expected.stdout", stdout);
         print_indented("expected.stderr", stderr);
-        panic!();
+        if out.ok {
+            panic!("expected command to fail");
+        } else if out.stdout != stdout {
+            panic!("expected stdout to match");
+        } else if out.stderr != stderr {
+            panic!("expected stderr to match");
+        } else {
+            unreachable!()
+        }
     }
 }
 
@@ -445,10 +453,16 @@ fn print_command(args: &[&str], out: &SanitizedOutput) {
 }
 
 fn print_indented(heading: &str, text: &str) {
+    let mut lines = text.lines().count();
+    // The standard library treats `a\n` and `a` as both being one line.
+    // This is confusing when the test fails because of a missing newline.
+    if !text.is_empty() && !text.ends_with('\n') {
+        lines -= 1;
+    }
     println!(
         "{} ({} lines):\n    {}",
         heading,
-        text.lines().count(),
+        lines,
         text.replace("\n", "\n    ")
     );
 }