Auto merge of #129176 - EnzymeAD:enzyme-backend, r=albertlarsan68

Autodiff Upstreaming - enzyme backend

Tracking issue: https://github.com/rust-lang/rust/issues/124509

Part of https://github.com/rust-lang/rust/pull/129175

This PR should allow building Enzyme from source on Tier 1 targets (when also building LLVM), except MSVC.
It's only a small fraction (~200 lines) of the whole upstream PR, but due to bootstrapping and the number of configurations in which rustc can be build I assume that this will be the hardest to merge, so I'm starting with it.
Happy to hear what changes are required to be able to upstream this code.

**Content:**
It contains a new configure flag `--enable-llvm-enzyme`, and will build the new Enzyme submodule when it is set.

**Discussion:**
Apparently Rust CI isn't able to clone repositories outside the rust-lang org? At least I'm seeing this error in CI:
```
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```
Does that mean we would need to mirror github.com/EnzymeAD/Enzyme in rust-lang, until LLVM upgrades Enzyme from an Incubator project to something that ships as part of the monorepo?

Tracking:

- https://github.com/rust-lang/rust/issues/124509

4 files changed, 4 insertions, 0 deletions

diff --git a/src/tools/enzyme b/src/tools/enzyme
new file mode 160000
+Subproject 2fe5164a2423dd67ef25e2c4fb204fd06362494
diff --git a/src/tools/tidy/config/black.toml b/src/tools/tidy/config/black.toml
index e73847a93ba..a1e1eef987a 100644
--- a/src/tools/tidy/config/black.toml
+++ b/src/tools/tidy/config/black.toml
@@ -12,5 +12,6 @@ extend-exclude = """(\
     src/llvm-project/|\
     src/doc/embedded-book/|\
     src/tools/rustc-perf/|\
+    src/tools/enzyme/|\
     library/backtrace/
     )"""
diff --git a/src/tools/tidy/config/ruff.toml b/src/tools/tidy/config/ruff.toml
index c87c17f7833..2be109010a0 100644
--- a/src/tools/tidy/config/ruff.toml
+++ b/src/tools/tidy/config/ruff.toml
@@ -16,6 +16,7 @@ extend-exclude = [
     "src/llvm-project/",
     "src/doc/embedded-book/",
     "library/backtrace/",
+    "src/tools/enzyme/",
     "src/tools/rustc-perf/",
     # Hack: CI runs from a subdirectory under the main checkout
     "../src/doc/nomicon/",
@@ -29,6 +30,7 @@ extend-exclude = [
     "../src/llvm-project/",
     "../src/doc/embedded-book/",
     "../library/backtrace/",
+    "../src/tools/enzyme/",
     "../src/tools/rustc-perf/",
 ]
 
diff --git a/src/tools/tidy/src/walk.rs b/src/tools/tidy/src/walk.rs
index d4721b5a5fd..4a69022ebdc 100644
--- a/src/tools/tidy/src/walk.rs
+++ b/src/tools/tidy/src/walk.rs
@@ -24,6 +24,7 @@ pub fn filter_dirs(path: &Path) -> bool {
         "src/tools/rust-analyzer",
         "src/tools/rustc-perf",
         "src/tools/rustfmt",
+        "src/tools/enzyme",
         "src/doc/book",
         "src/doc/edition-guide",
         "src/doc/embedded-book",