[Clang][Driver] New parameter allow-unrecognized-arguments

This parameter is used to suppress the ``Unknown argument '...'`` error
that clang will emit whenever it encounters an unknown argument.

This is probably an error to make sure the user fixes it's mistake by
either removing the argument or renaming it, but there are some cases
where it's not possible to fix the issue.
For instance, CMake now injects gcc-specific arguments in the clang-tidy
command that breaks static-analysis
(https://gitlab.kitware.com/cmake/cmake/-/issues/26283)

This will also allow users to run clang-tidy / clangd on a
gcc-based project without the need to maintain two separate
build commands to run llvm-based tools.

By enabling this parameter, the user is able to downgrade the error to a
warning (unknown-argument) that he can further silence using the
``-Qunused-arguments`` flag if needed.

Fixes: #108455

Author: felix642

clang/include/clang/Driver/Options.td

@@ -1011,6 +1011,9 @@ def : Flag<["-"], "Xcompiler">, IgnoredGCCCompat;
 def Z_Flag : Flag<["-"], "Z">, Group<Link_Group>;
 def all__load : Flag<["-"], "all_load">;
 def allowable__client : Separate<["-"], "allowable_client">;
+def allow_unrecognized_arguments : Flag<["--"], "allow-unrecognized-arguments">,
+    Visibility<[ClangOption, CLOption]>,
+    HelpText<"Ignore unrecognized command-line arguments instead of reporting an error.">;
 def ansi : Flag<["-", "--"], "ansi">, Group<CompileOnly_Group>;
 def arch__errors__fatal : Flag<["-"], "arch_errors_fatal">;
 def arch : Separate<["-"], "arch">, Flags<[NoXarchOption,TargetSpecific]>;

clang/lib/Driver/Driver.cpp

@@ -303,29 +303,31 @@ InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings,
     }
   }
 
-  for (const Arg *A : Args.filtered(options::OPT_UNKNOWN)) {
-    unsigned DiagID;
-    auto ArgString = A->getAsString(Args);
-    std::string Nearest;
-    if (getOpts().findNearest(ArgString, Nearest, VisibilityMask) > 1) {
-      if (!IsCLMode() &&
-          getOpts().findExact(ArgString, Nearest,
-                              llvm::opt::Visibility(options::CC1Option))) {
-        DiagID = diag::err_drv_unknown_argument_with_suggestion;
-        Diags.Report(DiagID) << ArgString << "-Xclang " + Nearest;
+  if (!Args.hasArg(options::OPT_allow_unrecognized_arguments)) {
+    for (const Arg *A : Args.filtered(options::OPT_UNKNOWN)) {
+      unsigned DiagID;
+      auto ArgString = A->getAsString(Args);
+      std::string Nearest;
+      if (getOpts().findNearest(ArgString, Nearest, VisibilityMask) > 1) {
+        if (!IsCLMode() &&
+            getOpts().findExact(ArgString, Nearest,
+                                llvm::opt::Visibility(options::CC1Option))) {
+          DiagID = diag::err_drv_unknown_argument_with_suggestion;
+          Diags.Report(DiagID) << ArgString << "-Xclang " + Nearest;
+        } else {
+          DiagID = IsCLMode() ? diag::warn_drv_unknown_argument_clang_cl
+                              : diag::err_drv_unknown_argument;
+          Diags.Report(DiagID) << ArgString;
+        }
       } else {
-        DiagID = IsCLMode() ? diag::warn_drv_unknown_argument_clang_cl
-                            : diag::err_drv_unknown_argument;
-        Diags.Report(DiagID) << ArgString;
+        DiagID = IsCLMode()
+                     ? diag::warn_drv_unknown_argument_clang_cl_with_suggestion
+                     : diag::err_drv_unknown_argument_with_suggestion;
+        Diags.Report(DiagID) << ArgString << Nearest;
       }
-    } else {
-      DiagID = IsCLMode()
-                   ? diag::warn_drv_unknown_argument_clang_cl_with_suggestion
-                   : diag::err_drv_unknown_argument_with_suggestion;
-      Diags.Report(DiagID) << ArgString << Nearest;
+      ContainsError |= Diags.getDiagnosticLevel(DiagID, SourceLocation()) >
+                       DiagnosticsEngine::Warning;
     }
-    ContainsError |= Diags.getDiagnosticLevel(DiagID, SourceLocation()) >
-                     DiagnosticsEngine::Warning;
   }
 
   for (const Arg *A : Args.filtered(options::OPT_o)) {

clang/lib/Frontend/CompilerInvocation.cpp

@@ -4991,15 +4991,17 @@ bool CompilerInvocation::CreateFromArgsImpl(
     Diags.Report(diag::err_drv_missing_argument)
         << Args.getArgString(MissingArgIndex) << MissingArgCount;
 
-  // Issue errors on unknown arguments.
-  for (const auto *A : Args.filtered(OPT_UNKNOWN)) {
-    auto ArgString = A->getAsString(Args);
-    std::string Nearest;
-    if (Opts.findNearest(ArgString, Nearest, VisibilityMask) > 1)
-      Diags.Report(diag::err_drv_unknown_argument) << ArgString;
-    else
-      Diags.Report(diag::err_drv_unknown_argument_with_suggestion)
-          << ArgString << Nearest;
+  if (!Args.hasArg(options::OPT_allow_unrecognized_arguments)) {
+    // Issue errors on unknown arguments.
+    for (const auto *A : Args.filtered(OPT_UNKNOWN)) {
+      auto ArgString = A->getAsString(Args);
+      std::string Nearest;
+      if (Opts.findNearest(ArgString, Nearest, VisibilityMask) > 1)
+        Diags.Report(diag::err_drv_unknown_argument) << ArgString;
+      else
+        Diags.Report(diag::err_drv_unknown_argument_with_suggestion)
+            << ArgString << Nearest;
+    }
   }
 
   ParseFileSystemArgs(Res.getFileSystemOpts(), Args, Diags);

clang/test/Driver/unsupported-option.c

@@ -32,3 +32,7 @@
 // RUN: not %clang -c -Qunused-arguments --target=aarch64-- -mfpu=crypto-neon-fp-armv8 %s 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=QUNUSED_ARGUMENTS
 // QUNUSED_ARGUMENTS: error: unsupported option '-mfpu=' for target 'aarch64--'
+
+// RUN: %clang %s -invalid --allow-unrecognized-arguments -### 2>&1 | \
+// RUN: FileCheck %s --check-prefix=UNKNOWN_ARGUMENT
+// UNKNOWN_ARGUMENT: warning: argument unused during compilation: '-invalid'