feat(cli): warn when removing the last/host target for a toolchain

Cargo.toml

@@ -57,6 +57,7 @@ flate2 = "1"
 fs_at.workspace = true
 git-testament = "0.2"
 home = "0.5.4"
+itertools = "0.12"
 libc = "0.2"
 once_cell.workspace = true
 opener = "0.6.0"

src/cli/rustup_mode.rs

@@ -10,6 +10,7 @@ use clap::{
     Arg, ArgAction, ArgGroup, ArgMatches, Command, ValueEnum,
 };
 use clap_complete::Shell;
+use itertools::Itertools;
 
 use crate::{
     cli::{
@@ -1334,11 +1335,23 @@ fn target_remove(cfg: &Cfg, m: &ArgMatches) -> Result<utils::ExitCode> {
     let distributable = DistributableToolchain::try_from(&toolchain)?;
 
     for target in m.get_many::<String>("target").unwrap() {
-        let new_component = Component::new(
-            "rust-std".to_string(),
-            Some(TargetTriple::new(target)),
-            false,
-        );
+        let target = TargetTriple::new(target);
+        let default_target = cfg.get_default_host_triple()?;
+        if target == default_target {
+            warn!("after removing the default host target, proc-macros and build scripts might no longer build");
+        }
+        let has_at_most_one_target = {
+            let components = distributable.components()?;
+            // Every component target that is not `None` (wildcard).
+            let targets = components
+                .iter()
+                .filter_map(|c| c.installed.then(|| c.component.target.clone()).flatten());
+            targets.unique().at_most_one().is_ok()
+        };
+        if has_at_most_one_target {
+            warn!("after removing the last target, no build targets will be available");
+        }
+        let new_component = Component::new("rust-std".to_string(), Some(target), false);
         distributable.remove_component(new_component)?;
     }
 

tests/suite/cli_v2.rs

@@ -952,9 +952,31 @@ fn remove_target_again() {
 #[test]
 fn remove_target_host() {
     setup(&|config| {
-        let trip = this_host_triple();
+        let host = this_host_triple();
         config.expect_ok(&["rustup", "default", "nightly"]);
-        config.expect_ok(&["rustup", "target", "remove", &trip]);
+        config.expect_ok(&["rustup", "target", "add", clitools::CROSS_ARCH1]);
+        config.expect_stderr_ok(
+            &["rustup", "target", "remove", &host], 
+            "after removing the default host target, proc-macros and build scripts might no longer build",
+        );
+        let path = format!("toolchains/nightly-{host}/lib/rustlib/{host}/lib/libstd.rlib");
+        assert!(!config.rustupdir.has(path));
+        let path = format!("toolchains/nightly-{host}/lib/rustlib/{host}/lib");
+        assert!(!config.rustupdir.has(path));
+        let path = format!("toolchains/nightly-{host}/lib/rustlib/{host}");
+        assert!(!config.rustupdir.has(path));
+    });
+}
+
+#[test]
+fn remove_target_last() {
+    setup(&|config| {
+        let host = this_host_triple();
+        config.expect_ok(&["rustup", "default", "nightly"]);
+        config.expect_stderr_ok(
+            &["rustup", "target", "remove", &host],
+            "after removing the last target, no build targets will be available",
+        );
     });
 }