Fix ICE on incompatible declarations of entry symbol

Fixes rust-lang#1313

Author: bjorn3

src/main_shim.rs

@@ -70,7 +70,13 @@ pub(crate) fn maybe_create_entry_wrapper(
         };
 
         let entry_name = tcx.sess.target.options.entry_name.as_ref();
-        let cmain_func_id = m.declare_function(entry_name, Linkage::Export, &cmain_sig).unwrap();
+        let cmain_func_id = match m.declare_function(entry_name, Linkage::Export, &cmain_sig) {
+            Ok(func_id) => func_id,
+            Err(err) => {
+                tcx.sess
+                    .fatal(&format!("entry symbol `{entry_name}` declared multiple times: {err}"));
+            }
+        };
 
         let instance = Instance::mono(tcx, rust_main_def_id).polymorphize(tcx);
 
@@ -162,7 +168,11 @@ pub(crate) fn maybe_create_entry_wrapper(
             bcx.seal_all_blocks();
             bcx.finalize();
         }
-        m.define_function(cmain_func_id, &mut ctx).unwrap();
+
+        if let Err(err) = m.define_function(cmain_func_id, &mut ctx) {
+            tcx.sess.fatal(&format!("entry symbol `{entry_name}` defined multiple times: {err}"));
+        }
+
         unwind_context.add_function(cmain_func_id, &ctx, m.isa());
     }
 }