Rollup merge of #137713 - vayunbiyani:fix-enzyme-build-errors, r=oli-obk

Fix enzyme build errors

After [this PR](https://github.com/rust-lang/rust/pull/136428) was merged, I switched to master and attempted building `./x.py build --stage 1 library` with the config mentioned in the enzyme rustbook but it resulted in some errors tho the config.example.toml build succeeded
The errors were re:
### 1. Use of ref in match patterns
The errors were related to match ergonomics in Rust 2024, where ref is no longer needed when matching on references. Examples:
```
error: binding modifiers may only be written when the default binding mode is `move`
   --> compiler/rustc_builtin_macros/src/autodiff.rs:136:31
    |
136 |             Annotatable::Item(ref iitem) => {
    |                               ^^^ binding modifier not allowed under `ref` default binding mode
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
note: matching on a reference type with a non-reference pattern changes the default binding mode
   --> compiler/rustc_builtin_macros/src/autodiff.rs:136:13
    |
136 |             Annotatable::Item(ref iitem) => {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
help: remove the unnecessary binding modifier
    |
136 -             Annotatable::Item(ref iitem) => {
136 +             Annotatable::Item(iitem) => {
    |

error: binding modifiers may only be written when the default binding mode is `move`
   --> compiler/rustc_builtin_macros/src/autodiff.rs:146:36
    |
146 |             Annotatable::AssocItem(ref assoc_item, _) => {
    |                                    ^^^ binding modifier not allowed under `ref` default binding mode
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
note: matching on a reference type with a non-reference pattern changes the default binding mode
   --> compiler/rustc_builtin_macros/src/autodiff.rs:146:13
    |
146 |             Annotatable::AssocItem(ref assoc_item, _) => {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
help: remove the unnecessary binding modifier
    |
146 -             Annotatable::AssocItem(ref assoc_item, _) => {
146 +             Annotatable::AssocItem(assoc_item, _) => {
    |

error: binding modifiers may only be written when the default binding mode is `move`
   --> compiler/rustc_builtin_macros/src/autodiff.rs:174:31
    |
174 | ...       Annotatable::Item(ref iitem) => (iitem.vis.clone(), iitem.ide...
    |                             ^^^ binding modifier not allowed under `ref` default binding mode
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
note: matching on a reference type with a non-reference pattern changes the default binding mode
   --> compiler/rustc_builtin_macros/src/autodiff.rs:174:13
    |
174 | ...   Annotatable::Item(ref iitem) => (iitem.vis.clone(), iitem.ident.c...
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
help: remove the unnecessary binding modifier
    |
174 -             Annotatable::Item(ref iitem) => (iitem.vis.clone(), iitem.ident.clone()),
174 +             Annotatable::Item(iitem) => (iitem.vis.clone(), iitem.ident.clone()),
    |

error: binding modifiers may only be written when the default binding mode is `move`
   --> compiler/rustc_builtin_macros/src/autodiff.rs:175:36
    |
175 |             Annotatable::AssocItem(ref assoc_item, _) => {
    |                                    ^^^ binding modifier not allowed under `ref` default binding mode
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
note: matching on a reference type with a non-reference pattern changes the default binding mode
   --> compiler/rustc_builtin_macros/src/autodiff.rs:175:13
    |
175 |             Annotatable::AssocItem(ref assoc_item, _) => {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
help: remove the unnecessary binding modifier
    |
175 -             Annotatable::AssocItem(ref assoc_item, _) => {
175 +             Annotatable::AssocItem(assoc_item, _) => {
    |

error: could not compile `rustc_builtin_macros` (lib) due to 4 previous errors
warning: build failed, waiting for other jobs to finish...
Build completed unsuccessfully in 0:19:39
```
### 2. the use of external C blocks without unsafe in compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs (I don't have the error message handy)

The first commit fixes the errors above

---
## Additional Improvement:
`@ZuseZ4` suggested we consolidate the variants under `#[cfg(llvm_enzyme)]` and `#[cfg(not(llvm_enzyme))]` by conditionally checking for `cfg!(llvm_enzyme)` instead. This way,  the autodiff code is compiled but not executed avoiding such regressions

r? `@ZuseZ4`
cc: `@oli-obk`

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
index 3c2c6964a3d..25ca3498803 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
@@ -42,10 +42,10 @@ pub use self::Enzyme_AD::*;
 #[cfg(llvm_enzyme)]
 pub mod Enzyme_AD {
     use libc::c_void;
-    extern "C" {
+    unsafe extern "C" {
         pub fn EnzymeSetCLBool(arg1: *mut ::std::os::raw::c_void, arg2: u8);
     }
-    extern "C" {
+    unsafe extern "C" {
         static mut EnzymePrintPerf: c_void;
         static mut EnzymePrintActivity: c_void;
         static mut EnzymePrintType: c_void;