Distinguish file-not-found errors from other errors

To implement config file search, I need to know if a file couldn't be
opened because it doesn't exist or if a file couldn't be opened because
of another reason. Have read_file return more precise information about
any errors encountered.

This feature isn't used anywhere yet (except tests).

Author: strager

src/file.cpp

@@ -205,8 +205,11 @@ read_file_result read_file(const char *path) {
       /*hTemplateFile=*/nullptr);
   if (handle == INVALID_HANDLE_VALUE) {
     DWORD error = ::GetLastError();
-    return read_file_result::failure(std::string("failed to open ") + path +
-                                     ": " + windows_error_message(error));
+    read_file_result result =
+        read_file_result::failure(std::string("failed to open ") + path + ": " +
+                                  windows_error_message(error));
+    result.is_not_found_error = error == ERROR_FILE_NOT_FOUND;
+    return result;
   }
   windows_handle_file file(handle);
   return read_file(path, file.ref());
@@ -223,8 +226,10 @@ read_file_result read_file(const char *path) {
   int fd = ::open(path, O_CLOEXEC | O_RDONLY);
   if (fd == -1) {
     int error = errno;
-    return read_file_result::failure(std::string("failed to open ") + path +
-                                     ": " + std::strerror(error));
+    read_file_result result = read_file_result::failure(
+        std::string("failed to open ") + path + ": " + std::strerror(error));
+    result.is_not_found_error = error == ENOENT;
+    return result;
   }
   posix_fd_file file(fd);
   return read_file(path, file.ref());

src/quick-lint-js/file.h

@@ -13,6 +13,7 @@ namespace quick_lint_js {
 struct read_file_result {
   padded_string content;
   std::string error;
+  bool is_not_found_error = false;
 
   bool ok() const noexcept { return this->error.empty(); }
   void exit_if_not_ok() const;

test/test-file.cpp

@@ -71,6 +71,7 @@ TEST_F(test_file, read_non_existing_file) {
 
   read_file_result file_content = read_file(temp_file_path.c_str());
   EXPECT_FALSE(file_content.ok());
+  EXPECT_TRUE(file_content.is_not_found_error);
   EXPECT_THAT(file_content.error, HasSubstr("does-not-exist.js"));
   EXPECT_THAT(file_content.error,
               AnyOf(HasSubstr("No such file"), HasSubstr("cannot find")));
@@ -81,6 +82,7 @@ TEST_F(test_file, read_directory) {
 
   read_file_result file_content = read_file(temp_file_path.c_str());
   EXPECT_FALSE(file_content.ok());
+  EXPECT_FALSE(file_content.is_not_found_error);
   EXPECT_THAT(
       file_content.error,
       testing::AnyOf(