Avoid runtime dependency on static libstdc++

Usually, we do want to use the static C++ library when building rustc_llvm, but do not want to have that dependency at compiler runtime. Change the defaults to Make It So.

config.toml.example

@@ -87,9 +87,10 @@ changelog-seen = 2
 # this flag will indicate that this version check should not be done.
 #version-check = true
 
-# Link libstdc++ statically into the rustc_llvm instead of relying on a
-# dynamic version to be available.
-#static-libstdcpp = true
+# When true, link libstdc++ statically into the rustc_llvm.
+# This is useful if you don't want to use the dynamic version of that
+# library provided by LLVM.
+#static-libstdcpp = false
 
 # Whether to use Ninja to build LLVM. This runs much faster than make.
 #ninja = true

src/bootstrap/config.rs

@@ -781,7 +781,7 @@ pub fn default_opts() -> Config {
         config.llvm_optimize = true;
         config.ninja_in_file = true;
         config.llvm_version_check = true;
-        config.llvm_static_stdcpp = true;
+        config.llvm_static_stdcpp = false;
         config.backtrace = true;
         config.rust_optimize = true;
         config.rust_optimize_tests = true;

src/ci/run.sh

@@ -123,6 +123,10 @@ else
   # (And PGO is its own can of worms).
   if [ "$NO_DOWNLOAD_CI_LLVM" = "" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set llvm.download-ci-llvm=if-available"
+  else
+    # When building for CI we want to use the static C++ Standard library
+    # included with LLVM, since a dynamic libstdcpp may not be available.
+    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set llvm.static-libstdcpp"
   fi
 fi