Make llvm_enzyme a regular cargo feature

compiler/rustc/Cargo.toml

@@ -30,6 +30,7 @@ features = ['unprefixed_malloc_on_supported_platforms']
 check_only = ['rustc_driver_impl/check_only']
 jemalloc = ['dep:tikv-jemalloc-sys']
 llvm = ['rustc_driver_impl/llvm']
+llvm_enzyme = ['rustc_driver_impl/llvm_enzyme']
 max_level_info = ['rustc_driver_impl/max_level_info']
 rustc_randomized_layouts = ['rustc_driver_impl/rustc_randomized_layouts']
 # tidy-alphabetical-end

compiler/rustc_builtin_macros/Cargo.toml

@@ -33,3 +33,8 @@ smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2.12"
 tracing = "0.1"
 # tidy-alphabetical-end
+
+[features]
+# tidy-alphabetical-start
+llvm_enzyme = []
+# tidy-alphabetical-end

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -209,7 +209,8 @@ mod llvm_enzyme {
         mut item: Annotatable,
         mode: DiffMode,
     ) -> Vec<Annotatable> {
-        if cfg!(not(llvm_enzyme)) {
+        // FIXME(bjorn3) maybe have the backend directly tell if autodiff is supported?
+        if cfg!(not(feature = "llvm_enzyme")) {
             ecx.sess.dcx().emit_err(errors::AutoDiffSupportNotBuild { span: meta_item.span });
             return vec![item];
         }

compiler/rustc_codegen_llvm/Cargo.toml

@@ -46,5 +46,6 @@ tracing = "0.1"
 [features]
 # tidy-alphabetical-start
 check_only = ["rustc_llvm/check_only"]
+llvm_enzyme = []
 # tidy-alphabetical-end
 

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -617,7 +617,7 @@ pub(crate) fn run_pass_manager(
         crate::builder::gpu_offload::handle_gpu_code(cgcx, &cx);
     }
 
-    if cfg!(llvm_enzyme) && enable_ad && !thin {
+    if cfg!(feature = "llvm_enzyme") && enable_ad && !thin {
         let opt_stage = llvm::OptStage::FatLTO;
         let stage = write::AutodiffStage::PostAD;
         if !config.autodiff.contains(&config::AutoDiff::NoPostopt) {

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -574,7 +574,8 @@ pub(crate) unsafe fn llvm_optimize(
     // FIXME(ZuseZ4): In a future update we could figure out how to only optimize individual functions getting
     // differentiated.
 
-    let consider_ad = cfg!(llvm_enzyme) && config.autodiff.contains(&config::AutoDiff::Enable);
+    let consider_ad =
+        cfg!(feature = "llvm_enzyme") && config.autodiff.contains(&config::AutoDiff::Enable);
     let run_enzyme = autodiff_stage == AutodiffStage::DuringAD;
     let print_before_enzyme = config.autodiff.contains(&config::AutoDiff::PrintModBefore);
     let print_after_enzyme = config.autodiff.contains(&config::AutoDiff::PrintModAfter);
@@ -740,7 +741,8 @@ pub(crate) fn optimize(
 
         // If we know that we will later run AD, then we disable vectorization and loop unrolling.
         // Otherwise we pretend AD is already done and run the normal opt pipeline (=PostAD).
-        let consider_ad = cfg!(llvm_enzyme) && config.autodiff.contains(&config::AutoDiff::Enable);
+        let consider_ad =
+            cfg!(feature = "llvm_enzyme") && config.autodiff.contains(&config::AutoDiff::Enable);
         let autodiff_stage = if consider_ad { AutodiffStage::PreAD } else { AutodiffStage::PostAD };
         // The embedded bitcode is used to run LTO/ThinLTO.
         // The bitcode obtained during the `codegen` phase is no longer suitable for performing LTO.

compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

@@ -59,10 +59,10 @@ pub(crate) enum LLVMRustVerifierFailureAction {
     LLVMReturnStatusAction = 2,
 }
 
-#[cfg(llvm_enzyme)]
+#[cfg(feature = "llvm_enzyme")]
 pub(crate) use self::Enzyme_AD::*;
 
-#[cfg(llvm_enzyme)]
+#[cfg(feature = "llvm_enzyme")]
 pub(crate) mod Enzyme_AD {
     use std::ffi::{CString, c_char};
 
@@ -134,10 +134,10 @@ pub(crate) mod Enzyme_AD {
     }
 }
 
-#[cfg(not(llvm_enzyme))]
+#[cfg(not(feature = "llvm_enzyme"))]
 pub(crate) use self::Fallback_AD::*;
 
-#[cfg(not(llvm_enzyme))]
+#[cfg(not(feature = "llvm_enzyme"))]
 pub(crate) mod Fallback_AD {
     #![allow(unused_variables)]
 

compiler/rustc_driver_impl/Cargo.toml

@@ -74,6 +74,7 @@ ctrlc = "3.4.4"
 # tidy-alphabetical-start
 check_only = ['rustc_interface/check_only']
 llvm = ['rustc_interface/llvm']
+llvm_enzyme = ['rustc_interface/llvm_enzyme']
 max_level_info = ['rustc_log/max_level_info']
 rustc_randomized_layouts = [
     'rustc_index/rustc_randomized_layouts',

compiler/rustc_interface/Cargo.toml

@@ -58,4 +58,5 @@ rustc_abi = { path = "../rustc_abi" }
 # tidy-alphabetical-start
 check_only = ['rustc_codegen_llvm?/check_only']
 llvm = ['dep:rustc_codegen_llvm']
+llvm_enzyme = ['rustc_builtin_macros/llvm_enzyme', 'rustc_codegen_llvm/llvm_enzyme']
 # tidy-alphabetical-end

src/bootstrap/src/core/build_steps/compile.rs

@@ -1357,10 +1357,6 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }
 
-    if builder.config.llvm_enzyme {
-        cargo.rustflag("--cfg=llvm_enzyme");
-    }
-
     // These conditionals represent a tension between three forces:
     // - For non-check builds, we need to define some LLVM-related environment
     //   variables, requiring LLVM to have been built.

src/bootstrap/src/lib.rs

@@ -87,9 +87,6 @@ const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     (Some(Mode::Codegen), "bootstrap", None),
     (Some(Mode::ToolRustcPrivate), "bootstrap", None),
     (Some(Mode::ToolStd), "bootstrap", None),
-    (Some(Mode::Rustc), "llvm_enzyme", None),
-    (Some(Mode::Codegen), "llvm_enzyme", None),
-    (Some(Mode::ToolRustcPrivate), "llvm_enzyme", None),
     (Some(Mode::ToolRustcPrivate), "rust_analyzer", None),
     (Some(Mode::ToolStd), "rust_analyzer", None),
     // Any library specific cfgs like `target_os`, `target_arch` should be put in
@@ -869,6 +866,9 @@ impl Build {
         if (self.config.llvm_enabled(target) || kind == Kind::Check) && check("llvm") {
             features.push("llvm");
         }
+        if self.config.llvm_enzyme {
+            features.push("llvm_enzyme");
+        }
         // keep in sync with `bootstrap/compile.rs:rustc_cargo_env`
         if self.config.rust_randomize_layout && check("rustc_randomized_layouts") {
             features.push("rustc_randomized_layouts");