cmd/go/internal/vet: print line numbers appropriately on list errors

Fixes #36173

For reasons that are unclear to me, this commit:
f1d5ce0
introduces a TestPackagesFor function that strips line numbers from error
messages. This commit introduces a new version of that function for 'go vet'
that always keeps the line numbers.

Author: nicks

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

@@ -395,6 +395,8 @@ func (e *importError) ImportPath() string {
 // An ImportStack is a stack of import paths, possibly with the suffix " (test)" appended.
 // The import path of a test package is the import path of the corresponding
 // non-test package with the suffix "_test" added.
+// Sometimes the first entry will be "command-line-arguments" to indicate
+// an import stack relative to the command-line working directory.
 type ImportStack []string
 
 func (s *ImportStack) Push(p string) {

src/cmd/go/internal/load/test.go

@@ -55,7 +55,18 @@ func TestPackagesFor(p *Package, cover *TestCover) (pmain, ptest, pxtest *Packag
 		}
 		if len(p1.DepsErrors) > 0 {
 			perr := p1.DepsErrors[0]
-			perr.Pos = "" // show full import stack
+			// A typical ImportStack looks like
+			// [ "command-line-arguments (test)", "pkg/with/error" ]
+			// or
+			// [ "command-line-arguments (test)", "parent/pkg", "pkg/with/error" ]
+			// so the package with the error has the highest index. So we want
+			// to check the second highest index to see if we should print the full stack.
+			direct := len(perr.ImportStack) < 2 ||
+				(perr.ImportStack[len(perr.ImportStack)-2] == p1.ImportPath ||
+					strings.HasPrefix(perr.ImportStack[len(perr.ImportStack)-2], "command-line-arguments"))
+			if !direct {
+				perr.Pos = "" // show full import stack
+			}
 			err = perr
 			break
 		}

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

@@ -0,0 +1,19 @@
+env GO111MODULE=off
+
+# Issue 36173. Verify that "go vet" prints line numbers on load errors.
+
+! go vet a/a_test.go
+stderr 'a_test.go:4:3: use of internal package'
+
+-- a/a.go --
+package a
+
+-- a/a_test.go --
+package a
+
+import (
+  _ "a/x/internal/y"
+)
+
+-- a/x/internal/y/y.go --
+package y