diff --git a/Makefile.config.in b/Makefile.config.in
index a4d641040f336..7af5d3c90200f 100644
--- a/Makefile.config.in
+++ b/Makefile.config.in
@@ -58,6 +58,22 @@ LLVM_OBJ_ROOT   := $(call realpath, @abs_top_builddir@)
 PROJ_SRC_ROOT   := $(LLVM_SRC_ROOT)
 PROJ_SRC_DIR    := $(LLVM_SRC_ROOT)$(patsubst $(PROJ_OBJ_ROOT)%,%,$(PROJ_OBJ_DIR))
 
+# FIXME: This is temporary during the grace period where in-source builds are
+# deprecated. Convert to a hard error when that period is up.
+#
+# See: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150323/268067.html
+ifeq ($(LLVM_SRC_ROOT), $(LLVM_OBJ_ROOT))
+  $(warning ######################################################################################)
+  $(warning #                                                                                    #)
+  $(warning #                                   WARNING                                          #)
+  $(warning #                                                                                    #)
+  $(warning #                        In-source builds are deprecated.                            #)
+  $(warning #                                                                                    #)
+  $(warning #               Please configure from a separate build directory!                    #)
+  $(warning #                                                                                    #)
+  $(warning ######################################################################################)
+endif
+
 ifneq ($(CLANG_SRC_ROOT),)
   CLANG_SRC_ROOT:= $(call realpath, $(CLANG_SRC_ROOT))
   PROJ_SRC_DIR  := $(patsubst $(LLVM_SRC_ROOT)/tools/clang%,$(CLANG_SRC_ROOT)%,$(PROJ_SRC_DIR))
diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 39ce9f353ee30..586f3f26cb313 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -73,6 +73,19 @@ if test ${srcdir} != "." ; then
   fi
 fi
 
+dnl Quit if it is an in-source build
+if test ${srcdir} == "." ; then
+  AC_MSG_WARN([**************************************************************************************])
+  AC_MSG_WARN([*                                                                                    *])
+  AC_MSG_WARN([*                                   WARNING                                          *])
+  AC_MSG_WARN([*                                                                                    *])
+  AC_MSG_WARN([*                        In-source builds are deprecated.                            *])
+  AC_MSG_WARN([*                                                                                    *])
+  AC_MSG_WARN([*               Please configure from a separate build directory!                    *])
+  AC_MSG_WARN([*                                                                                    *])
+  AC_MSG_WARN([**************************************************************************************])
+fi
+
 dnl Default to empty (i.e. assigning the null string to) CFLAGS and CXXFLAGS,
 dnl instead of the autoconf default (for example, '-g -O2' for CC=gcc).
 : ${CFLAGS=}
diff --git a/configure b/configure
index f51617639b3f9..37f3d7d95ae66 100755
--- a/configure
+++ b/configure
@@ -2030,6 +2030,27 @@ echo "$as_me: error: Already configured in ${srcdir}" >&2;}
   fi
 fi
 
+if test ${srcdir} == "." ; then
+  { echo "$as_me:$LINENO: WARNING: **************************************************************************************" >&5
+echo "$as_me: WARNING: **************************************************************************************" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                                                                                    *" >&5
+echo "$as_me: WARNING: *                                                                                    *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                                   WARNING                                          *" >&5
+echo "$as_me: WARNING: *                                   WARNING                                          *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                                                                                    *" >&5
+echo "$as_me: WARNING: *                                                                                    *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                        In-source builds are deprecated.                            *" >&5
+echo "$as_me: WARNING: *                        In-source builds are deprecated.                            *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                                                                                    *" >&5
+echo "$as_me: WARNING: *                                                                                    *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *               Please configure from a separate build directory!                    *" >&5
+echo "$as_me: WARNING: *               Please configure from a separate build directory!                    *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: *                                                                                    *" >&5
+echo "$as_me: WARNING: *                                                                                    *" >&2;}
+  { echo "$as_me:$LINENO: WARNING: **************************************************************************************" >&5
+echo "$as_me: WARNING: **************************************************************************************" >&2;}
+fi
+
 : ${CFLAGS=}
 : ${CXXFLAGS=}