lib/strings: Add makeIncludePath

lib/default.nix

@@ -97,7 +97,7 @@ let
     inherit (self.strings) concatStrings concatMapStrings concatImapStrings
       intersperse concatStringsSep concatMapStringsSep
       concatImapStringsSep concatLines makeSearchPath makeSearchPathOutput
-      makeLibraryPath makeBinPath optionalString
+      makeLibraryPath makeIncludePath makeBinPath optionalString
       hasInfix hasPrefix hasSuffix stringToCharacters stringAsChars escape
       escapeShellArg escapeShellArgs
       isStorePath isStringLike

lib/strings.nix

@@ -206,6 +206,18 @@ rec {
   */
   makeLibraryPath = makeSearchPathOutput "lib" "lib";
 
+  /* Construct an include search path (such as C_INCLUDE_PATH) containing the
+     header files for a set of packages or paths.
+
+     Example:
+       makeIncludePath [ "/usr" "/usr/local" ]
+       => "/usr/include:/usr/local/include"
+       pkgs = import <nixpkgs> { }
+       makeIncludePath [ pkgs.openssl pkgs.zlib ]
+       => "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/include:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8-dev/include"
+  */
+  makeIncludePath = makeSearchPathOutput "dev" "include";
+
   /* Construct a binary search path (such as $PATH) containing the
      binaries for a set of packages.
 

lib/tests/misc.nix

@@ -64,6 +64,7 @@ let
     lists
     listToAttrs
     makeExtensible
+    makeIncludePath
     makeOverridable
     mapAttrs
     matchAttrs
@@ -296,6 +297,30 @@ runTests {
     expected = "a\nb\nc\n";
   };
 
+  testMakeIncludePathWithPkgs = {
+    expr = (makeIncludePath [
+      { dev = "/nix/store/bash"; }
+      # lib.getOutput "dev" return out if "dev" is not found
+      { out = "/nix/store/openssl"; }
+    ]);
+    expected = "/nix/store/bash/include:/nix/store/openssl/include";
+  };
+
+  testMakeIncludePathWithEmptyList = {
+    expr = (makeIncludePath [ ]);
+    expected = "";
+  };
+
+  testMakeIncludePathWithOneString = {
+    expr = (makeIncludePath [ "/usr" ]);
+    expected = "/usr/include";
+  };
+
+  testMakeIncludePathWithManyString = {
+    expr = (makeIncludePath [ "/usr" "/usr/local" ]);
+    expected = "/usr/include:/usr/local/include";
+  };
+
   testReplicateString = {
     expr = strings.replicate 5 "hello";
     expected = "hellohellohellohellohello";