Fix enzyme build errors

[vayunbiyani]

After this PR 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

[rustbot]

Failed to set assignee to ZuseZ4: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit c85f038 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.