stabilize ci_rustc_if_unchanged_logic test for local environments

src/bootstrap/src/core/builder/tests.rs

@@ -261,8 +261,14 @@ fn ci_rustc_if_unchanged_logic() {
     // Make sure "if-unchanged" logic doesn't try to use CI rustc while there are changes
     // in compiler and/or library.
     if config.download_rustc_commit.is_some() {
-        let has_changes =
-            config.last_modified_commit(&["compiler", "library"], "download-rustc", true).is_none();
+        let mut paths = vec!["compiler"];
+
+        // Handle library tree the same way as in `Config::download_ci_rustc_commit`.
+        if build_helper::ci::CiEnv::is_ci() {
+            paths.push("library");
+        }
+
+        let has_changes = config.last_modified_commit(&paths, "download-rustc", true).is_none();
 
         assert!(
             !has_changes,

src/bootstrap/src/core/config/config.rs

@@ -2985,6 +2985,9 @@ impl Config {
         // these changes to speed up the build process for library developers. This provides consistent
         // functionality for library developers between `download-rustc=true` and `download-rustc="if-unchanged"`
         // options.
+        //
+        // If you update "library" logic here, update `builder::tests::ci_rustc_if_unchanged_logic` test
+        // logic accordingly.
         if !CiEnv::is_ci() {
             allowed_paths.push(":!library");
         }