Put mtime-on-use behind a feature flag.

src/cargo/core/compiler/fingerprint.rs

@@ -65,7 +65,8 @@ pub fn prepare_target<'a, 'cfg>(
     debug!("fingerprint at: {}", loc.display());
 
     let fingerprint = calculate(cx, unit)?;
-    let compare = compare_old_fingerprint(&loc, &*fingerprint);
+    let mtime_on_use = cx.bcx.config.cli_unstable().mtime_on_use;
+    let compare = compare_old_fingerprint(&loc, &*fingerprint, mtime_on_use);
     log_compare(unit, &compare);
 
     // If our comparison failed (e.g. we're going to trigger a rebuild of this
@@ -102,8 +103,10 @@ pub fn prepare_target<'a, 'cfg>(
                 .filter(|output| output.flavor != FileFlavor::DebugInfo)
                 .find(|output| {
                     if output.path.exists() {
-                        // update the mtime so other cleaners know we used it
-                        let _ = filetime::set_file_times(&output.path, t, t);
+                        if mtime_on_use {
+                            // update the mtime so other cleaners know we used it
+                            let _ = filetime::set_file_times(&output.path, t, t);
+                        }
                         false
                     } else {
                         true
@@ -555,7 +558,8 @@ pub fn prepare_build_cmd<'a, 'cfg>(
         rustc: util::hash_u64(&cx.bcx.rustc.verbose_version),
         ..Fingerprint::new()
     };
-    let compare = compare_old_fingerprint(&loc, &fingerprint);
+    let mtime_on_use = cx.bcx.config.cli_unstable().mtime_on_use;
+    let compare = compare_old_fingerprint(&loc, &fingerprint, mtime_on_use);
     log_compare(unit, &compare);
 
     // When we write out the fingerprint, we may want to actually change the
@@ -688,12 +692,18 @@ pub fn dep_info_loc<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> Pa
         .join(&format!("dep-{}", filename(cx, unit)))
 }
 
-fn compare_old_fingerprint(loc: &Path, new_fingerprint: &Fingerprint) -> CargoResult<()> {
+fn compare_old_fingerprint(
+    loc: &Path,
+    new_fingerprint: &Fingerprint,
+    mtime_on_use: bool,
+) -> CargoResult<()> {
     let old_fingerprint_short = paths::read(loc)?;
 
-    // update the mtime so other cleaners know we used it
-    let t = FileTime::from_system_time(SystemTime::now());
-    filetime::set_file_times(loc, t, t)?;
+    if mtime_on_use {
+        // update the mtime so other cleaners know we used it
+        let t = FileTime::from_system_time(SystemTime::now());
+        filetime::set_file_times(loc, t, t)?;
+    }
 
     let new_hash = new_fingerprint.hash();
 

src/cargo/core/features.rs

@@ -320,6 +320,7 @@ pub struct CliUnstable {
     pub package_features: bool,
     pub advanced_env: bool,
     pub config_profile: bool,
+    pub mtime_on_use: bool,
 }
 
 impl CliUnstable {
@@ -356,6 +357,7 @@ impl CliUnstable {
             "package-features" => self.package_features = true,
             "advanced-env" => self.advanced_env = true,
             "config-profile" => self.config_profile = true,
+            "mtime-on-use" => self.mtime_on_use = true,
             _ => failure::bail!("unknown `-Z` flag specified: {}", k),
         }
 

tests/testsuite/freshness.rs

@@ -1204,7 +1204,7 @@ fn simple_deps_cleaner(mut dir: PathBuf, timestamp: filetime::FileTime) {
 }
 
 #[test]
-fn simple_deps_cleaner_dose_not_rebuild() {
+fn simple_deps_cleaner_does_not_rebuild() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -1222,8 +1222,11 @@ fn simple_deps_cleaner_dose_not_rebuild() {
         .file("bar/src/lib.rs", "")
         .build();
 
-    p.cargo("build").run();
-    p.cargo("build")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
+        .run();
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .env("RUSTFLAGS", "-C target-cpu=native")
         .with_stderr(
             "\
@@ -1239,19 +1242,22 @@ fn simple_deps_cleaner_dose_not_rebuild() {
     if is_coarse_mtime() {
         sleep_ms(1000);
     }
-    // This dose not make new files, but it dose update the mtime.
-    p.cargo("build")
+    // This does not make new files, but it does update the mtime.
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .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")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .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")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .with_stderr(
             "\
 [COMPILING] bar v0.0.1 ([..])
@@ -1293,7 +1299,7 @@ fn fingerprint_cleaner(mut dir: PathBuf, timestamp: filetime::FileTime) {
 }
 
 #[test]
-fn fingerprint_cleaner_dose_not_rebuild() {
+fn fingerprint_cleaner_does_not_rebuild() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -1311,8 +1317,11 @@ fn fingerprint_cleaner_dose_not_rebuild() {
         .file("bar/src/lib.rs", "")
         .build();
 
-    p.cargo("build").run();
-    p.cargo("build")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
+        .run();
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .env("RUSTFLAGS", "-C target-cpu=native")
         .with_stderr(
             "\
@@ -1328,19 +1337,22 @@ fn fingerprint_cleaner_dose_not_rebuild() {
     if is_coarse_mtime() {
         sleep_ms(1000);
     }
-    // This dose not make new files, but it dose update the mtime.
-    p.cargo("build")
+    // This does not make new files, but it does update the mtime.
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .env("RUSTFLAGS", "-C target-cpu=native")
         .with_stderr("[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]")
         .run();
     fingerprint_cleaner(p.target_debug_dir(), timestamp);
     // This should not recompile!
-    p.cargo("build")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .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")
+    p.cargo("build -Z mtime-on-use")
+        .masquerade_as_nightly_cargo()
         .with_stderr(
             "\
 [COMPILING] bar v0.0.1 ([..])