[ci skip] (wip) Add unit tests for Cfg overriding

src/config.rs

@@ -1046,6 +1046,8 @@ enum ParseMode {
 mod tests {
     use rustup_macros::unit_test as test;
 
+    use crate::{cli::common::set_globals, utils::raw};
+
     use super::*;
 
     #[test]
@@ -1248,4 +1250,27 @@ channel = nightly
             Ok(OverrideFileConfigError::Parsing)
         ));
     }
+
+    /// Checks that `rust-toolchain.toml` configs can be overridden by `<proxy> +<toolchain>`.
+    /// See: <https://github.com/rust-lang/rustup/issues/3483>
+    #[test]
+    fn find_override_config_with_toolchain_override() {
+        let cwd = crate::test::test_dir().unwrap();
+        let toolchain_file = cwd.path().join("rust-toolchain.toml");
+        raw::write_file(
+            &toolchain_file,
+            r#"
+                    [toolchain]
+                    channel = "nightly"
+                    components = [ "rls" ]
+                "#,
+        )
+        .unwrap();
+
+        let mut cfg = set_globals(true, false).unwrap();
+        cfg.toolchain_override = Some("beta".try_into().unwrap());
+
+        let found_override = cfg.find_override_config(cwd.path()).unwrap();
+        assert!(dbg!(found_override).is_none());
+    }
 }