[Demangle] avoid more std::string_view::substr

[nickdesaulniers]

Summary

In D148959, I removed usage of std::string_view::substr because it may
throw, and libcxxabi cannot use such code. I missed one instance in
llvm::starts_with. That is blocking copying the code back upstream in
D148566.

Mark these helpers noexcept (as they are in C++20) as well, to remind
future travelers.

Make these changes upstream, and copy them back downstream using
libcxxabi/src/demangle/cp-to-llvm.sh.

[nickdesaulniers]

libcxxabi/src/demangle/StringViewExtras.h

24–25

front is not noexcept. Should I rewrite this to be &*self == C?

24–25

https://en.cppreference.com/w/cpp/string/basic_string_view/front

[MaskRay]

libcxxabi/src/demangle/StringViewExtras.h

24–25

Ah, *self == C looks good. It's a pity that front throws...

llvm/include/llvm/Demangle/StringViewExtras.h

32

size is more common than length.

[philnik]

libcxxabi/src/demangle/StringViewExtras.h

24–25

front() doesn't throw. All major implementation also mark it as noexcept as a conforming extension. It's just not required to be noexcept in case an implementation wants to throw in the UB case.

[nickdesaulniers]

• replace front() with *begin(), length() with size()

[MaskRay]

libcxxabi/src/demangle/StringViewExtras.h

30

if (needle.empty()) is a special case that can be handled by the generic code. It can be removed.

[nickdesaulniers]

• remove special case for empty needle; it's already handled

libcxxabi/src/demangle/StringViewExtras.h

 #define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
 
 #include <string_view>
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
-  return !self.empty() && self.front() == C;
+inline bool starts_with(std::string_view self, char C) noexcept {
+  return !self.empty() && *self.begin() == C;
 }
 
-inline bool starts_with(std::string_view haystack, std::string_view needle) {
-  return haystack.substr(0, needle.size()) == needle;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.size() > haystack.size())
+    return false;
+  haystack.remove_suffix(haystack.size() - needle.size());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END
 
 #endif

llvm/include/llvm/Demangle/StringViewExtras.h

 #define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
 
 #include <string_view>
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
-  return !self.empty() && self.front() == C;
+inline bool starts_with(std::string_view self, char C) noexcept {
+  return !self.empty() && *self.begin() == C;
 }
 
-inline bool starts_with(std::string_view haystack, std::string_view needle) {
-  return haystack.substr(0, needle.size()) == needle;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.size() > haystack.size())
+    return false;
+  haystack.remove_suffix(haystack.size() - needle.size());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END
 
 #endif