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, 5 insertions, 6 deletions

diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 6d4585cfbc6..daab6c85581 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -1,7 +1,9 @@
 use crate::dep_graph::{DepNode, WorkProduct, WorkProductId};
 use crate::ty::{GenericArgs, Instance, InstanceDef, SymbolName, TyCtxt};
 use rustc_attr::InlineAttr;
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::BaseNString;
+use rustc_data_structures::base_n::ToBaseN;
+use rustc_data_structures::base_n::CASE_INSENSITIVE;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::FxIndexMap;
@@ -337,14 +339,11 @@ impl<'tcx> CodegenUnit<'tcx> {
         self.is_code_coverage_dead_code_cgu = true;
     }
 
-    pub fn mangle_name(human_readable_name: &str) -> String {
-        // We generate a 80 bit hash from the name. This should be enough to
-        // avoid collisions and is still reasonably short for filenames.
+    pub fn mangle_name(human_readable_name: &str) -> BaseNString {
         let mut hasher = StableHasher::new();
         human_readable_name.hash(&mut hasher);
         let hash: Hash128 = hasher.finish();
-        let hash = hash.as_u128() & ((1u128 << 80) - 1);
-        base_n::encode(hash, base_n::CASE_INSENSITIVE)
+        hash.as_u128().to_base_fixed_len(CASE_INSENSITIVE)
     }
 
     pub fn compute_size_estimate(&mut self) {