enrich error info when tries to dlopen Enzyme

compiler/rustc_codegen_llvm/messages.ftl

@@ -1,4 +1,5 @@
-codegen_llvm_autodiff_component_unavailable = failed to load our autodiff backend. Did you install it via rustup?
+codegen_llvm_autodiff_component_unavailable = failed to load our autodiff backend.
+    .note = load error: {$err}
 
 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

compiler/rustc_codegen_llvm/src/errors.rs

@@ -34,7 +34,10 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
 
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_component_unavailable)]
-pub(crate) struct AutoDiffComponentUnavailable;
+#[note]
+pub(crate) struct AutoDiffComponentUnavailable {
+    pub err: String,
+}
 
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_without_lto)]

compiler/rustc_codegen_llvm/src/lib.rs

@@ -249,8 +249,10 @@ impl CodegenBackend for LlvmCodegenBackend {
 
             use crate::back::lto::enable_autodiff_settings;
             if sess.opts.unstable_opts.autodiff.contains(&AutoDiff::Enable) {
-                if let Err(_) = llvm::EnzymeWrapper::get_or_init(&sess.opts.sysroot) {
-                    sess.dcx().emit_fatal(crate::errors::AutoDiffComponentUnavailable);
+                if let Err(err) = llvm::EnzymeWrapper::get_or_init(&sess.opts.sysroot) {
+                    sess.dcx().emit_fatal(crate::errors::AutoDiffComponentUnavailable {
+                        err: format!("{err:?}"),
+                    });
                 }
                 enable_autodiff_settings(&sess.opts.unstable_opts.autodiff);
             }