Add tests for rust-toolchain.toml files

tests/cli-rustup.rs

@@ -2065,3 +2065,53 @@ warning: If you meant to build software to target that platform, perhaps try `ru
         );
     });
 }
+
+/// Checks that `rust-toolchain.toml` files are considered
+#[test]
+fn rust_toolchain_toml() {
+    setup(&|config| {
+        let cwd = config.current_dir();
+        let toolchain_file = cwd.join("rust-toolchain.toml");
+        raw::write_file(&toolchain_file, "[toolchain]\nchannel = \"nightly\"").unwrap();
+
+        expect_stdout_ok(config, &["rustc", "--version"], "hash-nightly-2");
+    });
+}
+
+/// Ensures that `rust-toolchain.toml` files (with `.toml` extension) only allow TOML contents
+#[test]
+fn only_toml_in_rust_toolchain_toml() {
+    setup(&|config| {
+        let cwd = config.current_dir();
+        let toolchain_file = cwd.join("rust-toolchain.toml");
+        raw::write_file(&toolchain_file, "nightly").unwrap();
+
+        expect_err(
+            config,
+            &["rustc", "--version"],
+            "error parsing override file",
+        );
+    });
+}
+
+/// Checks that a warning occurs if both `rust-toolchain` and `rust-toolchain.toml` files exist
+#[test]
+fn warn_on_duplicate_rust_toolchain_file() {
+    setup(&|config| {
+        let cwd = config.current_dir();
+        let toolchain_file_1 = cwd.join("rust-toolchain");
+        raw::write_file(&toolchain_file_1, "stable").unwrap();
+        let toolchain_file_2 = cwd.join("rust-toolchain.toml");
+        raw::write_file(&toolchain_file_2, "[toolchain]").unwrap();
+
+        expect_stderr_ok(
+            config,
+            &["rustc", "--version"],
+            &format!(
+                "warning: both `{0}` and `{1}` exist. Using `{0}`",
+                toolchain_file_1.display(),
+                toolchain_file_2.display(),
+            ),
+        );
+    });
+}