only skip mtime check on $CARGO_HOME/{git,registry}

Closes rust-lang#12090

Commit

Fix formatting

src/cargo/core/compiler/fingerprint/mod.rs

@@ -1857,14 +1857,27 @@ where
         Err(..) => return Some(StaleItem::MissingFile(reference.to_path_buf())),
     };
 
+    let skipable_dirs = if let Ok(cargo_home) = home::cargo_home() {
+        let skipable_dirs: Vec<_> = ["git", "registry"]
+            .into_iter()
+            .map(|subfolder| cargo_home.join(subfolder))
+            .collect();
+        Some(skipable_dirs)
+    } else {
+        None
+    };
+
     for path in paths {
         let path = path.as_ref();
 
-        // Assuming anything in cargo_home is immutable (see also #9455 about marking it readonly)
-        // which avoids rebuilds when CI caches $CARGO_HOME/registry/{index, cache} and
-        // $CARGO_HOME/git/db across runs, keeping the content the same but changing the mtime.
-        if let Ok(true) = home::cargo_home().map(|home| path.starts_with(home)) {
-            continue;
+        // Assuming anything in cargo_home/{git, registry} is immutable
+        // (see also #9455 about marking the src directory readonly) which avoids rebuilds when CI
+        // caches $CARGO_HOME/registry/{index, cache} and $CARGO_HOME/git/db across runs, keeping
+        // the content the same but changing the mtime.
+        if let Some(ref skipable_dirs) = skipable_dirs {
+            if skipable_dirs.iter().any(|dir| path.starts_with(dir)) {
+                continue;
+            }
         }
         let path_mtime = match mtime_cache.entry(path.to_path_buf()) {
             Entry::Occupied(o) => *o.get(),

tests/testsuite/build.rs

@@ -5972,6 +5972,62 @@ fn build_with_relative_cargo_home_path() {
     p.cargo("build").env("CARGO_HOME", "./cargo_home/").run();
 }
 
+#[cargo_test]
+fn skip_mtime_check_in_selected_cargo_home_subdirs() {
+    // Run once without cache
+    let p = project()
+        .at("cargo_home/registry/foo")
+        .file("Cargo.toml", &basic_bin_manifest("foo"))
+        .file("src/main.rs", r#"fn main() { println!("1"); }"#)
+        .build();
+    let project_root = p.root();
+    let cargo_home = project_root
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .to_str()
+        .unwrap();
+    p.cargo("run")
+        .env("CARGO_HOME", &cargo_home)
+        .with_stdout("1")
+        .run();
+    // Run with cache
+    p.change_file("src/main.rs", r#"fn main() { println!("2"); }"#);
+    p.cargo("run")
+        .env("CARGO_HOME", &cargo_home)
+        .with_stdout("1")
+        .run();
+}
+
+#[cargo_test]
+fn use_mtime_cache_in_cargo_home() {
+    println!("Without cache");
+    let p = project()
+        .at("cargo_home/foo")
+        .file("Cargo.toml", &basic_bin_manifest("foo"))
+        .file("src/main.rs", r#"fn main() { println!("1"); }"#)
+        .build();
+    let project_root = p.root();
+    let cargo_home = project_root
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .to_str()
+        .unwrap();
+    p.cargo("run")
+        .env("CARGO_HOME", &cargo_home)
+        .with_stdout("1")
+        .run();
+    println!("With cache");
+    p.change_file("src/main.rs", r#"fn main() { println!("2"); }"#);
+    p.cargo("run")
+        .env("CARGO_HOME", &cargo_home)
+        .with_stdout("2")
+        .run();
+}
+
 #[cargo_test]
 fn user_specific_cfgs_are_filtered_out() {
     let p = project()