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

Author: rami3l

src/config.rs

@@ -8,7 +8,7 @@ use anyhow::{Context, Result, anyhow, bail};
 use serde::Deserialize;
 use thiserror::Error as ThisError;
 use tokio_stream::StreamExt;
-use tracing::trace;
+use tracing::{info, trace, warn};
 
 use crate::dist::AutoInstallMode;
 use crate::{
@@ -387,12 +387,26 @@ impl<'a> Cfg<'a> {
     }
 
     pub(crate) fn should_auto_install(&self) -> Result<bool> {
-        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 res = 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 res
+            // We also need to suppress this warning if we're deep inside a recursive call.
+            && matches!(
+                self.process.var("RUST_RECURSION_COUNT").as_deref(),
+                Err(_) | Ok("0"),
+            )
+        {
+            warn!("auto-install is enabled, active toolchain will be installed if absent");
+            warn!("this behavior is deprecated and will be removed in a future version");
+            info!(
+                "you may opt out now with `RUSTUP_AUTO_INSTALL=0` or `rustup set auto-install disable`"
+            );
         }
+        Ok(res)
     }
 
     // Returns a profile, if one exists in the settings file.

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 behavior is deprecated and will be removed in a future version
+info: you may opt out now with `RUSTUP_AUTO_INSTALL=0` or `rustup set auto-install disable`
+...
+"#]])
+        .is_ok();
+}