Oops, minor typo. The "extracted\" path component should be removed from the above example.

Currently, in order to correctly generate environment files (environment.x64, environment.x86) you must also:

- set DEPOT_TOOLS_WIN_TOOLCHAIN=0
- set GYP_MSVS_OVERRIDE_PATH to the VS root directory.

In build/toolchain/win/setup_toolchain.py it loads the INCLUDE, PATH and LIB variables by parsing %GYP_MSVS_OVERRIDE_PATH%\VC\vcvarsall.bat. This doesn't work if you've packaged up Visual Studio into a zip and then extracted it to some temp location on the build machine. It would be better if the setup_toolchain.py script supported explicit definition of these values (e.g. read them from INCLUDE/PATH/LIB environment variables).

There's also a toolchain.ninja file that appears to be new with the GN build. I'm not sure where the VS paths in this file are coming from.

Below is a patch that I believe addresses the GN variable behaviors from the original comment. It does not address the setup_toolchain.py issues from comment #3.

diff --git a/build/config/win/visual_studio_version.gni b/build/config/win/visual_studio_version.gni
index 05b369f..8be3a47 100644
--- a/build/config/win/visual_studio_version.gni
+++ b/build/config/win/visual_studio_version.gni
@@ -12,9 +12,8 @@ declare_args() {
   # Use "2013" for Visual Studio 2013, or "2013e" for the Express version.
   visual_studio_version = ""

-  # Directory of the Windows driver kit. If visual_studio_path is empty, this
-  # will be auto-filled.
-  wdk_path = ""
+  # Path to Visual Studio runtime libraries.
+  visual_studio_runtime_dirs = ""

   # Full path to the Windows SDK, not including a backslash at the end.
   # This value is the default location, override if you have a different
@@ -33,12 +32,11 @@ if (visual_studio_path == "") {
   visual_studio_path = toolchain_data.vs_path
   windows_sdk_path = toolchain_data.sdk_path
   visual_studio_version = toolchain_data.vs_version
-  wdk_path = toolchain_data.wdk_dir
   visual_studio_runtime_dirs = toolchain_data.runtime_dirs
 } else {
   assert(visual_studio_version != "",
          "You must set the visual_studio_version if you set the path")
-  assert(wdk_path != "",
-         "You must set the wdk_path if you set the visual studio path")
-  visual_studio_runtime_dirs = []
+  assert(visual_studio_runtime_dirs != "",
+         "You must set the visual_studio_runtime_dirs if you set the visual " +
+         "studio path")
 }

Below is a patch to setup_toolchain.py that reads variables from the environment when %GYP_MSVS_OVERRIDE_PATH%\VC\vcvarsall.bat doesn't exist. This correctly populates the environment files (environment.x64, environment.x86) and toolchain.ninja based on what you set via INCLUDE, PATH and LIB.

diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index d58cb85..fd608ba 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -124,11 +124,14 @@ def _LoadToolchainEnv(cpu, sdk_dir):
     script_path = os.path.normpath(os.path.join(
                                        os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                        'VC/vcvarsall.bat'))
-    if not os.path.exists(script_path):
-      raise Exception('%s is missing - make sure VC++ tools are installed.' %
-                      script_path)
-    args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64']
-    variables = _LoadEnvFromBat(args)
+    if os.path.exists(script_path):
+      args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64']
+      variables = _LoadEnvFromBat(args)
+    else:
+      variables = []
+      for k in sorted(os.environ.keys()):
+        variables.append('%s=%s' % (str(k), str(os.environ[k])))
+      variables = '\n'.join(variables)
   return _ExtractImportantEnvironment(variables)

Here's a CL with the above changes: https://codereview.chromium.org/2128993002

I won't take ownership of this issue since I'm not sure what other changes might be required to support build environments different from my own.

Closing the above CL for now. The use of setup_toolchain.py, vs_toolchain.py, environment.* files, DEPOT_TOOLS_WIN_TOOLCHAIN, GYP_MSVS_OVERRIDE_PATH, various GN variables, and VS detection in general feels overly complicated. Maybe it would be better to agree on a single source of truth (JSON file or GN variables), and then just reference that from everywhere.

When DEPOT_TOOLS_WIN_TOOLCHAIN=0 the SetEnvironmentAndGetRuntimeDllDirs() function in build/vs_toolchain.py tries to copy toolchain binaries from system directories to build directories [1]. This fails when the toolchain is not installed locally. It should instead look for these binaries under visual_studio_runtime_dirs and visual_studio_path, or just assume that they're already available under PATH.

[1] https://cs.chromium.org/chromium/src/build/vs_toolchain.py?dr&q=vs_toolchain.py+Sysnative&sq=package:chromium&l=88