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

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.

Reviewers: majnemer

Subscribers: sanjoy, chandlerc, llvm-commits

Differential Revision: https://reviews.llvm.org/D22541

llvm-svn: 276058

llvm/include/llvm/ADT/ArrayRef.h

@@ -161,6 +161,7 @@ namespace llvm {
     }
 
     /// 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);
@@ -168,18 +169,21 @@ namespace llvm {
 
     /// 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);
@@ -279,6 +283,7 @@ namespace llvm {
     }
 
     /// 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);
@@ -286,17 +291,20 @@ namespace llvm {
 
     /// 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);

llvm/include/llvm/ADT/StringRef.h

@@ -422,6 +422,7 @@ namespace llvm {
     /// 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));
@@ -430,6 +431,7 @@ namespace llvm {
     /// 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);
@@ -438,6 +440,7 @@ namespace llvm {
     /// 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);
@@ -455,6 +458,7 @@ namespace llvm {
     /// 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);
@@ -549,36 +553,42 @@ namespace llvm {
 
     /// 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);
     }