autodiff: emit an error if we fail to find libEnzyme

compiler/rustc_codegen_llvm/messages.ftl

@@ -1,9 +1,10 @@
+codegen_llvm_autodiff_component_unavailable = failed to load our autodiff backend. Did you install it via rustup?
+
 codegen_llvm_autodiff_without_enable = using the autodiff feature requires -Z autodiff=Enable
 codegen_llvm_autodiff_without_lto = using the autodiff feature requires setting `lto="fat"` in your Cargo.toml
 
 codegen_llvm_copy_bitcode = failed to copy bitcode to object file: {$err}
 
-
 codegen_llvm_fixed_x18_invalid_arch = the `-Zfixed-x18` flag is not supported on the `{$arch}` architecture
 
 codegen_llvm_from_llvm_diag = {$message}

compiler/rustc_codegen_llvm/src/errors.rs

@@ -32,6 +32,11 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
     }
 }
 
+#[cfg(feature = "llvm_enzyme")]
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_autodiff_component_unavailable)]
+pub(crate) struct AutoDiffComponentUnavailable;
+
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_without_lto)]
 pub(crate) struct AutoDiffWithoutLto;

compiler/rustc_codegen_llvm/src/lib.rs

@@ -13,6 +13,7 @@
 #![feature(impl_trait_in_assoc_type)]
 #![feature(iter_intersperse)]
 #![feature(macro_derive)]
+#![feature(once_cell_try)]
 #![feature(trim_prefix_suffix)]
 #![feature(try_blocks)]
 // tidy-alphabetical-end
@@ -247,7 +248,9 @@ impl CodegenBackend for LlvmCodegenBackend {
 
             use crate::back::lto::enable_autodiff_settings;
             if sess.opts.unstable_opts.autodiff.contains(&AutoDiff::Enable) {
-                drop(llvm::EnzymeWrapper::get_or_init(&sess.opts.sysroot));
+                if let Err(_) = llvm::EnzymeWrapper::get_or_init(&sess.opts.sysroot) {
+                    sess.dcx().emit_fatal(crate::errors::AutoDiffComponentUnavailable);
+                }
                 enable_autodiff_settings(&sess.opts.unstable_opts.autodiff);
             }
         }

compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

@@ -199,15 +199,13 @@ pub(crate) mod Enzyme_AD {
         /// Safe to call multiple times - subsequent calls are no-ops due to OnceLock.
         pub(crate) fn get_or_init(
             sysroot: &rustc_session::config::Sysroot,
-        ) -> MutexGuard<'static, Self> {
-            ENZYME_INSTANCE
-                .get_or_init(|| {
-                    Self::call_dynamic(sysroot)
-                        .unwrap_or_else(|e| bug!("failed to load Enzyme: {e}"))
-                        .into()
-                })
-                .lock()
-                .unwrap()
+        ) -> Result<MutexGuard<'static, Self>, Box<dyn std::error::Error>> {
+            let mtx: &'static Mutex<EnzymeWrapper> = ENZYME_INSTANCE.get_or_try_init(|| {
+                let w = Self::call_dynamic(sysroot)?;
+                Ok::<_, Box<dyn std::error::Error>>(Mutex::new(w))
+            })?;
+
+            Ok(mtx.lock().unwrap())
         }
 
         /// Get the EnzymeWrapper instance. Panics if not initialized.
@@ -475,7 +473,7 @@ pub(crate) mod Fallback_AD {
     impl EnzymeWrapper {
         pub(crate) fn get_or_init(
             _sysroot: &rustc_session::config::Sysroot,
-        ) -> MutexGuard<'static, Self> {
+        ) -> Result<MutexGuard<'static, Self>, Box<dyn std::error::Error>> {
             unimplemented!("Enzyme not available: build with llvm_enzyme feature")
         }