[clang-tidy] Add support for NOLINTNEXTLINE.

[alexfh]

Awesome! LG with one nit.

clang-tidy/ClangTidyDiagnosticConsumer.cpp

300

Is it difficult to address right away?

I also suppose (https://en.wikipedia.org/wiki/Newline#Representations) that we only care about CR+LF or LF line endings, so just looking at LFs seems more than enough (and will make code slightly simpler). This is the approach we took in many places in clang-format.

[bkramer]

• Simplify code by not worrying about \r
• Don't allow blank lines between NOLINTNEXTLINE and the warning

clang-tidy/ClangTidyDiagnosticConsumer.cpp

 }
 
 static bool LineIsMarkedWithNOLINT(SourceManager &SM, SourceLocation Loc) {
   bool Invalid;
   const char *CharacterData = SM.getCharacterData(Loc, &Invalid);
   if (Invalid)
     return false;
 
+  // Check if there's a NOLINT on this line.
   const char *P = CharacterData;
   while (*P != '\0' && *P != '\r' && *P != '\n')
     ++P;
   StringRef RestOfLine(CharacterData, P - CharacterData + 1);
   // FIXME: Handle /\bNOLINT\b(\([^)]*\))?/ as cpplint.py does.
   if (RestOfLine.find("NOLINT") != StringRef::npos)
     return true;
+
+  // Check if there's a NOLINTNEXTLINE on the previous line.
+  const char *BufBegin =
+      SM.getCharacterData(SM.getLocForStartOfFile(SM.getFileID(Loc)), &Invalid);
+  if (Invalid || P == BufBegin)
+    return false;
+
+  // Scan backwards over the current line.
+  P = CharacterData;
+  while (P != BufBegin && *P != '\r' && *P != '\n')
+    --P;
+
+  // If we reached the begin of the file there is no line before it.
+  if (P == BufBegin)
+    return false;
+
+  // Now skip any newlines.
+  // FIXME: We want to skip over exactly one line, not an arbitrary number.
+  while (P != BufBegin && (*P == '\r' || *P == '\n'))
+    --P;
+
+  const char *LineEnd = P;
+
+  // Now we're on the previous line. Skip to the beginning of it.
+  while (P != BufBegin && *P != '\r' && *P != '\n')
+    --P;
+
+  RestOfLine = StringRef(P, LineEnd - P + 1);
+  if (RestOfLine.find("NOLINTNEXTLINE") != StringRef::npos)
+    return true;
+
   return false;
 }
 
 static bool LineIsMarkedWithNOLINTinMacro(SourceManager &SM,
                                           SourceLocation Loc) {
   while (true) {
     if (LineIsMarkedWithNOLINT(SM, Loc))
       return true;

test/clang-tidy/nolintnextline.cpp

+class A { A(int i); };
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: single-argument constructors must be marked explicit
+
+// NOLINTNEXTLINE
+class B { B(int i); };
+
+// NOLINTNEXTLINE(we-dont-care-about-categories-yet)
+class C { C(int i); };
+
+
+// NOLINTNEXTLINE
+
+
+class D { D(int i); };
+
+// NOLINTNEXTLINE
+//
+class E { E(int i); };
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: single-argument constructors must be marked explicit
+
+#define MACRO(X) class X { X(int i); };
+MACRO(F)
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: single-argument constructors must be marked explicit
+// NOLINTNEXTLINE
+MACRO(G)
+
+#define MACRO_NOARG class H { H(int i); };
+// NOLINTNEXTLINE
+MACRO_NOARG
+
+// CHECK-MESSAGES: Suppressed 5 warnings (5 NOLINT)
+
+// RUN: %check_clang_tidy %s google-explicit-constructor %t --