env GO111MODULE=auto
env CGO_CFLAGS=nodefault
env CGO_CPPFLAGS=nodefault

go env -changed
# linux output like GOTOOLCHAIN='local'
# windows output like GOTOOLCHAIN=local
stdout 'GOTOOLCHAIN=''?local''?'
stdout 'GOSUMDB=''?nodefault''?'
stdout 'GOPROXY=''?nodefault''?'
stdout 'GO111MODULE=''?auto''?'
stdout 'CGO_CFLAGS=''?nodefault''?'
stdout 'CGO_CPPFLAGS=''?nodefault''?'

go env -changed -json

# CGO_ENABLED is always present.
# Other flags are added if CGO_ENABLED is true.
env CGO_ENABLED=0
go build
go version -m m$GOEXE
stdout '^\tbuild\tCGO_ENABLED=0$'
! stdout CGO_CPPFLAGS|CGO_CFLAGS|CGO_CXXFLAGS|CGO_LDFLAGS

[cgo] env CGO_ENABLED=1
[cgo] env CGO_CPPFLAGS=-DFROM_CPPFLAGS=1
[cgo] env CGO_CFLAGS=-DFROM_CFLAGS=1
[cgo] env CGO_CXXFLAGS=-DFROM_CXXFLAGS=1
[cgo] env CGO_LDFLAGS=-L/extra/dir/does/not/exist

package cfg

// KnownEnv is a list of environment variables that affect the operation
// of the Go command.
const KnownEnv = `
	AR
	CC
	CGO_CFLAGS
	CGO_CFLAGS_ALLOW
	CGO_CFLAGS_DISALLOW
	CGO_CPPFLAGS
	CGO_CPPFLAGS_ALLOW
	CGO_CPPFLAGS_DISALLOW
	CGO_CXXFLAGS
	CGO_CXXFLAGS_ALLOW
	CGO_CXXFLAGS_DISALLOW
	CGO_ENABLED
	CGO_FFLAGS
	CGO_FFLAGS_ALLOW
	CGO_FFLAGS_DISALLOW
	CGO_LDFLAGS
	CGO_LDFLAGS_ALLOW

        GCCGO="gccgo"
        AR="ar"
        CC="clang"
        CXX="clang++"
        CGO_ENABLED="1"
        GOMOD="/dev/null"
        GOWORK=""
        CGO_CFLAGS="-O2 -g"
        CGO_CPPFLAGS=""
        CGO_CXXFLAGS="-O2 -g"
        CGO_FFLAGS="-O2 -g"
        CGO_LDFLAGS="-O2 -g"
        PKG_CONFIG="pkg-config"

//   - -compiler: the compiler toolchain flag used (typically "gc")
//   - CGO_ENABLED: the effective CGO_ENABLED environment variable
//   - CGO_CFLAGS: the effective CGO_CFLAGS environment variable
//   - CGO_CPPFLAGS: the effective CGO_CPPFLAGS environment variable
//   - CGO_CXXFLAGS:  the effective CGO_CXXFLAGS environment variable
//   - CGO_LDFLAGS: the effective CGO_LDFLAGS environment variable
//   - GOARCH: the architecture target

			return strings.Join(s, " ")
		}
		return q
	}

	ret := []cfg.EnvVar{
		// Note: Update the switch in runEnv below when adding to this list.
		{Name: "CGO_CFLAGS", Value: join(cflags)},
		{Name: "CGO_CPPFLAGS", Value: join(cppflags)},
		{Name: "CGO_CXXFLAGS", Value: join(cxxflags)},
		{Name: "CGO_FFLAGS", Value: join(fflags)},
		{Name: "CGO_LDFLAGS", Value: join(ldflags)},
		{Name: "PKG_CONFIG", Value: b.PkgconfigCmd()},

		case "runtime/race":
			args = append(args, "-import_syscall=false")
		}
	}
	args = append(args, "--")
	args = append(args, strings.Fields(os.Getenv("CGO_CPPFLAGS"))...)
	args = append(args, bp.CgoCPPFLAGS...)
	if len(bp.CgoPkgConfig) > 0 {
		cmd := exec.Command("pkg-config", append([]string{"--cflags"}, bp.CgoPkgConfig...)...)
		out, err := cmd.Output()
		if err != nil {

	CGO_CFLAGS_DISALLOW
		A regular expression specifying flags that must be disallowed
		from appearing in #cgo CFLAGS source code directives.
		Does not apply to the CGO_CFLAGS environment variable.
	CGO_CPPFLAGS, CGO_CPPFLAGS_ALLOW, CGO_CPPFLAGS_DISALLOW
		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
		but for the C preprocessor.
	CGO_CXXFLAGS, CGO_CXXFLAGS_ALLOW, CGO_CXXFLAGS_DISALLOW

permitted, notably alphanumeric characters and a few symbols, such as
'.', that will not be interpreted in unexpected ways. Attempts to use
forbidden characters will get a "malformed #cgo argument" error.

When building, the CGO_CFLAGS, CGO_CPPFLAGS, CGO_CXXFLAGS, CGO_FFLAGS and
CGO_LDFLAGS environment variables are added to the flags derived from
these directives. Package-specific flags should be set using the
directives, not the environment variables, so that builds work in

	// subset of flags that are known not to be paths?
	if cfg.BuildContext.CgoEnabled && !cfg.BuildTrimpath {
		for _, name := range []string{"CGO_CFLAGS", "CGO_CPPFLAGS", "CGO_CXXFLAGS", "CGO_LDFLAGS"} {
			appendSetting(name, cfg.Getenv(name))
		}
	}
	appendSetting("GOARCH", cfg.BuildContext.GOARCH)
	if cfg.RawGOEXPERIMENT != "" {