diff --git a/Makefile b/Makefile
index e0be7cdcd..2b8108150 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,7 @@
+-include Makefile.config
+clean-files += Makefile.config
+
+ifeq ($(ENABLE_BUILD), yes)
 makefiles = \
   mk/precompiled-headers.mk \
   local.mk \
@@ -18,15 +22,18 @@ makefiles = \
   misc/upstart/local.mk \
   doc/manual/local.mk \
   doc/internal-api/local.mk
+endif
 
--include Makefile.config
-
-ifeq ($(tests), yes)
+ifeq ($(ENABLE_BUILD)_$(ENABLE_TESTS), yes_yes)
 UNIT_TEST_ENV = _NIX_TEST_UNIT_DATA=unit-test-data
 makefiles += \
   tests/unit/libutil/local.mk \
   tests/unit/libutil-support/local.mk \
-  tests/unit/libstore/local.mk \
+  tests/unit/libstore/local.mk
+endif
+
+ifeq ($(ENABLE_TESTS), yes)
+makefiles += \
   tests/unit/libstore-support/local.mk \
   tests/unit/libexpr/local.mk \
   tests/unit/libexpr-support/local.mk \
diff --git a/Makefile.config.in b/Makefile.config.in
index 707cfe0e3..19992fa20 100644
--- a/Makefile.config.in
+++ b/Makefile.config.in
@@ -46,5 +46,6 @@ sandbox_shell = @sandbox_shell@
 storedir = @storedir@
 sysconfdir = @sysconfdir@
 system = @system@
-tests = @tests@
+ENABLE_BUILD = @ENABLE_BUILD@
+ENABLE_TESTS = @ENABLE_TESTS@
 internal_api_docs = @internal_api_docs@
diff --git a/configure.ac b/configure.ac
index 6d78237f0..225baf6b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -152,12 +152,17 @@ if test "x$GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC" = xyes; then
     LDFLAGS="-latomic $LDFLAGS"
 fi
 
+# Running the functional tests without building Nix is useful for testing
+# different pre-built versions of Nix against each other.
+AC_ARG_ENABLE(build, AS_HELP_STRING([--disable-build],[Do not build nix]),
+  ENABLE_BUILD=$enableval, ENABLE_BUILD=yes)
+AC_SUBST(ENABLE_BUILD)
 # Building without tests is useful for bootstrapping with a smaller footprint
 # or running the tests in a separate derivation. Otherwise, we do compile and
 # run them.
 AC_ARG_ENABLE(tests, AS_HELP_STRING([--disable-tests],[Do not build the tests]),
-  tests=$enableval, tests=yes)
-AC_SUBST(tests)
+  ENABLE_TESTS=$enableval, ENABLE_TESTS=yes)
+AC_SUBST(ENABLE_TESTS)
 
 # Building without API docs is the default as Nix' C++ interfaces are internal and unstable.
 AC_ARG_ENABLE(internal_api_docs, AS_HELP_STRING([--enable-internal-api-docs],[Build API docs for Nix's internal unstable C++ interfaces]),
@@ -289,7 +294,7 @@ if test "$gc" = yes; then
 fi
 
 
-if test "$tests" = yes; then
+if test "$ENABLE_TESTS" = yes; then
 
 # Look for gtest.
 PKG_CHECK_MODULES([GTEST], [gtest_main])
diff --git a/flake.nix b/flake.nix
index 1d03f1a1f..bc14941fd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -59,30 +59,42 @@
         # that would interfere with repo semantics.
         fileset.fileFilter (f: f.name != ".gitignore") ./.;
 
+      configureFiles = fileset.unions [
+        ./.version
+        ./bootstrap.sh
+        ./configure.ac
+        ./m4
+        # TODO: do we really need README.md? It doesn't seem used in the build.
+        ./README.md
+      ];
+
+      topLevelBuildFiles = fileset.unions [
+        ./local.mk
+        ./Makefile
+        ./Makefile.config.in
+        ./mk
+      ];
+
+      functionalTestFiles = fileset.unions [
+        ./tests/functional
+        ./tests/unit
+        (fileset.fileFilter (f: lib.strings.hasPrefix "nix-profile" f.name) ./scripts)
+      ];
+
       nixSrc = fileset.toSource {
         root = ./.;
         fileset = fileset.intersect baseFiles (fileset.unions [
-          ./.version
+          configureFiles
+          topLevelBuildFiles
           ./boehmgc-coroutine-sp-fallback.diff
-          ./bootstrap.sh
-          ./configure.ac
           ./doc
-          ./local.mk
-          ./m4
-          ./Makefile
-          ./Makefile.config.in
           ./misc
-          ./mk
           ./precompiled-headers.h
           ./src
-          ./tests/functional
-          ./tests/unit
           ./unit-test-data
           ./COPYING
           ./scripts/local.mk
-          (fileset.fileFilter (f: lib.strings.hasPrefix "nix-profile" f.name) ./scripts)
-          # TODO: do we really need README.md? It doesn't seem used in the build.
-          ./README.md
+          functionalTestFiles
         ]);
       };
 
