[ADT] add StringViewExtras llvm::starts_with for std::string_view

[nickdesaulniers]

Summary

std::string_view::starts_with isn't available until C++20. Create
llvm::starts_with for now; we can delete this when LLVM moves to C++20
one day.

To run the newly added unit test:
$ cd llvm/build; ninja ADTTests; cd -
$ ./llvm/build/unittests/ADT/ADTTests --gtest_filter=StringViewExtrasTest.\*

[nickdesaulniers]

• fix comment in new header

[nickdesaulniers]

• update description on how to run new unit tests

[erichkeane]

llvm/include/llvm/ADT/StringViewExtras.h

25

This might be a bit noisy depending on how compilers/libraries define this in C++17 mode. We might want a __cplusplus > (whatever the 2020 value is) here as well.

llvm/unittests/ADT/StringViewExtrasTest.cpp

23

Why is this running this 2x in a row?

[nickdesaulniers]

• remove test duplication, remove warning

llvm/include/llvm/ADT/StringViewExtras.h

25

Yeah, I'll just remove it.

llvm/include/llvm/ADT/StringViewExtras.h

+//===- llvm/ADT/StringViewExtras.h - Useful string_view functions C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains some functions that are useful when dealing with
+/// string_views.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_STRINGVIEWEXTRAS_H
+#define LLVM_ADT_STRINGVIEWEXTRAS_H
+
+#include <string_view>
+
+namespace llvm {
+
+// FIXME: std::string_view::starts_with is not available until C++20. Once LLVM
+// is upgraded to C++20, remove this header and users.
+
+inline constexpr bool starts_with(std::string_view self, char C) {
+  return !self.empty() && self.front() == C;
+}
+inline constexpr bool starts_with(std::string_view haystack,
+                                  std::string_view needle) {
+  return haystack.substr(0, needle.size()) == needle;
+}
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_STRINGVIEWEXTRAS_H

llvm/unittests/ADT/CMakeLists.txt

add_llvm_unittest(ADTTests
   SparseMultiSetTest.cpp
   SparseSetTest.cpp
   StatisticTest.cpp
   StringExtrasTest.cpp
   StringMapTest.cpp
   StringRefTest.cpp
   StringSetTest.cpp
   StringSwitchTest.cpp
+  StringViewExtrasTest.cpp
   TinyPtrVectorTest.cpp
   TwineTest.cpp
   TypeSwitchTest.cpp
   TypeTraitsTest.cpp
   )
 
 target_link_libraries(ADTTests PRIVATE LLVMTestingSupport)
 
 add_dependencies(ADTTests intrinsics_gen)

llvm/unittests/ADT/StringViewExtrasTest.cpp

+//===- StringExtrasTest.cpp - Unit tests for String extras ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringViewExtras.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+#include <string_view>
+
+using namespace llvm;
+
+TEST(StringViewExtrasTest, starts_with) {
+  std::string haystack = "hello world";
+  EXPECT_TRUE(llvm::starts_with(haystack, 'h'));
+  EXPECT_FALSE(llvm::starts_with(haystack, '\0'));
+  EXPECT_TRUE(llvm::starts_with(haystack, "hello"));
+  // TODO: should this differ from \0?
+  EXPECT_TRUE(llvm::starts_with(haystack, ""));
+
+  std::string empty;
+  EXPECT_FALSE(llvm::starts_with(empty, 'h'));
+  EXPECT_FALSE(llvm::starts_with(empty, '\0'));
+  EXPECT_FALSE(llvm::starts_with(empty, "hello"));
+  // TODO: should this differ from \0?
+  EXPECT_TRUE(llvm::starts_with(empty, ""));
+}