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

Cargo.toml

@@ -94,6 +94,7 @@ wait-timeout = "0.2"
 walkdir = { workspace = true, optional = true }
 xz2 = "0.1.3"
 zstd = "0.13"
+itertools = "0.12.0"
 
 [target."cfg(windows)".dependencies]
 cc = "1"

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 +1335,22 @@ 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().ok();
+        if default_target.as_ref() == Some(&target) {
+            warn!("after removing this default host target, proc-macros and build scripts might no longer build");
+        }
+        let at_most_one_target = distributable.components().ok().and_then(|cs| {
+            // Every component target that is not `None` (wildcard).
+            let targets = cs
+                .iter()
+                .filter_map(|c| c.installed.then(|| c.component.target.clone()).flatten());
+            targets.unique().at_most_one().ok()
+        });
+        if at_most_one_target.is_some() {
+            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)?;
     }