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..f46a0a396c0703e8a064d6a85fa7ec8fd43ae7eb 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -62,6 +62,24 @@ 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 MSVS builds require %PATH% to be set and just passing
+  # in a list of include directories isn't enough.
+  if (defined(invoker.sys_include_flags)) {
+    env_wrapper = ""
+
+    # -nostdlibinc so that clang-cl _only_ looks in sys_include_flags, instead

[scottmg, 2016/02/26 19:55:22]

-nostdlibinc doesn't appear to be mentioned in the flags below.

[Nico, 2016/02/26 19:58:22]

Yes, clang-cl doesn't expose that flag yet. It's not needed, it just makes
things not work by accident (by looking at sys includes) if I misspell -isystem
like I originally did.

+    # of in addition to eventual headers from an installed MSVC.
+    # Note trailing space.
+    sys_include_flags = "-isystem ..\..\third_party\llvm-build\Release+Asserts\lib\clang\3.9.0\include ${invoker.sys_include_flags} "

[scottmg, 2016/02/26 19:55:22]

I guess we should have an assert(invoker.is_clang) in this branch now.

[Nico, 2016/02/26 19:58:22]

Yes, if I go ahead with that I'll add that. But it's kinda gross to hardcode
this path here anyhow, so I'll think about different approaches for a bit first.

+  } else {
+    # clang-cl doesn't need this env hoop, so omit it there.
+    assert(!invoker.is_clang)
+    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 +92,13 @@ 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 +107,13 @@ 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 +257,13 @@ 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 +281,7 @@ if (target_cpu == "x86") {
                                      windows_sdk_path,
                                      visual_studio_runtime_dirs,
                                      "x86",
+                                     "${sys_include_prefix}",
                                    ],
                                    "scope")
 
@@ -274,6 +300,7 @@ 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 +312,7 @@ x64_toolchain_data = exec_script("setup_toolchain.py",
                                    windows_sdk_path,
                                    visual_studio_runtime_dirs,
                                    "x64",
+                                   "${sys_include_prefix}",
                                  ],
                                  "scope")
 
@@ -313,6 +341,7 @@ 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" ])
   }