* Set the NIX_STORE and NIX_BUILD_TOP environment variables in

builders to point to the store and the temporary build directory,
  respectively.  Useful for purity checking.
* Also set TEMPDIR, TMPDIR, TEMP, and TEMP to NIX_BUILD_TOP to make
  sure that tools in the builder store temporary files in the right
  location.

src/libstore/exec.cc

@@ -17,7 +17,7 @@ static string pathNullDevice = "/dev/null";
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env,
+    const Strings & args, Environment env,
     const string & logFileName)
 {
     /* Create a log file. */
@@ -32,10 +32,20 @@ void runProgram(const string & program,
 
     /* Create a temporary directory where the build will take
        place. */
-    string tmpDir = createTempDir();
+    Path tmpDir = createTempDir();
 
     AutoDelete delTmpDir(tmpDir);
 
+    /* For convenience, set an environment pointing to the top build
+       directory. */
+    env["NIX_BUILD_TOP"] = tmpDir;
+
+    /* Also set TMPDIR and variants to point to this directory. */
+    env["TMPDIR"] = tmpDir;
+    env["TEMPDIR"] = tmpDir;
+    env["TMP"] = tmpDir;
+    env["TEMP"] = tmpDir;
+
     /* Fork a child to build the package. */
     pid_t pid;
     switch (pid = fork()) {

src/libstore/exec.hh

@@ -15,7 +15,7 @@ typedef map<string, string> Environment;
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env,
+    const Strings & args, Environment env,
     const string & logFileName);
 
 

src/libstore/normalise.cc

@@ -124,6 +124,12 @@ Path normaliseStoreExpr(const Path & _nePath, PathSet pending)
        non-existing path. */
     env["HOME"] = "/homeless-shelter";
 
+    /* Tell the builder where the Nix store is.  Usually they
+       shouldn't care, but this is useful for purity checking (e.g.,
+       the compiler or linker might only want to accept paths to files
+       in the store or in the build directory). */
+    env["NIX_STORE"] = nixStore;
+
     /* Build the environment. */
     for (StringPairs::iterator i = ne.derivation.env.begin();
          i != ne.derivation.env.end(); i++)