@@ -253,7 +265,6 @@
       testNixVersions = pkgs: client: daemon: with commonDeps { inherit pkgs; }; with pkgs.lib; pkgs.stdenv.mkDerivation {
         NIX_DAEMON_PACKAGE = daemon;
         NIX_CLIENT_PACKAGE = client;
-        HAVE_LOCAL_NIX_BUILD = false;
         name =
           "nix-tests"
           + optionalString
@@ -262,7 +273,14 @@
             "-${client.version}-against-${daemon.version}";
         inherit version;
 
-        src = nixSrc;
+        src = fileset.toSource {
+          root = ./.;
+          fileset = fileset.intersect baseFiles (fileset.unions [
+            configureFiles
+            topLevelBuildFiles
+            functionalTestFiles
+          ]);
+        };
 
         VERSION_SUFFIX = versionSuffix;
 
@@ -272,19 +290,20 @@
 
         enableParallelBuilding = true;
 
-        configureFlags = testConfigureFlags; # otherwise configure fails
+        configureFlags =
+          testConfigureFlags # otherwise configure fails
+          ++ [ "--disable-build" ];
+        dontBuild = true;
         doInstallCheck = true;
 
-        buildPhase = ''
-          # Remove the source files to make sure that we're not accidentally rebuilding Nix
-          rm src/**/*.cc
-        '';
-
         installPhase = ''
           mkdir -p $out
         '';
 
-        installCheckPhase = "make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES";
+        installCheckPhase = ''
+          mkdir -p src/nix-channel
+          make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES
+        '';
       };
 
       binaryTarball = nix: pkgs:
@@ -460,7 +479,15 @@
             passthru.perl-bindings = with final; perl.pkgs.toPerlModule (currentStdenv.mkDerivation {
               name = "nix-perl-${version}";
 
-              src = self;
+              src = fileset.toSource {
+                root = ./.;
+                fileset = fileset.intersect baseFiles (fileset.unions [
+                  ./perl
+                  ./.version
+                  ./m4
+                  ./mk
+                ]);
+              };
 
               nativeBuildInputs =
                 [ buildPackages.autoconf-archive
diff --git a/local.mk b/local.mk
index 6951c179e..3f3abb9f0 100644
--- a/local.mk
+++ b/local.mk
@@ -1,5 +1,3 @@
-clean-files += Makefile.config
-
 GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch
 # Allow switch-enum to be overridden for files that do not support it, usually because of dependency headers.
 ERROR_SWITCH_ENUM = -Werror=switch-enum
diff --git a/perl/Makefile b/perl/Makefile
index c2c95f255..832668dd1 100644
--- a/perl/Makefile
+++ b/perl/Makefile
@@ -1,6 +1,12 @@
 makefiles = local.mk
 
-GLOBAL_CXXFLAGS += -g -Wall -std=c++2a -I ../src
+GLOBAL_CXXFLAGS += -g -Wall -std=c++2a
+
+# A convenience for concurrent development of Nix and its Perl bindings.
+# Not needed in a standalone build of the Perl bindings.
+ifneq ("$(wildcard ../src)", "")
+  GLOBAL_CXXFLAGS += -I ../src
+endif
 
 -include Makefile.config
 
diff --git a/tests/functional/local.mk b/tests/functional/local.mk
index 5eec6df9e..a2f900a9e 100644
--- a/tests/functional/local.mk
+++ b/tests/functional/local.mk
@@ -1,7 +1,3 @@
-# whether to run the tests that assume that we have a local build of
-# Nix
-HAVE_LOCAL_NIX_BUILD ?= 1
-
 nix_tests = \
   test-infra.sh \
   init.sh \
@@ -130,7 +126,7 @@ ifeq ($(HAVE_LIBCPUID), 1)
 	nix_tests += compute-levels.sh
 endif
 
-ifeq ($(HAVE_LOCAL_NIX_BUILD), 1)
+ifeq ($(ENABLE_BUILD), yes)
 	nix_tests += test-libstoreconsumer.sh
 
 	ifeq ($(BUILD_SHARED_LIBS), 1)