add a test for touching deps

tests/testsuite/freshness.rs

@@ -1,7 +1,9 @@
 use std::fs::{self, File, OpenOptions};
 use std::io::prelude::*;
 use std::net::TcpListener;
+use std::path::PathBuf;
 use std::thread;
+use std::time::SystemTime;
 
 use crate::support::paths::CargoPathExt;
 use crate::support::registry::Package;
@@ -1178,6 +1180,83 @@ fn changing_rustflags_is_cached() {
         .run();
 }
 
+fn simple_deps_cleaner(mut dir: PathBuf, timestamp: filetime::FileTime) {
+    // Cargo is experimenting with letting outside projects develop some
+    // limited forms of GC for target_dir. This is one of the forms.
+    // Specifically, Cargo is updating the mtime of files in
+    // target/profile/deps each time it uses the file.
+    // So a cleaner can remove files older then a time stamp without
+    // effecting any builds that happened since that time stamp.
+    let mut cleand = false;
+    dir.push("deps");
+    for dep in fs::read_dir(&dir).unwrap() {
+        let dep = dep.unwrap();
+        if filetime::FileTime::from_last_modification_time(&dep.metadata().unwrap()) <= timestamp {
+            fs::remove_file(dep.path()).unwrap();
+            cleand = true;
+        }
+    }
+    assert!(
+        cleand,
+        "called simple_deps_cleaner, but there was nothing to remove"
+    );
+}
+
+#[test]
+fn simple_deps_cleaner_dose_not_rebuild() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+
+            [dependencies]
+            bar = { path = "bar" }
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.0.1"))
+        .file("bar/src/lib.rs", "")
+        .build();
+
+    p.cargo("build").run();
+    p.cargo("build")
+        .env("RUSTFLAGS", "-C target-cpu=native")
+        .with_stderr(
+            "\
+[COMPILING] bar v0.0.1 ([..])
+[COMPILING] foo v0.0.1 ([..])
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]",
+        )
+        .run();
+    if is_coarse_mtime() {
+        sleep_ms(1000);
+    }
+    let timestamp = filetime::FileTime::from_system_time(SystemTime::now());
+    // This dose not make new files, but it dose update the mtime.
+    p.cargo("build")
+        .env("RUSTFLAGS", "-C target-cpu=native")
+        .with_stderr("[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]")
+        .run();
+    simple_deps_cleaner(p.target_debug_dir(), timestamp);
+    // This should not recompile!
+    p.cargo("build")
+        .env("RUSTFLAGS", "-C target-cpu=native")
+        .with_stderr("[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]")
+        .run();
+    // But this should be cleaned and so need a rebuild
+    p.cargo("build")
+        .with_stderr(
+            "\
+[COMPILING] bar v0.0.1 ([..])
+[COMPILING] foo v0.0.1 ([..])
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]",
+        )
+        .run();
+}
+
 #[test]
 fn reuse_panic_build_dep_test() {
     let p = project()