cmd/go: pass GoVersion in vet config

When invoking a vet tool with -vettool (or vet itself),
we need to pass the package's GoVersion to use when
analyzing the package.

The test of this behavior is in the x/tools/go/analysis CL 507880.

For #61176.

Change-Id: I3b5a90fcd19a0adc7fb29366e106e18f722fc061
Reviewed-on: https://go-review.googlesource.com/c/go/+/507976
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>

src/cmd/go/internal/work/exec.go

@@ -1115,6 +1115,7 @@ type vetConfig struct {
 	PackageVetx map[string]string // map package path to vetx data from earlier vet run
 	VetxOnly    bool              // only compute vetx data; don't report detected problems
 	VetxOutput  string            // write vetx data to this output file
+	GoVersion   string            // Go version for package
 
 	SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
 }
@@ -1149,6 +1150,13 @@ func buildVetConfig(a *Action, srcfiles []string) {
 		PackageFile:  make(map[string]string),
 		Standard:     make(map[string]bool),
 	}
+	if a.Package.Module != nil {
+		v := a.Package.Module.GoVersion
+		if v == "" {
+			v = gover.DefaultGoModVersion
+		}
+		vcfg.GoVersion = "go" + v
+	}
 	a.vetCfg = vcfg
 	for i, raw := range a.Package.Internal.RawImports {
 		final := a.Package.Imports[i]

src/cmd/go/internal/work/gc.go

@@ -85,19 +85,7 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 	if p.Module != nil {
 		v := p.Module.GoVersion
 		if v == "" {
-			// We started adding a 'go' directive to the go.mod file unconditionally
+			v = gover.DefaultGoModVersion
-			// as of Go 1.12, so any module that still lacks such a directive must
-			// either have been authored before then, or have a hand-edited go.mod
-			// file that hasn't been updated by cmd/go since that edit.
-			//
-			// Unfortunately, through at least Go 1.16 we didn't add versions to
-			// vendor/modules.txt. So this could also be a vendored 1.16 dependency.
-			//
-			// Fortunately, there were no breaking changes to the language between Go
-			// 1.11 and 1.16, so if we assume Go 1.16 semantics we will not introduce
-			// any spurious errors — we will only mask errors, and not particularly
-			// important ones at that.
-			v = "1.16"
 		}
 		if allowedVersion(v) {
 			defaultGcFlags = append(defaultGcFlags, "-lang=go"+gover.Lang(v))