feat(config): warn user if auto-install is enabled

src/cli/rustup_mode.rs

@@ -1096,7 +1096,7 @@ async fn run(
     install: bool,
 ) -> Result<ExitStatus> {
     let toolchain = toolchain.resolve(&cfg.default_host_tuple()?)?;
-    let toolchain = Toolchain::from_local(toolchain, install, cfg).await?;
+    let toolchain = Toolchain::from_local(toolchain, || Ok(install), cfg).await?;
     let cmd = toolchain.command(&command[0])?;
     command::run_command_for_dir(cmd, &command[0], &command[1..])
 }

src/config.rs

@@ -717,13 +717,10 @@ impl<'a> Cfg<'a> {
         name: Option<(LocalToolchainName, ActiveSource)>,
     ) -> Result<(Toolchain<'_>, ActiveSource)> {
         match name {
-            Some((tc, source)) => {
-                let install_if_missing = self.should_auto_install()?;
-                Ok((
-                    Toolchain::from_local(tc, install_if_missing, self).await?,
-                    source,
-                ))
-            }
+            Some((tc, src)) => Ok((
+                Toolchain::from_local(tc, || self.should_auto_install(), self).await?,
+                src,
+            )),
             None => {
                 let (tc, source) = self
                     .maybe_ensure_active_toolchain(None)

src/test/clitools.rs

@@ -314,6 +314,12 @@ impl Config {
             "/bogus-config-file.toml",
         );
 
+        // Clear current recursion count to avoid messing up related logic
+        cmd.env("RUST_RECURSION_COUNT", "");
+
+        // Clear override for auto installation of active toolchain unless explicitly requested
+        cmd.env("RUSTUP_AUTO_INSTALL", "");
+
         // Pass `RUSTUP_CI` over to the test process in case it is required downstream
         if let Some(ci) = env::var_os("RUSTUP_CI") {
             cmd.env("RUSTUP_CI", ci);

src/toolchain.rs

@@ -15,7 +15,7 @@ use std::{
 use anyhow::{Context, anyhow, bail};
 use fs_at::OpenOptions;
 use same_file::is_same_file;
-use tracing::info;
+use tracing::{info, warn};
 use url::Url;
 use wait_timeout::ChildExt;
 
@@ -52,15 +52,27 @@ pub(crate) struct Toolchain<'a> {
 impl<'a> Toolchain<'a> {
     pub(crate) async fn from_local(
         name: LocalToolchainName,
-        install_if_missing: bool,
+        install_if_missing: impl Fn() -> anyhow::Result<bool>,
         cfg: &'a Cfg<'a>,
     ) -> anyhow::Result<Toolchain<'a>> {
         match Self::new(cfg, name) {
             Ok(tc) => Ok(tc),
             Err(RustupError::ToolchainNotInstalled {
                 name: ToolchainName::Official(desc),
                 ..
-            }) if install_if_missing => {
+            }) if install_if_missing()? => {
+                // Emit a warning about auto-installation if we aren't deep inside a recursive call.
+                let recursions = cfg.process.var("RUST_RECURSION_COUNT");
+                if !recursions.is_ok_and(|it| it != "0") {
+                    warn!("auto-install is enabled, active toolchain will be installed if absent");
+                    warn!(
+                        "this might cause rustup commands to take longer time to finish than expected"
+                    );
+                    info!(
+                        "you may opt out with `RUSTUP_AUTO_INSTALL=0` or `rustup set auto-install disable`"
+                    );
+                }
+
                 let options = DistOptions::new(&[], &[], &desc, cfg.get_profile()?, true, cfg)?;
                 Ok(DistributableToolchain::install(options).await?.1.toolchain)
             }

tests/suite/cli_misc.rs

@@ -64,7 +64,7 @@ async fn rustc_with_bad_rustup_toolchain_env_var() {
         .expect_with_env(["rustc"], [("RUSTUP_TOOLCHAIN", "bogus")])
         .await
         .with_stderr(snapbox::str![[r#"
-error: override toolchain 'bogus' is not installed[..]
+error:[..] toolchain 'bogus' is not installed[..]
 
