[clang][Driver] Improve multilib custom error reporting.

If `multilib.yaml` reports a custom error message for some unsupported
configuration, it's not very helpful to display that error message
_first_, and then follow it up with a huge list of all the multilib
configurations that _are_ supported.

In interactive use, the list is likely to scroll the most important
message off the top of the user's window, leaving them with just a
long list of available libraries, without a visible explanation of
_why_ clang just printed that long list. Also, in general, it makes
more intuitive sense to print the message last that shows why
compilation can't continue, because that's where users are most likely
to look for the reason why something stopped.

Author: statham-arm

clang/lib/Driver/Multilib.cpp

@@ -99,6 +99,7 @@ bool MultilibSet::select(const Driver &D, const Multilib::flags_list &Flags,
                          llvm::SmallVectorImpl<Multilib> &Selected) const {
   llvm::StringSet<> FlagSet(expandFlags(Flags));
   Selected.clear();
+  bool AnyErrors = false;
 
   // Decide which multilibs we're going to select at all.
   llvm::DenseSet<StringRef> ExclusiveGroupsSelected;
@@ -123,12 +124,11 @@ bool MultilibSet::select(const Driver &D, const Multilib::flags_list &Flags,
     }
 
     // If this multilib is actually a placeholder containing a fatal
-    // error message written by the multilib.yaml author, display that
-    // error message, and return failure.
-    if (M.isError()) {
-      D.Diag(clang::diag::err_drv_multilib_custom_error) << M.getErrorMessage();
-      return false;
-    }
+    // error message written by the multilib.yaml author, then set a
+    // flag that will cause a failure return. Our caller will display
+    // the error message.
+    if (M.isError())
+      AnyErrors = true;
 
     // Select this multilib.
     Selected.push_back(M);
@@ -138,7 +138,7 @@ bool MultilibSet::select(const Driver &D, const Multilib::flags_list &Flags,
   // round.
   std::reverse(Selected.begin(), Selected.end());
 
-  return !Selected.empty();
+  return !AnyErrors && !Selected.empty();
 }
 
 llvm::StringSet<>

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -186,10 +186,27 @@ static void findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
     return;
   D.Diag(clang::diag::warn_drv_missing_multilib) << llvm::join(Flags, " ");
   std::stringstream ss;
+
+  // If multilib selection didn't complete successfully, report a list
+  // of all the configurations the user could have provided.
   for (const Multilib &Multilib : Result.Multilibs)
     if (!Multilib.isError())
       ss << "\n" << llvm::join(Multilib.flags(), " ");
   D.Diag(clang::diag::note_drv_available_multilibs) << ss.str();
+
+  // Now report any custom error messages requested by the YAML. We do
+  // this after displaying the list of available multilibs, because
+  // that list is probably large, and (in interactive use) risks
+  // scrolling the useful error message off the top of the user's
+  // terminal.
+  for (const Multilib &Multilib : Result.SelectedMultilibs)
+    if (Multilib.isError())
+      D.Diag(clang::diag::err_drv_multilib_custom_error)
+          << Multilib.getErrorMessage();
+
+  // If there was an error, clear the SelectedMultilibs vector, in
+  // case it contains partial data.
+  Result.SelectedMultilibs.clear();
 }
 
 static constexpr llvm::StringLiteral MultilibFilename = "multilib.yaml";