Rollup merge of rust-lang#137713 - vayunbiyani:fix-enzyme-build-error…

…s, r=oli-obk

Fix enzyme build errors

After [this PR](rust-lang#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`

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -3,7 +3,6 @@
 //! configs (autodiff enabled or disabled), so we have to add cfg's to each import.
 //! FIXME(ZuseZ4): Remove this once we have a smarter linter.
 
-#[cfg(llvm_enzyme)]
 mod llvm_enzyme {
     use std::str::FromStr;
     use std::string::String;
@@ -130,10 +129,14 @@ mod llvm_enzyme {
         meta_item: &ast::MetaItem,
         mut item: Annotatable,
     ) -> Vec<Annotatable> {
+        if cfg!(not(llvm_enzyme)) {
+            ecx.sess.dcx().emit_err(errors::AutoDiffSupportNotBuild { span: meta_item.span });
+            return vec![item];
+        }
         let dcx = ecx.sess.dcx();
         // first get the annotable item:
         let (sig, is_impl): (FnSig, bool) = match &item {
-            Annotatable::Item(ref iitem) => {
+            Annotatable::Item(iitem) => {
                 let sig = match &iitem.kind {
                     ItemKind::Fn(box ast::Fn { sig, .. }) => sig,
                     _ => {
@@ -143,7 +146,7 @@ mod llvm_enzyme {
                 };
                 (sig.clone(), false)
             }
-            Annotatable::AssocItem(ref assoc_item, _) => {
+            Annotatable::AssocItem(assoc_item, _) => {
                 let sig = match &assoc_item.kind {
                     ast::AssocItemKind::Fn(box ast::Fn { sig, .. }) => sig,
                     _ => {
@@ -171,8 +174,8 @@ mod llvm_enzyme {
         let sig_span = ecx.with_call_site_ctxt(sig.span);
 
         let (vis, primal) = match &item {
-            Annotatable::Item(ref iitem) => (iitem.vis.clone(), iitem.ident.clone()),
-            Annotatable::AssocItem(ref assoc_item, _) => {
+            Annotatable::Item(iitem) => (iitem.vis.clone(), iitem.ident.clone()),
+            Annotatable::AssocItem(assoc_item, _) => {
                 (assoc_item.vis.clone(), assoc_item.ident.clone())
             }
             _ => {
@@ -801,25 +804,4 @@ mod llvm_enzyme {
     }
 }
 
-#[cfg(not(llvm_enzyme))]
-mod ad_fallback {
-    use rustc_ast::ast;
-    use rustc_expand::base::{Annotatable, ExtCtxt};
-    use rustc_span::Span;
-
-    use crate::errors;
-    pub(crate) fn expand(
-        ecx: &mut ExtCtxt<'_>,
-        _expand_span: Span,
-        meta_item: &ast::MetaItem,
-        item: Annotatable,
-    ) -> Vec<Annotatable> {
-        ecx.sess.dcx().emit_err(errors::AutoDiffSupportNotBuild { span: meta_item.span });
-        return vec![item];
-    }
-}
-
-#[cfg(not(llvm_enzyme))]
-pub(crate) use ad_fallback::expand;
-#[cfg(llvm_enzyme)]
 pub(crate) use llvm_enzyme::expand;

compiler/rustc_builtin_macros/src/errors.rs

@@ -144,10 +144,8 @@ pub(crate) struct AllocMustStatics {
     pub(crate) span: Span,
 }
 
-#[cfg(llvm_enzyme)]
 pub(crate) use autodiff::*;
 
-#[cfg(llvm_enzyme)]
 mod autodiff {
     use super::*;
     #[derive(Diagnostic)]
@@ -203,9 +201,7 @@ mod autodiff {
     }
 }
 
-#[cfg(not(llvm_enzyme))]
 pub(crate) use ad_fallback::*;
-#[cfg(not(llvm_enzyme))]
 mod ad_fallback {
     use super::*;
     #[derive(Diagnostic)]

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;