 "#]])
         .is_err();
@@ -1381,7 +1381,10 @@ async fn which_asking_uninstalled_toolchain() {
 "#]])
         .is_ok();
     cx.config
-        .expect(["rustup", "which", "--toolchain=nightly", "rustc"])
+        .expect_with_env(
+            ["rustup", "which", "--toolchain=nightly", "rustc"],
+            [("RUSTUP_AUTO_INSTALL", "1")],
+        )
         .await
         .with_stderr(snapbox::str![[r#"
 error: toolchain 'nightly-[HOST_TUPLE]' is not installed
@@ -1746,3 +1749,26 @@ info: falling back to "[EXTERN_PATH]"
 "#]])
         .is_ok();
 }
+
+#[tokio::test]
+async fn warn_auto_install() {
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+    cx.config
+        .expect_with_env(
+            ["rustc", "--version"],
+            [("RUSTUP_TOOLCHAIN", "stable"), ("RUSTUP_AUTO_INSTALL", "1")],
+        )
+        .await
+        .with_stdout(snapbox::str![[r#"
+1.1.0 (hash-stable-1.1.0)
+
+"#]])
+        .with_stderr(snapbox::str![[r#"
+...
+warn: auto-install is enabled, active toolchain will be installed if absent
+warn: this might cause rustup commands to take longer time to finish than expected
+info: you may opt out with `RUSTUP_AUTO_INSTALL=0` or `rustup set auto-install disable`
+...
+"#]])
+        .is_ok();
+}

tests/suite/cli_rustup.rs

@@ -1247,7 +1247,7 @@ async fn show_toolchain_override_not_installed() {
         .await
         .is_ok();
     cx.config
-        .expect(["rustup", "show"])
+        .expect_with_env(["rustup", "show"], [("RUSTUP_AUTO_INSTALL", "1")])
         .await
         .extend_redactions([("[RUSTUP_DIR]", &cx.config.rustupdir.to_string())])
         .with_stdout(snapbox::str![[r#"
@@ -1349,7 +1349,13 @@ installed targets:
 async fn show_toolchain_env_not_installed() {
     let cx = CliTestContext::new(Scenario::SimpleV2).await;
     cx.config
-        .expect_with_env(["rustup", "show"], [("RUSTUP_TOOLCHAIN", "nightly")])
+        .expect_with_env(
+            ["rustup", "show"],
+            [
+                ("RUSTUP_TOOLCHAIN", "nightly"),
+                ("RUSTUP_AUTO_INSTALL", "1"),
+            ],
+        )
         .await
         .extend_redactions([("[RUSTUP_DIR]", &cx.config.rustupdir.to_string())])
         .is_ok()

tests/suite/cli_v1.rs

@@ -271,7 +271,7 @@ async fn remove_override_toolchain_err_handling() {
         .await
         .is_ok();
     cx.config
-        .expect(["rustc", "--version"])
+        .expect_with_env(["rustc", "--version"], [("RUSTUP_AUTO_INSTALL", "1")])
         .await
         .with_stdout(snapbox::str![[r#"
 1.2.0 (hash-beta-1.2.0)

tests/suite/cli_v2.rs

@@ -478,7 +478,7 @@ async fn remove_override_toolchain_err_handling() {
         .await
         .is_ok();
     cx.config
-        .expect(["rustc", "--version"])
+        .expect_with_env(["rustc", "--version"], [("RUSTUP_AUTO_INSTALL", "1")])
         .await
         .with_stdout(snapbox::str![[r#"
 1.2.0 (hash-beta-1.2.0)
@@ -511,7 +511,7 @@ async fn file_override_toolchain_err_handling() {
     let toolchain_file = cwd.join("rust-toolchain");
     rustup::utils::raw::write_file(&toolchain_file, "beta").unwrap();
     cx.config
-        .expect(["rustc", "--version"])
+        .expect_with_env(["rustc", "--version"], [("RUSTUP_AUTO_INSTALL", "1")])
         .await
         .with_stdout(snapbox::str![[r#"
 1.2.0 (hash-beta-1.2.0)
@@ -553,7 +553,10 @@ error: toolchain 'beta-[HOST_TUPLE]' is not installed
 "#]])
         .is_err();
     cx.config
-        .expect(["rustc", "+beta", "--version"])
+        .expect_with_env(
+            ["rustc", "+beta", "--version"],
+            [("RUSTUP_AUTO_INSTALL", "1")],
+        )
         .await
         .with_stdout(snapbox::str![[r#"
 1.2.0 (hash-beta-1.2.0)