Rollup merge of #82047 - the8472:fast-rename, r=davidtwco

bypass auto_da_alloc for metadata files

This saves about 0.7% when rerunning the UI test suite. I.e. when the metadata files exist and will be overwritten. No improvements expected for a clean build. So it might show up in incr-patched perf results.
```
regular rename:

Benchmark #1: touch src/tools/compiletest/src/main.rs ; RUSTC_WRAPPER="" schedtool -B -e ./x.py test src/test/ui
  Time (mean ± σ):     47.305 s ±  0.170 s    [User: 1631.540 s, System: 412.648 s]
  Range (min … max):   47.125 s … 47.856 s    20 runs

non-durable rename:

Benchmark #1: touch src/tools/compiletest/src/main.rs ; RUSTC_WRAPPER="" schedtool -B -e ./x.py test src/test/ui
  Time (mean ± σ):     46.930 s ±  0.064 s    [User: 1634.344 s, System: 396.038 s]
  Range (min … max):   46.759 s … 47.043 s    20 runs
```

There are more places that trigger auto_da_alloc behavior by overwriting existing files with O_TRUNC, but those are much harder to locate because `O_TRUNC` is set on `open()` but the writeback is triggered on `close()`. The latter is the part which shows up in profiles.

2 files changed, 19 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 5217066bbef..fb6ef7d6b25 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -988,7 +988,7 @@ fn encode_and_write_metadata(
             .unwrap_or_else(|err| tcx.sess.fatal(&format!("couldn't create a temp dir: {}", err)));
         let metadata_tmpdir = MaybeTempDir::new(metadata_tmpdir, tcx.sess.opts.cg.save_temps);
         let metadata_filename = emit_metadata(tcx.sess, &metadata, &metadata_tmpdir);
-        if let Err(e) = fs::rename(&metadata_filename, &out_filename) {
+        if let Err(e) = util::non_durable_rename(&metadata_filename, &out_filename) {
             tcx.sess.fatal(&format!("failed to write {}: {}", out_filename.display(), e));
         }
         if tcx.sess.opts.json_artifact_notifications {
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index 798996263c7..0a30eda1ec4 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -694,6 +694,24 @@ pub fn build_output_filenames(
     }
 }
 
+#[cfg(not(target_os = "linux"))]
+pub fn non_durable_rename(src: &Path, dst: &Path) -> std::io::Result<()> {
+    std::fs::rename(src, dst)
+}
+
+/// This function attempts to bypass the auto_da_alloc heuristic implemented by some filesystems
+/// such as btrfs and ext4. When renaming over a file that already exists then they will "helpfully"
+/// write back the source file before committing the rename in case a developer forgot some of
+/// the fsyncs in the open/write/fsync(file)/rename/fsync(dir) dance for atomic file updates.
+///
+/// To avoid triggering this heuristic we delete the destination first, if it exists.
+/// The cost of an extra syscall is much lower than getting descheduled for the sync IO.
+#[cfg(target_os = "linux")]
+pub fn non_durable_rename(src: &Path, dst: &Path) -> std::io::Result<()> {
+    let _ = std::fs::remove_file(dst);
+    std::fs::rename(src, dst)
+}
+
 // Note: Also used by librustdoc, see PR #43348. Consider moving this struct elsewhere.
 //
 // FIXME: Currently the `everybody_loops` transformation is not applied to: