cc1: Report an error for multiple actions unless separated by -main-file-name (#91140)

When multiple actions are specified, the last one is used and others are
overridden. This might lead to confusion if the user is used to driver's
`-S -emit-llvm` behavior.

```
%clang_cc1 -S -emit-llvm a.c     # -S is overridden
%clang_cc1 -emit-llvm -S a.c     # -emit-llvm is overridden
%clang_cc1 -fsyntax-only -S a.c  # -fsyntax-only is overridden
```

However, we want to continue supporting overriding the driver action
with -Xclang:

* `clang -c -Xclang -ast-dump a.c` (`%clang -cc1 -emit-obj ...
-main-file-name a.c ... -ast-dump`)
* `clang -c -xc++ -Xclang -emit-module stl.modulemap`

As an exception, we allow -ast-dump* options to be composed together
(e.g. `-ast-dump -ast-dump-lookups` in AST/ast-dump-lookups.cpp).

1 files changed, 24 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 8312abc..948fe08 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2841,6 +2841,30 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     }
 
     Opts.ProgramAction = *ProgramAction;
+
+    // Catch common mistakes when multiple actions are specified for cc1 (e.g.
+    // -S -emit-llvm means -emit-llvm while -emit-llvm -S means -S). However, to
+    // support driver `-c -Xclang ACTION` (-cc1 -emit-llvm file -main-file-name
+    // X ACTION), we suppress the error when the two actions are separated by
+    // -main-file-name.
+    //
+    // As an exception, accept composable -ast-dump*.
+    if (!A->getSpelling().starts_with("-ast-dump")) {
+      const Arg *SavedAction = nullptr;
+      for (const Arg *AA :
+           Args.filtered(OPT_Action_Group, OPT_main_file_name)) {
+        if (AA->getOption().matches(OPT_main_file_name)) {
+          SavedAction = nullptr;
+        } else if (!SavedAction) {
+          SavedAction = AA;
+        } else {
+          if (!A->getOption().matches(OPT_ast_dump_EQ))
+            Diags.Report(diag::err_fe_invalid_multiple_actions)
+                << SavedAction->getSpelling() << A->getSpelling();
+          break;
+        }
+      }
+    }
   }
 
   if (const Arg* A = Args.getLastArg(OPT_plugin)) {