cmd/go: populate available module information for packages in vendor mode

Updates #33848

Change-Id: I10b4c79faef8bc3dee2ceba14d496fa049e84fb2
Reviewed-on: https://go-review.googlesource.com/c/go/+/202977
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/build.go

@@ -119,14 +119,8 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 		info.GoVersion = loaded.goVersion[m.Path]
 	}
 
-	if cfg.BuildMod == "vendor" {
-		// The vendor directory doesn't contain enough information to reconstruct
-		// anything more about the module.
-		return info
-	}
-
-	// complete fills in the extra fields in m.
-	complete := func(m *modinfo.ModulePublic) {
+	// completeFromModCache fills in the extra fields in m using the module cache.
+	completeFromModCache := func(m *modinfo.ModulePublic) {
 		if m.Version != "" {
 			if q, err := Query(m.Path, m.Version, "", nil); err != nil {
 				m.Error = &modinfo.ModuleError{Err: err.Error()}
@@ -152,13 +146,21 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 	}
 
 	if !fromBuildList {
-		complete(info)
+		completeFromModCache(info) // Will set m.Error in vendor mode.
 		return info
 	}
 
 	r := Replacement(m)
 	if r.Path == "" {
-		complete(info)
+		if cfg.BuildMod == "vendor" {
+			// It's tempting to fill in the "Dir" field to point within the vendor
+			// directory, but that would be misleading: the vendor directory contains
+			// a flattened package tree, not complete modules, and it can even
+			// interleave packages from different modules if one module path is a
+			// prefix of the other.
+		} else {
+			completeFromModCache(info)
+		}
 		return info
 	}
 
@@ -178,9 +180,11 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 			info.Replace.Dir = filepath.Join(ModRoot(), r.Path)
 		}
 	}
-	complete(info.Replace)
-	info.Dir = info.Replace.Dir
-	info.GoMod = filepath.Join(info.Dir, "go.mod")
+	if cfg.BuildMod != "vendor" {
+		completeFromModCache(info.Replace)
+		info.Dir = info.Replace.Dir
+		info.GoMod = filepath.Join(info.Dir, "go.mod")
+	}
 	return info
 }
 

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

@@ -21,6 +21,11 @@ stdout '.*[/\\]vendor[/\\]rsc.io[/\\]quote[/\\]v3'
 ! stderr 'finding'
 ! stderr 'lookup disabled'
 
+# 'go list' should provide the original replacement directory as the module's
+# replacement path.
+go list -mod=vendor -f '{{with .Module}}{{with .Replace}}{{.Path}}{{end}}{{end}}' rsc.io/quote/v3
+stdout '.*[/\\]not-rsc.io[/\\]quote[/\\]v3'
+
 # The same module can't be used as two different paths.
 cd multiple-paths
 ! go mod vendor
@@ -58,4 +63,4 @@ require (
 	rsc.io/quote/v3 v3.0.0
 	not-rsc.io/quote/v3 v3.0.0
 )
-replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0
\ No newline at end of file
+replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0