go/build: don't include imports from cgo files when CGO_ENABLED=0

Fixes #35873
Fixes #35946

Change-Id: I9f9a9c09006f8957569db6e5cc13382b9b28f829
Reviewed-on: https://go-review.googlesource.com/c/go/+/209660
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/testdata/script/list_constraints.txt

+# Check that files and their imports are not included in 'go list' output
+# when they are excluded by build constraints.
+
+# Linux and cgo files should be included when building in that configuration.
+env GOOS=linux
+env CGO_ENABLED=1
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^cgotag.go empty.go suffix_linux.go tag.go $'
+go list -f '{{range .CgoFiles}}{{.}} {{end}}'
+stdout '^cgoimport.go $'
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^C cgoimport cgotag suffix tag $'
+
+# Disabling cgo should exclude cgo files and their imports.
+env CGO_ENABLED=0
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout 'empty.go suffix_linux.go tag.go'
+go list -f '{{range .CgoFiles}}{{.}} {{end}}'
+! stdout .
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^suffix tag $'
+
+# Changing OS should exclude linux sources.
+env GOOS=darwin
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^empty.go $'
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^$'
+
+# Enabling a tag should include files that require it.
+go list -tags=extra -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^empty.go extra.go $'
+go list -tags=extra -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^extra $'
+
+# Packages that require a tag should not be listed unless the tag is on.
+! go list ./tagonly
+go list -tags=extra ./tagonly
+stdout m/tagonly
+
+-- go.mod --
+module m
+
+go 1.13
+
+-- empty.go --
+package p
+
+-- extra.go --
+// +build extra
+
+package p
+
+import _ "extra"
+
+-- suffix_linux.go --
+package p
+
+import _ "suffix"
+
+-- tag.go --
+// +build linux
+
+package p
+
+import _ "tag"
+
+-- cgotag.go --
+// +build cgo
+
+package p
+
+import _ "cgotag"
+
+-- cgoimport.go --
+package p
+
+import "C"
+
+import _ "cgoimport"
+
+-- tagonly/tagonly.go --
+// +build extra
+
+package tagonly

src/cmd/go/testdata/script/list_tags.txt

-env GO111MODULE=off
-
-# go list supports -tags
-go list -tags=thetag ./my...
-stdout mypkg
-
--- mypkg/x.go --
-// +build thetag
-
-package mypkg

src/go/build/build.go

@@ -905,6 +905,11 @@ Found:
 		}
 
 		// Record imports and information about cgo.
+		type importPos struct {
+			path string
+			pos  token.Pos
+		}
+		var fileImports []importPos
 		isCgo := false
 		for _, decl := range pf.Decls {
 			d, ok := decl.(*ast.GenDecl)
@@ -921,13 +926,7 @@ Found:
 				if err != nil {
 					log.Panicf("%s: parser returned invalid quoted string: <%s>", filename, quoted)
 				}
-				if isXTest {
-					xTestImported[path] = append(xTestImported[path], fset.Position(spec.Pos()))
-				} else if isTest {
-					testImported[path] = append(testImported[path], fset.Position(spec.Pos()))
-				} else {
-					imported[path] = append(imported[path], fset.Position(spec.Pos()))
-				}
+				fileImports = append(fileImports, importPos{path, spec.Pos()})
 				if path == "C" {
 					if isTest {
 						badFile(fmt.Errorf("use of cgo in test %s not supported", filename))
@@ -946,21 +945,37 @@ Found:
 				}
 			}
 		}
-		if isCgo {
+
+		var fileList *[]string
+		var importMap map[string][]token.Position
+		switch {
+		case isCgo:
 			allTags["cgo"] = true
 			if ctxt.CgoEnabled {
-				p.CgoFiles = append(p.CgoFiles, name)
+				fileList = &p.CgoFiles
+				importMap = imported
 			} else {
-				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+				// Ignore imports from cgo files if cgo is disabled.
+				fileList = &p.IgnoredGoFiles
+			}
+		case isXTest:
+			fileList = &p.XTestGoFiles
+			importMap = xTestImported
+		case isTest:
+			fileList = &p.TestGoFiles
+			importMap = testImported
+		default:
+			fileList = &p.GoFiles
+			importMap = imported
+		}
+		*fileList = append(*fileList, name)
+		if importMap != nil {
+			for _, imp := range fileImports {
+				importMap[imp.path] = append(importMap[imp.path], fset.Position(imp.pos))
 			}
-		} else if isXTest {
-			p.XTestGoFiles = append(p.XTestGoFiles, name)
-		} else if isTest {
-			p.TestGoFiles = append(p.TestGoFiles, name)
-		} else {
-			p.GoFiles = append(p.GoFiles, name)
 		}
 	}
+
 	if badGoError != nil {
 		return p, badGoError
 	}

src/go/build/build_test.go

@@ -526,3 +526,20 @@ func TestMissingImportErrorRepetition(t *testing.T) {
 		t.Fatalf("package path %q appears in error %d times; should appear once\nerror: %v", pkgPath, n, err)
 	}
 }
+
+// TestCgoImportsIgnored checks that imports in cgo files are not included
+// in the imports list when cgo is disabled.
+// Verifies golang.org/issue/35946.
+func TestCgoImportsIgnored(t *testing.T) {
+	ctxt := Default
+	ctxt.CgoEnabled = false
+	p, err := ctxt.ImportDir("testdata/cgo_disabled", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, path := range p.Imports {
+		if path == "should/be/ignored" {
+			t.Errorf("found import %q in ignored cgo file", path)
+		}
+	}
+}

src/go/build/testdata/cgo_disabled/cgo_disabled.go

+package cgo_disabled
+
+import "C"
+
+import _ "should/be/ignored"

src/go/build/testdata/cgo_disabled/empty.go

+package cgo_disabled