chore: Support overriding the fallback settings file path

src/config.rs

@@ -196,7 +196,14 @@ impl Cfg {
 
         // Centralised file for multi-user systems to provide admin/distributor set initial values.
         let fallback_settings = if cfg!(not(windows)) {
-            FallbackSettings::new(PathBuf::from(UNIX_FALLBACK_SETTINGS))?
+            // If present, use the RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS environment
+            // variable as settings path, or UNIX_FALLBACK_SETTINGS otherwise
+            FallbackSettings::new(
+                match process().var("RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS") {
+                    Ok(s) => PathBuf::from(s),
+                    Err(_) => PathBuf::from(UNIX_FALLBACK_SETTINGS),
+                },
+            )?
         } else {
             None
         };

tests/cli-rustup.rs

@@ -1940,3 +1940,41 @@ fn check_host_goes_away() {
         );
     })
 }
+
+#[cfg(unix)]
+#[test]
+fn check_unix_settings_fallback() {
+    setup(&|config| {
+        // No default toolchain specified yet
+        expect_err(
+            config,
+            &["rustup", "default"],
+            r"no default toolchain configured",
+        );
+
+        // Default toolchain specified in fallback settings file
+        let mock_settings_file = config.current_dir().join("mock_fallback_settings.toml");
+        raw::write_file(
+            &mock_settings_file,
+            for_host!(r"default_toolchain = 'nightly-{0}'"),
+        )
+        .unwrap();
+
+        let mut cmd = clitools::cmd(config, "rustup", &["default"]);
+        clitools::env(config, &mut cmd);
+
+        // Override the path to the fallback settings file to be the mock file
+        cmd.env("RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS", mock_settings_file);
+
+        let out = cmd.output().unwrap();
+        assert!(out.status.success());
+        let stdout = String::from_utf8(out.stdout).unwrap();
+        assert_eq!(
+            &stdout,
+            for_host!(
+                r"nightly-{0} (default)
+"
+            )
+        );
+    });
+}

tests/mock/clitools.rs

@@ -446,6 +446,13 @@ pub fn env<E: rustup_test::Env>(config: &Config, cmd: &mut E) {
             .join("tests/mock/signing-key.pub.asc"),
     );
 
+    // The unix fallback settings file may be present in the test environment, so override
+    // the path to the settings file with a non-existing path to avoid intereference
+    cmd.env(
+        "RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS",
+        "/bogus-config-file.toml",
+    );
+
     if let Some(root) = config.rustup_update_root.as_ref() {
         cmd.env("RUSTUP_UPDATE_ROOT", root);
     }