test nogo/coverage: test generated code

This add a test that demonstrate the current behavior of nogo today:
code which was generated under "bazel coverage" to capture coverage
information is going through static analysis.

This behavior is not ideal as end users have very little control over
the way rules_go generated these code and would not be able to fix them
to please the analyzer.

In a future patch, we will introduce a mechanism to alleviate this pain
and flip the expectation in this test.

tests/core/nogo/coverage/BUILD.bazel

@@ -4,3 +4,8 @@ go_bazel_test(
     name = "coverage_test",
     srcs = ["coverage_test.go"],
 )
+
+go_bazel_test(
+    name = "gen_code_test",
+    srcs = ["gen_code_test.go"],
+)

tests/core/nogo/coverage/README.rst

@@ -16,3 +16,13 @@ handle libraries like this that do not have serialized facts. Verifies `#1940`_.
 
 Also checks that `nogo`_ itself can be built with coverage enabled.
 Verifies `#2146`_.
+
+gen_code_test
+-------------
+Checks how `nogo`_ would interact with source code that was generated as part of
+rules_go's coverage implementation.  Currently `nogo`_ would be run over these
+generated source code, which is not desirable as end-user have very little control
+over how these code were generated.
+
+In a future version, we shall flip this behavior so that `nogo`_ would not run
+over source files that users have no control over.

tests/core/nogo/coverage/gen_code_test.go

@@ -0,0 +1,142 @@
+// Copyright 2019 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gen_code_test
+
+import (
+	"errors"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/bazelbuild/rules_go/go/tools/bazel_testing"
+)
+
+func TestMain(m *testing.M) {
+	bazel_testing.TestMain(m, bazel_testing.Args{
+		Main: `
+-- BUILD.bazel --
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test", "nogo")
+
+go_library(
+	  name = "simple",
+    srcs = ["simple.go"],
+		importpath = "simple"
+)
+
+go_test(
+    name = "simple_test",
+    srcs = ["simple_test.go"],
+		embed = [":simple"]
+)
+
+nogo(
+    name = "nogo",
+		deps = ["//nocover"],
+    visibility = ["//visibility:public"],
+)
+-- simple.go --
+package simple
+
+func Foo() string {
+	return "foo"
+}
+-- simple_test.go --
+package simple
+
+import "testing"
+
+func TestFoo(t *testing.T) {
+	if actual, expected := Foo(), "foo"; actual != expected {
+		t.Errorf("Foo() should return foo")
+	}
+}
+-- nocover/BUILD.bazel --
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "nocover",
+    srcs = ["analyzer.go"],
+    importpath = "nocover",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@org_golang_x_tools//go/analysis",
+        "@org_golang_x_tools//go/analysis/passes/inspect",
+        "@org_golang_x_tools//go/ast/inspector",
+    ],
+)
+-- nocover/analyzer.go --
+package nocover
+
+import (
+	"fmt"
+	"go/ast"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+)
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "nocover",
+	Doc:      "nocover ensure that source code was not a generated file created by rules_go's coverage implementation",
+	Run:      run,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspector := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+
+	inspector.Preorder([]ast.Node{(*ast.ValueSpec)(nil)}, func(node ast.Node) {
+		valueSpec := node.(*ast.ValueSpec)
+		if len(valueSpec.Names) != 1 {
+			return
+		}
+
+		varName := valueSpec.Names[0].Name
+
+		// check for coverage variable name that matches this pattern: CoverZ%sZ%dZ%s
+		// see go/tools/builders/compilepkg.go -> coverVar for more information
+		if strings.HasPrefix(varName, "CoverZ") && strings.Count(varName, "Z") >= 3 {
+			pass.Report(analysis.Diagnostic{
+				Pos:     valueSpec.Pos(),
+				End:     valueSpec.End(),
+				Message: fmt.Sprintf("variable %s was generated by rules_go", varName),
+			})
+		}
+	})
+
+	return nil, nil
+}
+`,
+		Nogo: `@//:nogo`,
+	})
+}
+
+func TestNogoCoverGenCode(t *testing.T) {
+	out, err := bazel_testing.BazelOutput("coverage", "//:simple_test")
+	if err == nil {
+		t.Fatal("test should fail")
+	}
+
+	var eErr *exec.ExitError
+	if errors.As(err, &eErr) && strings.Contains(string(eErr.Stderr), "was generated by rules_go (nocover)") {
+		// Expected failure
+		return
+	}
+
+	println(string(out))
+	t.Fatal(err)
+}