Add staticcheck and fix issues

This adds staticcheck as a linter to Zap and fixes issues found by it.
Specifically, it found a few unused unexported types and functions.

Makefile

@@ -1,6 +1,7 @@
 export GOBIN ?= $(shell pwd)/bin
 
 GOLINT = $(GOBIN)/golint
+STATICCHECK = $(GOBIN)/staticcheck
 BENCH_FLAGS ?= -cpuprofile=cpu.pprof -memprofile=mem.pprof -benchmem
 
 # Directories containing independent Go modules.
@@ -17,14 +18,16 @@ GO_FILES := $(shell \
 all: lint test
 
 .PHONY: lint
-lint: $(GOLINT)
+lint: $(GOLINT) $(STATICCHECK)
 	@rm -rf lint.log
 	@echo "Checking formatting..."
 	@gofmt -d -s $(GO_FILES) 2>&1 | tee lint.log
 	@echo "Checking vet..."
 	@$(foreach dir,$(MODULE_DIRS),(cd $(dir) && go vet ./... 2>&1) &&) true | tee -a lint.log
 	@echo "Checking lint..."
 	@$(foreach dir,$(MODULE_DIRS),(cd $(dir) && $(GOLINT) ./... 2>&1) &&) true | tee -a lint.log
+	@echo "Checking staticcheck..."
+	@$(foreach dir,$(MODULE_DIRS),(cd $(dir) && $(STATICCHECK) ./... 2>&1) &&) true | tee -a lint.log
 	@echo "Checking for unresolved FIXMEs..."
 	@git grep -i fixme | grep -v -e Makefile | tee -a lint.log
 	@echo "Checking for license headers..."
@@ -34,6 +37,9 @@ lint: $(GOLINT)
 $(GOLINT):
 	go install golang.org/x/lint/golint
 
+$(STATICCHECK):
+	go install honnef.co/go/tools/cmd/staticcheck
+
 .PHONY: test
 test:
 	@$(foreach dir,$(MODULE_DIRS),(cd $(dir) && go test -race ./...) &&) true

go.mod

@@ -8,4 +8,5 @@ require (
 	go.uber.org/atomic v1.6.0
 	go.uber.org/multierr v1.5.0
 	golang.org/x/lint v0.0.0-20190930215403-16217165b5de
+	honnef.co/go/tools v0.0.1-2019.2.3
 )

sugar_test.go

@@ -159,10 +159,6 @@ func TestSugarFieldsInvalidPairs(t *testing.T) {
 	})
 }
 
-type stringerF func() string
-
-func (f stringerF) String() string { return f() }
-
 func TestSugarStructuredLogging(t *testing.T) {
 	tests := []struct {
 		msg       string

tools_test.go

@@ -25,4 +25,5 @@ package zap
 import (
 	// Tools we use during development.
 	_ "golang.org/x/lint/golint"
+	_ "honnef.co/go/tools/cmd/staticcheck"
 )

zapcore/encoder_test.go

@@ -67,14 +67,6 @@ func humanEncoderConfig() EncoderConfig {
 	return cfg
 }
 
-func withJSONEncoder(f func(Encoder)) {
-	f(NewJSONEncoder(testEncoderConfig()))
-}
-
-func withConsoleEncoder(f func(Encoder)) {
-	f(NewConsoleEncoder(humanEncoderConfig()))
-}
-
 func capitalNameEncoder(loggerName string, enc PrimitiveArrayEncoder) {
 	enc.AppendString(strings.ToUpper(loggerName))
 }

zapcore/error.go

@@ -66,11 +66,6 @@ type errorGroup interface {
 	Errors() []error
 }
 
-type causer interface {
-	// Provides access to the error that caused this error.
-	Cause() error
-}
-
 // Note that errArry and errArrayElem are very similar to the version
 // implemented in the top-level error.go file. We can't re-use this because
 // that would require exporting errArray as part of the zapcore API.