[flang][driver] deprecate manual usage of -lFortran_main (#79016)

Intended to warn users of the 18.x release not to do this.

A better solution should be found for the 19.x release. See discussion
in #78152.

Unfortunately there is no warning on Windows currently. I am rushing to
get this landed before 18.x branches.

Author: tblah

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -427,6 +427,8 @@ def warn_drv_clang_unsupported : Warning<
   "the clang compiler does not support '%0'">;
 def warn_drv_deprecated_arg : Warning<
   "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
+def warn_drv_deprecated_custom : Warning<
+  "argument '%0' is deprecated, %1">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<
   "unknown platform, assuming -mfloat-abi=%0">;
 def warn_drv_unsupported_float_abi_by_lib : Warning<

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1194,19 +1194,31 @@ static void addFortranMain(const ToolChain &TC, const ArgList &Args,
   }
 
   // 2. GNU and similar
+  const Driver &D = TC.getDriver();
+  const char *FortranMainLinkFlag = "-lFortran_main";
+
+  // Warn if the user added `-lFortran_main` - this library is an implementation
+  // detail of Flang and should be handled automaticaly by the driver.
+  for (const char *arg : CmdArgs) {
+    if (strncmp(arg, FortranMainLinkFlag, strlen(FortranMainLinkFlag)) == 0)
+      D.Diag(diag::warn_drv_deprecated_custom)
+          << FortranMainLinkFlag
+          << "see the Flang driver documentation for correct usage";
+  }
+
   // The --whole-archive option needs to be part of the link line to make
   // sure that the main() function from Fortran_main.a is pulled in by the
   // linker. However, it shouldn't be used if it's already active.
   // TODO: Find an equivalent of `--whole-archive` for Darwin and AIX.
   if (!isWholeArchivePresent(Args) && !TC.getTriple().isMacOSX() &&
       !TC.getTriple().isOSAIX()) {
     CmdArgs.push_back("--whole-archive");
-    CmdArgs.push_back("-lFortran_main");
+    CmdArgs.push_back(FortranMainLinkFlag);
     CmdArgs.push_back("--no-whole-archive");
     return;
   }
 
-  CmdArgs.push_back("-lFortran_main");
+  CmdArgs.push_back(FortranMainLinkFlag);
 }
 
 /// Add Fortran runtime libs

flang/test/Driver/linker-flags.f90

@@ -11,6 +11,7 @@
 ! RUN: %flang -### --target=x86_64-unknown-dragonfly %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,UNIX
 ! RUN: %flang -### --target=x86_64-unknown-haiku %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,HAIKU
 ! RUN: %flang -### --target=x86_64-windows-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MINGW
+! RUN: %flang -### --target=aarch64-unknown-linux-gnu %S/Inputs/hello.f90 -lFortran_main 2>&1 | FileCheck %s --check-prefixes=DEPRECATED
 
 ! NOTE: Clang's driver library, clangDriver, usually adds 'oldnames' on Windows,
 !       but it is not needed when compiling Fortran code and they might bring in
@@ -53,3 +54,6 @@
 ! MSVC-LABEL: link
 ! MSVC-SAME: /subsystem:console
 ! MSVC-SAME: "[[object_file]]"
+
+! Check that we warn when using -lFortran_main
+! DEPRECATED: warning: argument '-lFortran_main' is deprecated, see the Flang driver documentation for correct usage [-Wdeprecated]