feat(cli): warn when removing the default/active toolchain

src/cli/rustup_mode.rs

@@ -1281,8 +1281,18 @@ async fn toolchain_link(
 }
 
 fn toolchain_remove(cfg: &mut Cfg<'_>, opts: UninstallOpts) -> Result<utils::ExitCode> {
+    let default_toolchain = cfg.get_default().ok().flatten();
+    let active_toolchain = cfg.find_active_toolchain().ok().flatten().map(|(it, _)| it);
+
     for toolchain_name in &opts.toolchain {
         let toolchain_name = toolchain_name.resolve(&cfg.get_default_host_triple()?)?;
+
+        match (default_toolchain.as_ref(), active_toolchain.as_ref()) {
+            (Some(n), _) if n == &toolchain_name => Some("default"),
+            (_, Some(n)) if n == &toolchain_name => Some("active"),
+            _ => None,
+        }.inspect(|status| warn!("after removing the {status} toolchain, proc-macros and build scripts might no longer build"));
+
         Toolchain::ensure_removed(cfg, (&toolchain_name).into())?;
     }
     Ok(utils::ExitCode(0))

src/toolchain/names.rs

@@ -359,6 +359,15 @@ from_variant!(
     LocalToolchainName::Path
 );
 
+impl PartialEq<ToolchainName> for LocalToolchainName {
+    fn eq(&self, other: &ToolchainName) -> bool {
+        match self {
+            LocalToolchainName::Named(n) => n == other,
+            _ => false,
+        }
+    }
+}
+
 impl Display for LocalToolchainName {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {

tests/suite/cli_v2.rs

@@ -212,18 +212,35 @@ async fn list_toolchains_with_none() {
 }
 
 #[tokio::test]
-async fn remove_toolchain() {
+async fn remove_toolchain_default() {
     let mut cx = CliTestContext::new(Scenario::SimpleV2).await;
     cx.config.expect_ok(&["rustup", "update", "nightly"]).await;
-    cx.config
-        .expect_ok(&["rustup", "toolchain", "remove", "nightly"])
-        .await;
+    cx.config.expect_stderr_ok(
+        &["rustup", "toolchain", "remove", "nightly"],
+        "after removing the default toolchain, proc-macros and build scripts might no longer build",
+    ).await;
     cx.config.expect_ok(&["rustup", "toolchain", "list"]).await;
     cx.config
         .expect_stdout_ok(&["rustup", "toolchain", "list"], "no installed toolchains")
         .await;
 }
 
+#[tokio::test]
+async fn remove_toolchain_active() {
+    let mut cx = CliTestContext::new(Scenario::SimpleV2).await;
+    cx.config.expect_ok(&["rustup", "default", "nightly"]).await;
+    cx.config
+        .expect_ok(&["rustup", "override", "set", "stable"])
+        .await;
+    cx.config.expect_stderr_ok(
+        &["rustup", "toolchain", "remove", "stable"],
+        "after removing the active toolchain, proc-macros and build scripts might no longer build",
+    ).await;
+    cx.config
+        .expect_stdout_ok(&["rustup", "toolchain", "list"], "nightly")
+        .await;
+}
+
 // Issue #2873
 #[tokio::test]
 async fn remove_toolchain_ignore_trailing_slash() {