Add a FatalUserError macro to help print user-caused errors. (#668)

executable_semantics/common/BUILD

@@ -4,6 +4,24 @@
 
 package(default_visibility = ["//executable_semantics:__subpackages__"])
 
+cc_library(
+    name = "error",
+    hdrs = ["error.h"],
+    deps = [
+        "@llvm-project//llvm:Support",
+    ],
+)
+
+cc_test(
+    name = "error_test",
+    srcs = ["error_test.cpp"],
+    deps = [
+        ":error",
+        "@llvm-project//llvm:gtest",
+        "@llvm-project//llvm:gtest_main",
+    ],
+)
+
 cc_library(
     name = "tracing_flag",
     srcs = ["tracing_flag.cpp"],

executable_semantics/common/error.h

@@ -0,0 +1,38 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_COMMON_ERROR_H_
+#define EXECUTABLE_SEMANTICS_COMMON_ERROR_H_
+
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/Signals.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace Carbon {
+
+// Prints an error and exits. This should be used for non-recoverable errors
+// with user input.
+//
+// For example:
+//   FatalUserError() << "Input is not valid!";
+class FatalUserError {
+ public:
+  FatalUserError() { llvm::errs() << "ERROR: "; }
+  ~FatalUserError() {
+    // Finish with a newline.
+    llvm::errs() << "\n";
+    exit(-1);
+  }
+
+  // Forward output to llvm::errs.
+  template <typename T>
+  FatalUserError& operator<<(const T& message) {
+    llvm::errs() << message;
+    return *this;
+  }
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_COMMON_ERROR_H_

executable_semantics/common/error_test.cpp

@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "executable_semantics/common/error.h"
+
+#include "gtest/gtest.h"
+
+namespace Carbon {
+
+TEST(ErrorTest, FatalUserError) {
+  ASSERT_DEATH({ FatalUserError() << "test"; }, "ERROR: test\n");
+}
+
+}  // namespace Carbon