SA1019: account for all combinations of test packages using each other

A single Go package results in three packages when testing: the package
under test with its internal tests, a package for external tests, and a
package main that runs the tests.

We need to allow external tests to import the package under test, and
the test's main to import both the package under test as well as its
external tests, and to use the objects defined in them.

Closes gh-1285

staticcheck/lint.go

@@ -3166,6 +3166,8 @@ func CheckDeprecated(pass *analysis.Pass) (interface{}, error) {
 		if fn, ok := node.(*ast.FuncDecl); ok {
 			tfn = pass.TypesInfo.ObjectOf(fn.Name)
 		}
+
+		// FIXME(dh): this misses dot-imported objects
 		sel, ok := node.(*ast.SelectorExpr)
 		if !ok {
 			return true
@@ -3178,8 +3180,26 @@ func CheckDeprecated(pass *analysis.Pass) (interface{}, error) {
 		if obj.Pkg() == nil {
 			return true
 		}
-		if pass.Pkg == obj.Pkg() || obj.Pkg().Path()+"_test" == pass.Pkg.Path() {
-			// Don't flag stuff in our own package
+
+		if obj.Pkg() == pass.Pkg {
+			// A package is allowed to use its own deprecated objects
+			return true
+		}
+
+		// A package "foo" has two related packages "foo_test" and "foo.test", for external tests and the package main
+		// generated by 'go test' respectively. "foo_test" can import and use "foo", "foo.test" imports and uses "foo"
+		// and "foo_test".
+
+		if strings.TrimSuffix(pass.Pkg.Path(), "_test") == obj.Pkg().Path() {
+			// foo_test (the external tests of foo) can use objects from foo.
+			return true
+		}
+		if strings.TrimSuffix(pass.Pkg.Path(), ".test") == obj.Pkg().Path() {
+			// foo.test (the main package of foo's tests) can use objects from foo.
+			return true
+		}
+		if strings.TrimSuffix(pass.Pkg.Path(), ".test") == strings.TrimSuffix(obj.Pkg().Path(), "_test") {
+			// foo.test (the main package of foo's tests) can use objects from foo's external tests.
 			return true
 		}
 
@@ -3208,6 +3228,19 @@ func CheckDeprecated(pass *analysis.Pass) (interface{}, error) {
 				}
 			}
 
+			if strings.TrimSuffix(pass.Pkg.Path(), "_test") == path {
+				// foo_test can import foo
+				return
+			}
+			if strings.TrimSuffix(pass.Pkg.Path(), ".test") == path {
+				// foo.test can import foo
+				return
+			}
+			if strings.TrimSuffix(pass.Pkg.Path(), ".test") == strings.TrimSuffix(path, "_test") {
+				// foo.test can import foo_test
+				return
+			}
+
 			handleDeprecation(depr, spec.Path, path, path, nil)
 		}
 	}

staticcheck/testdata/src/example.com/CheckDeprecated/CheckDeprecated.go

@@ -1,3 +1,4 @@
+// Deprecated: this package is deprecated.
 package pkg
 
 import _ "example.com/CheckDeprecated.assist"          //@ diag(`Alas, it is deprecated.`)
@@ -8,4 +9,14 @@ import "example.com/AnotherCheckDeprecated.assist"     //@ diag(`Alas, it is dep
 func init() {
 	foo.Fn()
 	AnotherCheckDeprecatedassist.Fn()
+
+	// Field is deprecated, but we're using it from the same package, which is fine.
+	var s S
+	_ = s.Field
+}
+
+
+type S struct {
+	 // Deprecated: this is deprecated.
+	 Field int
 }

staticcheck/testdata/src/example.com/CheckDeprecated/CheckDeprecated_test.go

@@ -0,0 +1,18 @@
+package pkg
+
+import "testing"
+
+// Deprecated: deprecating tests is silly but possible.
+func TestFoo(t *testing.T) {
+	var s S
+	// Internal tests can use deprecated objects from the package they test.
+	_ = s.Field
+}
+
+// This test isn't deprecated, to test that s.Field doesn't get flagged because it's from the package under test. If
+// TestBar was itself deprecated, it could use any deprecated objects it wanted.
+func TestBar(t *testing.T) {
+	var s S
+	// Internal tests can use deprecated objects from the package under test
+	_ = s.Field
+}

staticcheck/testdata/src/example.com/CheckDeprecated/external_test.go

@@ -0,0 +1,21 @@
+// Deprecated: deprecating external test packages is silly but possible.
+package pkg_test
+
+import (
+	"testing"
+
+	// External tests can import deprecated packages under test.
+	pkg "example.com/CheckDeprecated"
+)
+
+// Deprecated: deprecating tests is silly but possible.
+func TestFoo(t *testing.T) {
+}
+
+// This test isn't deprecated, to test that s.Field doesn't get flagged because it's from the package under test. If
+// TestBar was itself deprecated, it could use any deprecated objects it wanted.
+func TestBar(t *testing.T) {
+	var s pkg.S
+	// External tests can use deprecated objects from the package under test
+	_ = s.Field
+}