[ADT] Warn on unused results from ArrayRef and StringRef functions that read like they might mutate.

[jlebar]

Summary

Functions like "slice" and "drop_front" sound like they might mutate the
underlying object, but they don't. Warning on unused results would have
saved me an hour yesterday, and I'm sure I'm not the only one.

LLVM and Clang are clean wrt this warning after D22540.

[majnemer]

LGTM!

[chandlerc]

LGTM too, this is a great idea.

llvm/trunk/include/llvm/ADT/ArrayRef.h

public:
     /// equals - Check for element-wise equality.
     bool equals(ArrayRef RHS) const {
       if (Length != RHS.Length)
         return false;
       return std::equal(begin(), end(), RHS.begin());
     }
 
     /// slice(n) - Chop off the first N elements of the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     ArrayRef<T> slice(size_t N) const {
       assert(N <= size() && "Invalid specifier");
       return ArrayRef<T>(data()+N, size()-N);
     }
 
     /// slice(n, m) - Chop off the first N elements of the array, and keep M
     /// elements in the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     ArrayRef<T> slice(size_t N, size_t M) const {
       assert(N+M <= size() && "Invalid specifier");
       return ArrayRef<T>(data()+N, M);
     }
 
     /// \brief Drop the first \p N elements of the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     ArrayRef<T> drop_front(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return slice(N, size() - N);
     }
 
     /// \brief Drop the last \p N elements of the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     ArrayRef<T> drop_back(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return slice(0, size() - N);
     }
 
     /// @}
     /// @name Operator Overloads
     /// @{
public:
 
     /// back - Get the last element.
     T &back() const {
       assert(!this->empty());
       return data()[this->size()-1];
     }
 
     /// slice(n) - Chop off the first N elements of the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     MutableArrayRef<T> slice(size_t N) const {
       assert(N <= this->size() && "Invalid specifier");
       return MutableArrayRef<T>(data()+N, this->size()-N);
     }
 
     /// slice(n, m) - Chop off the first N elements of the array, and keep M
     /// elements in the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     MutableArrayRef<T> slice(size_t N, size_t M) const {
       assert(N+M <= this->size() && "Invalid specifier");
       return MutableArrayRef<T>(data()+N, M);
     }
 
     /// \brief Drop the first \p N elements of the array.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     MutableArrayRef<T> drop_front(size_t N = 1) const {
       assert(this->size() >= N && "Dropping more elements than exist");
       return slice(N, this->size() - N);
     }
 
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     MutableArrayRef<T> drop_back(size_t N = 1) const {
       assert(this->size() >= N && "Dropping more elements than exist");
       return slice(0, this->size() - N);
     }
 
     /// @}
     /// @name Operator Overloads
     /// @{

llvm/trunk/include/llvm/ADT/StringRef.h

public:
     /// \param Start The index of the starting character in the substring; if
     /// the index is npos or greater than the length of the string then the
     /// empty substring will be returned.
     ///
     /// \param N The number of characters to included in the substring. If N
     /// exceeds the number of characters remaining in the string, the string
     /// suffix (starting with \p Start) will be returned.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef substr(size_t Start, size_t N = npos) const {
       Start = std::min(Start, Length);
       return StringRef(Data + Start, std::min(N, Length - Start));
     }
 
     /// Return a StringRef equal to 'this' but with the first \p N elements
     /// dropped.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef drop_front(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return substr(N);
     }
 
     /// Return a StringRef equal to 'this' but with the last \p N elements
     /// dropped.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef drop_back(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return substr(0, size()-N);
     }
 
     /// Return a reference to the substring from [Start, End).
     ///
     /// \param Start The index of the starting character in the substring; if
     /// the index is npos or greater than the length of the string then the
     /// empty substring will be returned.
     ///
     /// \param End The index following the last character to include in the
     /// substring. If this is npos or exceeds the number of characters
     /// remaining in the string, the string suffix (starting with \p Start)
     /// will be returned. If this is less than \p Start, an empty string will
     /// be returned.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef slice(size_t Start, size_t End) const {
       Start = std::min(Start, Length);
       End = std::min(std::max(Start, End), Length);
       return StringRef(Data + Start, End - Start);
     }
 
     /// Split into two substrings around the first occurrence of a separator
     /// character.
std::pair<StringRef, StringRef> rsplit(char Separator) const {
       size_t Idx = rfind(Separator);
       if (Idx == npos)
         return std::make_pair(*this, StringRef());
       return std::make_pair(slice(0, Idx), slice(Idx+1, npos));
     }
 
     /// Return string with consecutive \p Char characters starting from the
     /// the left removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef ltrim(char Char) const {
       return drop_front(std::min(Length, find_first_not_of(Char)));
     }
 
     /// Return string with consecutive characters in \p Chars starting from
     /// the left removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef ltrim(StringRef Chars = " \t\n\v\f\r") const {
       return drop_front(std::min(Length, find_first_not_of(Chars)));
     }
 
     /// Return string with consecutive \p Char characters starting from the
     /// right removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef rtrim(char Char) const {
       return drop_back(Length - std::min(Length, find_last_not_of(Char) + 1));
     }
 
     /// Return string with consecutive characters in \p Chars starting from
     /// the right removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef rtrim(StringRef Chars = " \t\n\v\f\r") const {
       return drop_back(Length - std::min(Length, find_last_not_of(Chars) + 1));
     }
 
     /// Return string with consecutive \p Char characters starting from the
     /// left and right removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef trim(char Char) const {
       return ltrim(Char).rtrim(Char);
     }
 
     /// Return string with consecutive characters in \p Chars starting from
     /// the left and right removed.
+    LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef trim(StringRef Chars = " \t\n\v\f\r") const {
       return ltrim(Chars).rtrim(Chars);
     }
 
     /// @}
   };
 
   /// @name StringRef Comparison Operators