Linting: add unused lint

BUILD.bazel

@@ -317,6 +317,8 @@ nogo(
             "//dev/linters/gocritic",
             "//dev/linters/ineffassign",
             "//dev/linters/unparam",
+            # Disabled because we currently have a lot of unused code
+            # "//dev/linters/unused",
         ] + STATIC_CHECK_ANALYZERS,
     }),
 )

dev/linters/unused/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "unused",
+    srcs = ["unused.go"],
+    importpath = "github.com/sourcegraph/sourcegraph/dev/linters/unused",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//dev/linters/nolint",
+        "@co_honnef_go_tools//unused",
+        "@org_golang_x_tools//go/analysis",
+    ],
+)

dev/linters/unused/unused.go

@@ -0,0 +1,31 @@
+package unused
+
+import (
+	"fmt"
+	"go/token"
+	"reflect"
+
+	"github.com/sourcegraph/sourcegraph/dev/linters/nolint"
+	"golang.org/x/tools/go/analysis"
+	"honnef.co/go/tools/unused"
+)
+
+var Analyzer = nolint.Wrap(&analysis.Analyzer{
+	Name: "unused",
+	Doc:  "Unused code",
+	Run: func(pass *analysis.Pass) (interface{}, error) {
+		// This is just a lightweight wrapper around the default unused
+		// analyzer that reports a diagnostic error rather than just returning
+		// a result
+		allUnused := pass.ResultOf[unused.Analyzer.Analyzer].(unused.Result).Unused
+		for _, u := range allUnused {
+			pass.Report(analysis.Diagnostic{
+				Pos:     token.Pos(u.Position.Offset),
+				Message: fmt.Sprintf("%s is unused", u.Name),
+			})
+		}
+		return nil, nil
+	},
+	Requires:   []*analysis.Analyzer{unused.Analyzer.Analyzer},
+	ResultType: reflect.TypeOf(nil),
+})