cmd/go: report the imports of CompiledGoFiles in ImportMap

Ideally we should encode the load.PackageInternal data in a way that
doesn't rely on 1:1 correlations of slices, but this is a minimal fix
to unblock Go 1.17.

Fixes #46462

Change-Id: I6e029c69f757aadc54d4be02c01d6b294c217542
Reviewed-on: https://go-review.googlesource.com/c/go/+/326610
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/internal/list/list.go

@@ -724,8 +724,18 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 
 	// Record non-identity import mappings in p.ImportMap.
 	for _, p := range pkgs {
-		for i, srcPath := range p.Internal.RawImports {
-			path := p.Imports[i]
+		nRaw := len(p.Internal.RawImports)
+		for i, path := range p.Imports {
+			var srcPath string
+			if i < nRaw {
+				srcPath = p.Internal.RawImports[i]
+			} else {
+				// This path is not within the raw imports, so it must be an import
+				// found only within CompiledGoFiles. Those paths are found in
+				// CompiledImports.
+				srcPath = p.Internal.CompiledImports[i-nRaw]
+			}
+
 			if path != srcPath {
 				if p.ImportMap == nil {
 					p.ImportMap = make(map[string]string)

src/cmd/go/internal/load/pkg.go

@@ -194,8 +194,8 @@ type PackageInternal struct {
 	// Unexported fields are not part of the public API.
 	Build             *build.Package
 	Imports           []*Package           // this package's direct imports
-	CompiledImports   []string             // additional Imports necessary when using CompiledGoFiles (all from standard library)
-	RawImports        []string             // this package's original imports as they appear in the text of the program
+	CompiledImports   []string             // additional Imports necessary when using CompiledGoFiles (all from standard library); 1:1 with the end of PackagePublic.Imports
+	RawImports        []string             // this package's original imports as they appear in the text of the program; 1:1 with the end of PackagePublic.Imports
 	ForceLibrary      bool                 // this package is a library (even if named "main")
 	CmdlineFiles      bool                 // package built from files listed on command line
 	CmdlinePkg        bool                 // package listed on command line

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

@@ -0,0 +1,38 @@
+# Regression test for https://golang.org/issue/46462.
+#
+# The "runtime/cgo" import found in synthesized .go files (reported in
+# the CompiledGoFiles field) should have a corresponding entry in the
+# ImportMap field when a runtime/cgo variant (such as a test variant)
+# will be used.
+
+[short] skip  # -compiled can be slow (because it compiles things)
+[!cgo] skip
+
+env CGO_ENABLED=1
+env GOFLAGS=-tags=netcgo  # Force net to use cgo even on Windows.
+
+
+# "runtime/cgo [runtime.test]" appears in the the test dependencies of "runtime",
+# because "runtime/cgo" itself depends on "runtime"
+
+go list -deps -test -compiled -f '{{if eq .ImportPath "net [runtime.test]"}}{{printf "%q" .Imports}}{{end}}' runtime
+
+	# Control case: the explicitly-imported package "sync" is a test variant,
+	# because "sync" depends on "runtime".
+stdout '"sync \[runtime\.test\]"'
+! stdout '"sync"'
+
+	# Experiment: the implicitly-imported package "runtime/cgo" is also a test variant,
+	# because "runtime/cgo" also depends on "runtime".
+stdout '"runtime/cgo \[runtime\.test\]"'
+! stdout '"runtime/cgo"'
+
+
+# Because the import of "runtime/cgo" in the cgo-generated file actually refers
+# to "runtime/cgo [runtime.test]", the latter should be listed in the ImportMap.
+# BUG(#46462): Today, it is not.
+
+go list -deps -test -compiled -f '{{if eq .ImportPath "net [runtime.test]"}}{{printf "%q" .ImportMap}}{{end}}' runtime
+
+stdout '"sync":"sync \[runtime\.test\]"'                # control
+stdout '"runtime/cgo":"runtime/cgo \[runtime\.test\]"'  # experiment