Make LLVMRustGetOrInsertGlobal always return a GlobalVariable

`Module::getOrInsertGlobal` returns a `Constant*`, which is a super
class of `GlobalVariable`, but if the given type doesn't match an
existing declaration, it returns a bitcast of that global instead.
This causes UB when we pass that to `LLVMGetVisibility` which
unconditionally casts the opaque argument to a `GlobalValue*`.

Instead, we can do our own get-or-insert without worrying whether
existing types match exactly. It's not relevant when we're just trying
to get/set the linkage and visibility, and if types are needed we can
bitcast or error nicely from `rustc_codegen_llvm` instead.

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -124,8 +124,18 @@ extern "C" LLVMValueRef LLVMRustGetOrInsertFunction(LLVMModuleRef M,
 
 extern "C" LLVMValueRef
 LLVMRustGetOrInsertGlobal(LLVMModuleRef M, const char *Name, size_t NameLen, LLVMTypeRef Ty) {
+  Module *Mod = unwrap(M);
   StringRef NameRef(Name, NameLen);
-  return wrap(unwrap(M)->getOrInsertGlobal(NameRef, unwrap(Ty)));
+
+  // We don't use Module::getOrInsertGlobal because that returns a Constant*,
+  // which may either be the real GlobalVariable*, or a constant bitcast of it
+  // if our type doesn't match the original declaration. We always want the
+  // GlobalVariable* so we can access linkage, visibility, etc.
+  GlobalVariable *GV = Mod->getGlobalVariable(NameRef, true);
+  if (!GV)
+    GV = new GlobalVariable(*Mod, unwrap(Ty), false,
+                            GlobalValue::ExternalLinkage, nullptr, NameRef);
+  return wrap(GV);
 }
 
 extern "C" LLVMValueRef

src/test/ui/issues/issue-91050.rs

@@ -0,0 +1,34 @@
+// build-pass
+// compile-flags: --crate-type lib -Ccodegen-units=1
+
+// This test declares globals by the same name with different types, which
+// caused problems because Module::getOrInsertGlobal would return a Constant*
+// bitcast instead of a GlobalVariable* that could access linkage/visibility.
+// In alt builds with LLVM assertions this would fail:
+//
+// rustc: /checkout/src/llvm-project/llvm/include/llvm/Support/Casting.h:269:
+// typename cast_retty<X, Y *>::ret_type llvm::cast(Y *) [X = llvm::GlobalValue, Y = llvm::Value]:
+// Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
+//
+// In regular builds, the bad cast was UB, like "Invalid LLVMRustVisibility value!"
+
+pub mod before {
+    #[no_mangle]
+    pub static GLOBAL1: [u8; 1] = [1];
+}
+
+pub mod inner {
+    extern "C" {
+        pub static GLOBAL1: u8;
+        pub static GLOBAL2: u8;
+    }
+
+    pub fn call() {
+        drop(unsafe { (GLOBAL1, GLOBAL2) });
+    }
+}
+
+pub mod after {
+    #[no_mangle]
+    pub static GLOBAL2: [u8; 1] = [2];
+}