Slow post-compilation file copy steps

[orlp]

A colleague of mine uses a Linux machine running on an Ext4 disk, and is noticing long (~8 seconds) post-compilation steps (with the latest version of maturin). Using profiling I've identified that the full multi-gigabyte artifact (Polars with debug symbols) is copied multiple times.

One copy occurs here:

maturin/src/binding_generator/mod.rs

Lines 190 to 192 in da71d84

	// 2a. Install the artifact
	debug!("Installing {} from {}", target.display(), source.display());
	fs::copy(&source, &target).with_context(|| {

Another copy occurs here:

maturin/src/build_context/repair.rs

Lines 335 to 344 in da71d84

	// Rename succeeded — we now own the only copy. Put a copy
	// back at the original location for users who expect the
	// artifact at the standard cargo output path. Skip if a
	// new file already appeared (cargo / rust-analyzer rebuilt).
	if artifact_path.exists() {
	tracing::debug!(
	"Skipping copy-back: {} was recreated by another process",
	artifact_path.display()
	);
	} else if let Err(err) = reflink_or_copy(&new_artifact_path, artifact_path) {

Can these copies be replaced with renames / softlinks, or avoided altogether? Reflink is not enough, his filesystem (Ext4, which is really common) does not support them.

For example to avoid the second copy, I believe the "copy back" can just be a "rename back" step after the modification/reading process is complete. A complete file copy just to then be able to 'safely' mmap the file to read it seems rather silly.