clang/gn/win: Stop running the compiler through `ninja -t msvc -e environment.foo`

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 1934e9b19ad43287d8435364e318d8dd0b4b3d8f..cf664b7349f7791834bdffd90ee37ad433c609ad 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -62,6 +62,18 @@ template("msvc_toolchain") {
 
   cl = invoker.cl
 
+  # If possible, pass system includes as flags to the compiler.  When that's
+  # not possible, load a full environment file (containing %INCLUDE% and
+  # %PATH) -- e.g. 32-bit builds require %PATH% to be set and just passing

[scottmg, 2016/02/25 22:56:37]

"%PATH%" in the first instance.

[scottmg, 2016/02/25 22:56:37]

"32-bit cl.exe builds..."

[Nico, 2016/02/26 03:00:44]

Done.

[Nico, 2016/02/26 03:00:44]

Done.

+  # in a list of include directories isn't enough.
+  if (defined(invoker.sys_include_flags)) {

[scottmg, 2016/02/25 22:56:37]

I think this would be more clear as is_clang.

[Nico, 2016/02/26 03:00:44]

Hm, but then if we ever get rid of the 32-bit build and move the 64-bit msvs
build (if it's still around then) to this system too, then we have to change two
lines instead of just one…I kind of like that this doesn't duplicate the logic. 
I added an `assert(!is_clang)` to the else block, how's that? (Just hitting *
while the cursor is on sys_include_flags finds quickly where this is set.)

If you really do want is_clang I'll change it though, of course :-)

[scottmg, 2016/02/26 03:32:03]

Wow, I feel like we'll add a 128-bit build before that happens... :)
assert sgtm.

+    env_wrapper = ""
+    sys_include_flags = "${invoker.sys_include_flags} "  # Note trailing space.
+  } else {
+    env_wrapper = "ninja -t msvc -e $env -- "  # Note trailing space.
+    sys_include_flags = ""
+  }
+
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
@@ -74,13 +86,13 @@ template("msvc_toolchain") {
       rspfile = "{{output}}.rsp"
       precompiled_header_type = "msvc"
       pdbname = "{{target_out_dir}}/{{label_name}}_c.pdb"
-      command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
+      command = "$env_wrapper$cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CC {{output}}"
       outputs = [
         "$object_subdir/{{source_name_part}}.obj",
       ]
-      rspfile_content = "{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
+      rspfile_content = "$sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
     }
 
     tool("cxx") {
@@ -89,13 +101,13 @@ template("msvc_toolchain") {
 
       # The PDB name needs to be different between C and C++ compiled files.
       pdbname = "{{target_out_dir}}/{{label_name}}_cc.pdb"
-      command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
+      command = "$env_wrapper$cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CXX {{output}}"
       outputs = [
         "$object_subdir/{{source_name_part}}.obj",
       ]
-      rspfile_content = "{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
+      rspfile_content = "$sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
     }
 
     tool("rc") {
@@ -239,6 +251,13 @@ template("msvc_toolchain") {
   }
 }
 
+if (is_clang) {
+  sys_include_prefix = "-isystem="
+} else {
+  # MSVC doesn't have the concept of system headers.
+  sys_include_prefix = "/I"
+}
+
 if (host_os == "win") {
   clang_cl = "clang-cl.exe"
 } else {
@@ -256,6 +275,7 @@ if (target_cpu == "x86") {
                                      windows_sdk_path,
                                      visual_studio_runtime_dirs,
                                      "x86",
+                                     "${sys_include_prefix}",
                                    ],
                                    "scope")
 
@@ -274,6 +294,7 @@ if (target_cpu == "x86") {
     cl = "${goma_prefix}$prefix/${clang_cl}"
     toolchain_os = "win"
     is_clang = true
+    sys_include_flags = "${x86_toolchain_data.include_flags}"
   }
 }
 
@@ -285,6 +306,7 @@ x64_toolchain_data = exec_script("setup_toolchain.py",
                                    windows_sdk_path,
                                    visual_studio_runtime_dirs,
                                    "x64",
+                                   "${sys_include_prefix}",
                                  ],
                                  "scope")
 
@@ -313,6 +335,7 @@ template("win_x64_toolchains") {
     cl = "${goma_prefix}$prefix/${clang_cl}"
     toolchain_os = "win"
     is_clang = true
+    sys_include_flags = "${x64_toolchain_data.include_flags}"
 
     forward_variables_from(invoker, [ "is_component_build" ])
   }