modload: provide a clearer error for standard library packages from newer releases

An older version of go compiling a main module that references a
standard library package from a newer release (e.g. net/netip added in
go 1.18) currently produces a confusing error message. This changes adds
a new error message including go version diagnostics.

Fixes #48966

Change-Id: I1e8319dafcf1f67d1b1ca869fe84190c3b3f3c3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/432075
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modload/import.go

@@ -41,6 +41,10 @@ type ImportMissingError struct {
 	// modules.
 	isStd bool
 
+	// importerGoVersion is the version the module containing the import error
+	// specified. It is only set when isStd is true.
+	importerGoVersion string
+
 	// replaced the highest replaced version of the module where the replacement
 	// contains the package. replaced is only set if the replacement is unused.
 	replaced module.Version
@@ -53,7 +57,11 @@ type ImportMissingError struct {
 func (e *ImportMissingError) Error() string {
 	if e.Module.Path == "" {
 		if e.isStd {
-			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
+			msg := fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
+			if e.importerGoVersion != "" {
+				msg += fmt.Sprintf("\nnote: imported by a module that requires go %s", e.importerGoVersion)
+			}
+			return msg
 		}
 		if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
 			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)

src/cmd/go/internal/modload/load.go

@@ -984,7 +984,7 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 	if ld.GoVersion == "" {
 		ld.GoVersion = MainModules.GoVersion()
 
-		if ld.Tidy && semver.Compare("v"+ld.GoVersion, "v"+LatestGoVersion()) > 0 {
+		if ld.Tidy && versionLess(LatestGoVersion(), ld.GoVersion) {
 			ld.errorf("go: go.mod file indicates go %s, but maximum version supported by tidy is %s\n", ld.GoVersion, LatestGoVersion())
 			base.ExitIfErrors()
 		}
@@ -993,7 +993,7 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 	if ld.Tidy {
 		if ld.TidyCompatibleVersion == "" {
 			ld.TidyCompatibleVersion = priorGoVersion(ld.GoVersion)
-		} else if semver.Compare("v"+ld.TidyCompatibleVersion, "v"+ld.GoVersion) > 0 {
+		} else if versionLess(ld.GoVersion, ld.TidyCompatibleVersion) {
 			// Each version of the Go toolchain knows how to interpret go.mod and
 			// go.sum files produced by all previous versions, so a compatibility
 			// version higher than the go.mod version adds nothing.
@@ -1184,11 +1184,19 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 			}
 		}
 
-		if ld.SilencePackageErrors {
-			continue
+		if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) && stdErr.isStd {
+			// Add importer go version information to import errors of standard
+			// library packages arising from newer releases.
+			if importer := pkg.stack; importer != nil {
+				if v, ok := rawGoVersion.Load(importer.mod); ok && versionLess(LatestGoVersion(), v.(string)) {
+					stdErr.importerGoVersion = v.(string)
+				}
+			}
+			if ld.SilenceMissingStdImports {
+				continue
+			}
 		}
-		if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) &&
-			stdErr.isStd && ld.SilenceMissingStdImports {
+		if ld.SilencePackageErrors {
 			continue
 		}
 		if ld.SilenceNoGoErrors && errors.Is(pkg.err, imports.ErrNoGo) {
@@ -1202,6 +1210,12 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 	return ld
 }
 
+// versionLess returns whether a < b according to semantic version precedence.
+// Both strings are interpreted as go version strings, e.g. "1.19".
+func versionLess(a, b string) bool {
+	return semver.Compare("v"+a, "v"+b) < 0
+}
+
 // updateRequirements ensures that ld.requirements is consistent with the
 // information gained from ld.pkgs.
 //

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

@@ -0,0 +1,17 @@
+# Go should indicate the version the module requires when a standard library
+# import is missing. See golang.org/issue/48966.
+
+! go build .
+stderr '^main.go:3:8: package nonexistent is not in GOROOT \(.*\)$'
+stderr '^note: imported by a module that requires go 1.99999$'
+
+-- go.mod --
+module example
+
+go 1.99999
+-- main.go --
+package main
+
+import _ "nonexistent"
+
+func main() {}