Make rustup default not error if no default

`rustup default` will no longer error and print to stderr if there is no
default toolchain configured. It will print to stdout and exit
successfully instead.

src/cli/rustup_mode.rs

@@ -872,10 +872,15 @@ fn default_(cfg: &Cfg, m: &ArgMatches) -> Result<utils::ExitCode> {
             }
         }
     } else {
-        let default_toolchain = cfg
-            .get_default()?
-            .ok_or_else(|| anyhow!("no default toolchain configured"))?;
-        writeln!(process().stdout().lock(), "{default_toolchain} (default)")?;
+        match cfg.get_default()? {
+            Some(default_toolchain) => {
+                writeln!(process().stdout().lock(), "{default_toolchain} (default)")?;
+            }
+            None => writeln!(
+                process().stdout().lock(),
+                "no default toolchain is configured"
+            )?,
+        }
     }
 
     Ok(utils::ExitCode(0))

tests/suite/cli_exact.rs

@@ -577,6 +577,13 @@ fn default_none() {
             &["rustup", "default", "none"],
             "info: default toolchain unset",
         );
+
+        config.expect_ok_ex(
+            &["rustup", "default"],
+            "no default toolchain is configured\n",
+            "",
+        );
+
         config.expect_err_ex(
             &["rustc", "--version"],
             "",

tests/suite/cli_misc.rs

@@ -477,13 +477,6 @@ fn toolchains_are_resolved_early() {
     });
 }
 
-#[test]
-fn no_panic_on_default_toolchain_missing() {
-    setup(&|config| {
-        config.expect_err(&["rustup", "default"], "no default toolchain configured");
-    });
-}
-
 // #190
 #[test]
 fn proxies_pass_empty_args() {

tests/suite/cli_rustup.rs

@@ -2482,7 +2482,7 @@ fn check_unix_settings_fallback() {
     test(&|config| {
         config.with_scenario(Scenario::SimpleV2, &|config| {
             // No default toolchain specified yet
-            config.expect_err(&["rustup", "default"], r"no default toolchain configured");
+            config.expect_stdout_ok(&["rustup", "default"], "no default toolchain is configured");
 
             // Default toolchain specified in fallback settings file
             let mock_settings_file = config.current_dir().join("mock_fallback_settings.toml");