Auto merge of #128320 - saethlin:link-me-maybe, r=compiler-errors

Avoid no-op unlink+link dances in incr comp

Incremental compilation scales quite poorly with the number of CGUs. This PR improves one reason for that.

The incr comp process hard-links all the files from an old session into a new one, then it runs the backend, which may just hard-link the new session files into the output directory. Then codegen hard-links all the output files back to the new session directory.

This PR (perhaps unimaginatively) fixes the silliness that ensues in the last step. The old `link_or_copy` implementation would be passed pairs of paths which are already the same inode, then it would blindly delete the destination and re-create the hard-link that it just deleted. This PR lets us skip both those operations. We don't skip the other two hard-links.

`cargo +stage1 b && touch crates/core/main.rs && strace -cfw -elink,linkat,unlink,unlinkat cargo +stage1 b` before and then after on `ripgrep-13.0.0`:
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 52.56    0.024950          25       978       485 unlink
 34.38    0.016318          22       727           linkat
 13.06    0.006200          24       249           unlinkat
------ ----------- ----------- --------- --------- ----------------
100.00    0.047467          24      1954       485 total
```
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 42.83    0.014521          57       252           unlink
 38.41    0.013021          26       486           linkat
 18.77    0.006362          25       249           unlinkat
------ ----------- ----------- --------- --------- ----------------
100.00    0.033904          34       987           total
```

This reduces the number of hard-links that are causing perf troubles, noted in https://github.com/rust-lang/rust/issues/64291 and https://github.com/rust-lang/rust/issues/137560

1 files changed, 6 insertions, 1 deletions

diff --git a/compiler/rustc_incremental/src/persist/work_product.rs b/compiler/rustc_incremental/src/persist/work_product.rs
index 048981f0d5c..7b1eb0a82e3 100644
--- a/compiler/rustc_incremental/src/persist/work_product.rs
+++ b/compiler/rustc_incremental/src/persist/work_product.rs
@@ -3,7 +3,7 @@
 //! [work products]: WorkProduct
 
 use std::fs as std_fs;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 use rustc_data_structures::unord::UnordMap;
 use rustc_fs_util::link_or_copy;
@@ -20,6 +20,7 @@ pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
     sess: &Session,
     cgu_name: &str,
     files: &[(&'static str, &Path)],
+    known_links: &[PathBuf],
 ) -> Option<(WorkProductId, WorkProduct)> {
     debug!(?cgu_name, ?files);
     sess.opts.incremental.as_ref()?;
@@ -28,6 +29,10 @@ pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
     for (ext, path) in files {
         let file_name = format!("{cgu_name}.{ext}");
         let path_in_incr_dir = in_incr_comp_dir_sess(sess, &file_name);
+        if known_links.contains(&path_in_incr_dir) {
+            let _ = saved_files.insert(ext.to_string(), file_name);
+            continue;
+        }
         match link_or_copy(path, &path_in_incr_dir) {
             Ok(_) => {
                 let _ = saved_files.insert(ext.to_string(), file_name);