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

Author: rami3l

src/config.rs

@@ -384,12 +384,26 @@ impl<'a> Cfg<'a> {
             return Ok(false);
         }
 
-        if let Ok(mode) = self.process.var("RUSTUP_AUTO_INSTALL") {
-            Ok(mode != "0")
-        } else {
-            self.settings_file
-                .with(|s| Ok(s.auto_install != Some(AutoInstallMode::Disable)))
+        let should_auto = match self.process.var("RUSTUP_AUTO_INSTALL") {
+            Ok(mode) => mode != "0",
+            Err(_) => self
+                .settings_file
+                .with(|s| Ok(s.auto_install != Some(AutoInstallMode::Disable)))?,
+        };
+        if !should_auto {
+            return Ok(false);
         }
+
+        // We also need to suppress this warning if we're deep inside a recursive call.
+        let recursions = self.process.var("RUST_RECURSION_COUNT");
+        if recursions.is_ok_and(|it| it != "0") {
+            return Ok(true);
+        }
+
+        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`");
+        Ok(true)
     }
 
     // Returns a profile, if one exists in the settings file.

src/test/clitools.rs

@@ -314,6 +314,9 @@ impl Config {
             "/bogus-config-file.toml",
         );
 
+        // Clear current recursion count to avoid messing up related logic
+        cmd.env("RUST_RECURSION_COUNT", "");
+
         // 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);

tests/suite/cli_misc.rs

@@ -1746,3 +1746,30 @@ 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"),
+                ("RUST_RECURSION_COUNT", ""),
+            ],
+        )
+        .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();
+}