Unexpected behaviour of go vet -unusedresult

Filed as a Go bug at golang/go#14972

Issue Summary

There are 4 functions in pizza.go and 4 functions in string.go that go vet cannot report as an unused result as of 21cc49bd (March 25, 2016).

As an example, the functions in pizza.go all share the same implementation:

return Pizza{
  toppings: append(pizza.toppings, topping),
}

... although they have four different signatures:

func (pizza Pizza) addToppingMethod(topping string) Pizza
func (pizza Pizza) PublicAddToppingMethod(topping string) Pizza
func addToppingBareFunction(pizza Pizza, topping string) Pizza
func PublicAddToppingBareFunction(pizza Pizza, topping string) Pizza

It is not possible to check for unused results of these functions using go vet with the -unusedresult flag.

Based on the commit that introduced -unusedresult, it seems this was originally designed to catch "pure" functions. However, the documentation does not make this clear at all.

Also:

• It's not obvious which function types -unusedresult can check for, which functions to pass to -unusedfuncs or -unusedstringmethods, and how to format and qualify them.
• It's not possible to get go vet to output the default values for -unusedfuncs and -unusedstringmethods and the documentation is vague ("By default, this includes functions like fmt.Errorf and fmt.Sprintf and methods like String and Error."). In addition, there is no way to use the default values in addition to custom values in a single invocation, which makes it easy to miss out on vetting the default values by accident. The only way to do this seems to be to run go vet multiple times` with different arguments.

Reproduction Steps

# Get the code
go get github.com/lgarron/pizza
cd "$GOPATH/src/github.com/lgarron/pizza"

# Get `go vet`
go get golang.org/x/tools/cmd/vet

# Sanity check that the code compiles and runs.
go test -v

# Run `go vet` with default settings.
go tool vet -unusedresult *_test.go

# Run `go vet` using qualified *and* unqualified versions for each function.
go tool vet -unusedresult -unusedfuncs="makeStringMethod,PublicMakeStringMethod,makeStringBareFunction,PublicMakeStringBareFunction,addToppingMethod,PublicAddToppingMethod,addToppingBareFunction,PublicAddToppingBareFunction,pizza.Pizza.makeStringMethod,pizza.Pizza.PublicMakeStringMethod,pizza.makeStringBareFunction,pizza.PublicMakeStringBareFunction,pizza.Pizza.addToppingMethod,pizza.Pizza.PublicAddToppingMethod,pizza.addToppingBareFunction,pizza.PublicAddToppingBareFunction,errors.New,fmt.Errorf,fmt.Sprintf,fmt.Sprint,sort.Reverse" -unusedstringmethods="makeStringMethod,PublicMakeStringMethod,makeStringBareFunction,PublicMakeStringBareFunction,addToppingMethod,PublicAddToppingMethod,addToppingBareFunction,PublicAddToppingBareFunction,pizza.Pizza.makeStringMethod,pizza.Pizza.PublicMakeStringMethod,pizza.makeStringBareFunction,pizza.PublicMakeStringBareFunction,pizza.Pizza.addToppingMethod,pizza.Pizza.PublicAddToppingMethod,pizza.addToppingBareFunction,pizza.PublicAddToppingBareFunction,Error,String" *_test.go

Debugging using manual traces:

The relevant file int he go vet source is called unused.go. We will use a modified version of unused.go to show us what is happening.

# Check out a known-bad version of the source for `go vet`.
cd "$GOPATH/src/golang.org/x/tools"
git checkout 21cc49bd030cf5c6ebaca2fa0e3323628efed6d8
cd "$GOPATH/src/github.com/lgarron/pizza"

# Overwrite the relevant `go vet` code with an annotated version.
cp modified-vet-src/unused.go "$GOPATH/src/golang.org/x/tools/cmd/vet/unused.go"

# Build a new version of `vet` in the current project directory.
go build -a golang.org/x/tools/cmd/vet

# View output.
./vet -unusedresult *_test.go

Here is the output I get:

unusedFuncsFlag: errors.New,fmt.Errorf,fmt.Sprintf,fmt.Sprint,sort.Reverse
unusedStringMethodsFlag: Error,String

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[checking for unusedFuncs]
REPORT: qualified name `fmt.Sprintf` is in unusedFuncs

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[pkg.uses was not okay]

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[pkg.uses was not okay]

[start]
[checking for conversion]
[checking for (not method + unqualified)]
NO REPORT: not method + unqualified

[start]
[checking for conversion]
[checking for (not method + unqualified)]
NO REPORT: not method + unqualified

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[checking for unusedFuncs]
REPORT: qualified name `fmt.Sprintf` is in unusedFuncs

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[checking for unusedFuncs]
REPORT: qualified name `fmt.Sprintf` is in unusedFuncs

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[pkg.uses was not okay]

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[pkg.uses was not okay]

[start]
[checking for conversion]
[checking for (not method + unqualified)]
NO REPORT: not method + unqualified

[start]
[checking for conversion]
[checking for (not method + unqualified)]
NO REPORT: not method + unqualified

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[checking for unusedFuncs]
REPORT: qualified name `fmt.Sprintf` is in unusedFuncs

[start]
[checking for conversion]
[checking for (not method + unqualified)]
[checking pkg.selectors]
[pkg.selectors not okay]
[checking pkg.uses]
[checking for unusedFuncs]
NO REPORT BUT REPORTABLE: qualified name `fmt.Printf` is not in unusedFuncs

Suggestions

• 1a) Remove some of the restrictions and assumptions in the source to allow reporting these functions as unused, or 1b) explicitly document the limited behaviour.
• 2. Document which functions can be passed in, with clear examples of how to format them.

I tried 1a), but I don't know the code well enough to contribute a fix yet.