added comments

Author: nebulark

compiler/rustc_codegen_llvm/src/back/target_machine_wrapper.rs

@@ -45,6 +45,7 @@ impl TargetMachineWrapper {
             "The last character must be a null terminator."
         );
 
+        // SAFETY: llvm::LLVMRustCreateTargetMachine copies pointed to data
         let tm_ptr = unsafe {
             llvm::LLVMRustCreateTargetMachine(
                 triple.as_ptr(),

compiler/rustc_codegen_llvm/src/context.rs

@@ -160,6 +160,7 @@ pub unsafe fn create_module<'ll>(
 
     // Ensure the data-layout values hardcoded remain the defaults.
     if sess.target.is_builtin {
+        // tm is disposed by its drop impl
         let tm = crate::back::write::create_informational_target_machine(tcx.sess);
         llvm::LLVMRustSetDataLayoutFromTargetMachine(llmod, &tm);
 

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2112,6 +2112,8 @@ extern "C" {
     );
 
     pub fn LLVMRustGetHostCPUName(len: *mut usize) -> *const c_char;
+
+    // This function makes copies of pointed to data, so the data's lifetime may end after this function returns
     pub fn LLVMRustCreateTargetMachine(
         Triple: *const c_char,
         CPU: *const c_char,