Provide a way of accessing the ThinLTO module import map in rustc.

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -363,6 +363,10 @@ extern { pub type ThinLTOData; }
 /// LLVMRustThinLTOBuffer
 extern { pub type ThinLTOBuffer; }
 
+// LLVMRustModuleNameCallback
+pub type ThinLTOModuleNameCallback =
+    unsafe extern "C" fn(*mut c_void, *const c_char, *const c_char);
+
 /// LLVMRustThinLTOModule
 #[repr(C)]
 pub struct ThinLTOModule {
@@ -1622,6 +1626,11 @@ extern "C" {
         Data: &ThinLTOData,
         Module: &Module,
     ) -> bool;
+    pub fn LLVMRustGetThinLTOModuleImports(
+        Data: *const ThinLTOData,
+        ModuleNameCallback: ThinLTOModuleNameCallback,
+        CallbackPayload: *mut c_void,
+    );
     pub fn LLVMRustFreeThinLTOData(Data: &'static mut ThinLTOData);
     pub fn LLVMRustParseBitcodeForThinLTO(
         Context: &Context,

src/rustllvm/PassWrapper.cpp

@@ -1123,6 +1123,28 @@ LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
   return true;
 }
 
+extern "C" typedef void (*LLVMRustModuleNameCallback)(void*, // payload
+                                                      const char*, // importing module name
+                                                      const char*); // imported module name
+
+// Calls `module_name_callback` for each module import done by ThinLTO.
+// The callback is provided with regular null-terminated C strings.
+extern "C" void
+LLVMRustGetThinLTOModuleImports(const LLVMRustThinLTOData *data,
+                                LLVMRustModuleNameCallback module_name_callback,
+                                void* callback_payload) {
+  for (const auto& importing_module : data->ImportLists) {
+    const std::string importing_module_id = importing_module.getKey().str();
+    const auto& imports = importing_module.getValue();
+    for (const auto& imported_module : imports) {
+      const std::string imported_module_id = imported_module.getKey().str();
+      module_name_callback(callback_payload,
+                           importing_module_id.c_str(),
+                           imported_module_id.c_str());
+    }
+  }
+}
+
 // This struct and various functions are sort of a hack right now, but the
 // problem is that we've got in-memory LLVM modules after we generate and
 // optimize all codegen-units for one compilation in rustc. To be compatible
@@ -1288,6 +1310,11 @@ LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
   report_fatal_error("ThinLTO not available");
 }
 
+extern "C" LLVMRustThinLTOModuleImports
+LLVMRustGetLLVMRustThinLTOModuleImports(const LLVMRustThinLTOData *Data) {
+  report_fatal_error("ThinLTO not available");
+}
+
 extern "C" void
 LLVMRustFreeThinLTOData(LLVMRustThinLTOData *Data) {
   report_fatal_error("ThinLTO not available");