[clang-tidy] Fix misc-unused-parameters on params with attrs (#122286)

Don't suggest to comment-out the parameter name if the parameter has an
attribute that's spelled after the parameter name.

This prevents the parameter's attributes from being wrongly applied to
the parameter's type.

This fixes #122191.

Author: emaxx-google

clang-tools-extra/clang-tidy/misc/UnusedParametersCheck.cpp

@@ -9,8 +9,11 @@
 #include "UnusedParametersCheck.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTLambda.h"
+#include "clang/AST/Attr.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Basic/SourceManager.h"
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/STLExtras.h"
 #include <unordered_map>
@@ -26,6 +29,17 @@ bool isOverrideMethod(const FunctionDecl *Function) {
     return MD->size_overridden_methods() > 0 || MD->hasAttr<OverrideAttr>();
   return false;
 }
+
+bool hasAttrAfterParam(const SourceManager *SourceManager,
+                       const ParmVarDecl *Param) {
+  for (const auto *Attr : Param->attrs()) {
+    if (SourceManager->isBeforeInTranslationUnit(Param->getLocation(),
+                                                 Attr->getLocation())) {
+      return true;
+    }
+  }
+  return false;
+}
 } // namespace
 
 void UnusedParametersCheck::registerMatchers(MatchFinder *Finder) {
@@ -189,6 +203,11 @@ void UnusedParametersCheck::check(const MatchFinder::MatchResult &Result) {
     if (Param->isUsed() || Param->isReferenced() || !Param->getDeclName() ||
         Param->hasAttr<UnusedAttr>())
       continue;
+    if (hasAttrAfterParam(Result.SourceManager, Param)) {
+      // Due to how grammar works, attributes would be wrongly applied to the
+      // type if we remove the preceding parameter name.
+      continue;
+    }
 
     // In non-strict mode ignore function definitions with empty bodies
     // (constructor initializer counts for non-empty body).

clang-tools-extra/test/clang-tidy/checkers/misc/unused-parameters.cpp

@@ -33,6 +33,16 @@ void f(void (*fn)()) {;}
 // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: parameter 'fn' is unused [misc-unused-parameters]
 // CHECK-FIXES: {{^}}void f(void (* /*fn*/)()) {;}{{$}}
 
+int *k([[clang::lifetimebound]] int *i) {;}
+// CHECK-MESSAGES: :[[@LINE-1]]:38: warning: parameter 'i' is unused [misc-unused-parameters]
+// CHECK-FIXES: {{^}}int *k({{\[\[clang::lifetimebound\]\]}} int * /*i*/) {;}{{$}}
+
+#define ATTR_BEFORE(x) [[clang::lifetimebound]] x
+int* m(ATTR_BEFORE(const int *i)) { return nullptr; }
+// CHECK-MESSAGES: :[[@LINE-1]]:31: warning: parameter 'i' is unused [misc-unused-parameters]
+// CHECK-FIXES: {{^}}int* m(ATTR_BEFORE(const int * /*i*/)) { return nullptr; }{{$}}
+#undef ATTR_BEFORE
+
 // Unchanged cases
 // ===============
 void f(int i); // Don't remove stuff in declarations
@@ -42,6 +52,12 @@ void s(int i[1]);
 void u(void (*fn)());
 void w(int i) { (void)i; } // Don't remove used parameters
 
+// Don't reanchor the attribute to the type:
+int *x(int *i [[clang::lifetimebound]]) { return nullptr; }
+#define ATTR_AFTER(x) x [[clang::lifetimebound]]
+int* y(ATTR_AFTER(const int *i)) { return nullptr; }
+#undef ATTR_AFTER
+
 bool useLambda(int (*fn)(int));
 static bool static_var = useLambda([] (int a) { return a; });