cmd/go: fix 'go test' and 'go fmt' with files outside a module

Use the actual loader result in findModule instead of making
assumptions about nesting in the build list.
As a side-effect, this produces much clearer error messages for
packages that (for one reason or another) failed to load.

Adjust the package and module path outside a module to
"command-line-arguments". That string already appears in the output of
a number of (module-mode and GOPATH-mode) commands for file arguments,
and as far as I can tell operation outside a module is currently the
only case in which the module path of a package is not actually a
prefix of the import path.

Fixes #28011
Fixes #27099
Fixes #28943
Updates #27102
Updates #28459
Updates #27063

Change-Id: I61d5556df7b1b7d1efdaffa892f0e3e95b612d87
Reviewed-on: https://go-review.googlesource.com/c/153459
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 8 insertions, 9 deletions

diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index 616adcc57a..228be07f24 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -997,10 +997,12 @@ func disallowInternal(srcDir string, importer *Package, importerPath string, p *
 	} else {
 		// p is in a module, so make it available based on the importer's import path instead
 		// of the file path (https://golang.org/issue/23970).
-		if importerPath == "." {
+		if importer.Internal.CmdlineFiles {
 			// The importer is a list of command-line files.
 			// Pretend that the import path is the import path of the
 			// directory containing them.
+			// If the directory is outside the main module, this will resolve to ".",
+			// which is not a prefix of any valid module.
 			importerPath = ModDirImportPath(importer.Dir)
 		}
 		parentOfInternal := p.ImportPath[:i]
@@ -1515,11 +1517,13 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	}
 
 	if cfg.ModulesEnabled {
-		if !p.Internal.CmdlineFiles {
-			p.Module = ModPackageModuleInfo(p.ImportPath)
+		mainPath := p.ImportPath
+		if p.Internal.CmdlineFiles {
+			mainPath = "command-line-arguments"
 		}
+		p.Module = ModPackageModuleInfo(mainPath)
 		if p.Name == "main" {
-			p.Internal.BuildInfo = ModPackageBuildInfo(p.ImportPath, p.Deps)
+			p.Internal.BuildInfo = ModPackageBuildInfo(mainPath, p.Deps)
 		}
 	}
 }
@@ -1986,11 +1990,6 @@ func GoFilesPackage(gofiles []string) *Package {
 	}
 
 	bp, err := ctxt.ImportDir(dir, 0)
-	if ModDirImportPath != nil {
-		// Use the effective import path of the directory
-		// for deciding visibility during pkg.load.
-		bp.ImportPath = ModDirImportPath(dir)
-	}
 	pkg := new(Package)
 	pkg.Internal.Local = true
 	pkg.Internal.CmdlineFiles = true