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

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    ")
     );
 }