Auto merge of #123441 - saethlin:fixed-len-file-names, r=oli-obk

Stabilize the size of incr comp object file names

The current implementation does not produce stable-length paths, and we create the paths in a way that makes our allocation behavior is nondeterministic. I think `@eddyb` fixed a number of other cases like this in the past, and this PR fixes another one. Whether that actually matters I have no idea, but we still have bimodal behavior in rustc-perf and the non-uniformity in `find` and `ls` was bothering me.

I've also removed the truncation of the mangled CGU names. Before this PR incr comp paths look like this:
```
target/debug/incremental/scratch-38izrrq90cex7/s-gux6gz0ow8-1ph76gg-ewe1xj434l26w9up5bedsojpd/261xgo1oqnd90ry5.o
```
And after, they look like this:
```
target/debug/incremental/scratch-035omutqbfkbw/s-gux6borni0-16r3v1j-6n64tmwqzchtgqzwwim5amuga/55v2re42sztc8je9bva6g8ft3.o
```

On the one hand, I'm sure this will break some people's builds because they're on Windows and only a few bytes from the path length limit. But if we're that seriously worried about the length of our file names, I have some other ideas on how to make them smaller. And last time I deleted some hash truncations from the compiler, there was a huge drop in the number if incremental compilation ICEs that were reported: https://github.com/rust-lang/rust/pull/110367https://github.com/rust-lang/rust/pull/110367

---

Upon further reading, this PR actually fixes a bug. This comment says the CGU names are supposed to be a fixed-length hash, and before this PR they aren't: https://github.com/rust-lang/rust/blob/ca7d34efa94afe271accf2bd3d44152a5bd6fff1/compiler/rustc_monomorphize/src/partitioning.rs#L445-L448

1 files changed, 10 insertions, 2 deletions

diff --git a/compiler/rustc_data_structures/src/base_n/tests.rs b/compiler/rustc_data_structures/src/base_n/tests.rs
index 2be2f0532c9..148d8dde02a 100644
--- a/compiler/rustc_data_structures/src/base_n/tests.rs
+++ b/compiler/rustc_data_structures/src/base_n/tests.rs
@@ -1,9 +1,17 @@
 use super::*;
 
 #[test]
-fn test_encode() {
+fn limits() {
+    assert_eq!(Ok(u128::MAX), u128::from_str_radix(&u128::MAX.to_base(36), 36));
+    assert_eq!(Ok(u64::MAX), u64::from_str_radix(&u64::MAX.to_base(36), 36));
+    assert_eq!(Ok(u32::MAX), u32::from_str_radix(&u32::MAX.to_base(36), 36));
+}
+
+#[test]
+fn test_to_base() {
     fn test(n: u128, base: usize) {
-        assert_eq!(Ok(n), u128::from_str_radix(&encode(n, base), base as u32));
+        assert_eq!(Ok(n), u128::from_str_radix(&n.to_base(base), base as u32));
+        assert_eq!(Ok(n), u128::from_str_radix(&n.to_base_fixed_len(base), base as u32));
     }
 
     for base in 2..37 {