analyzers.md

Gopls: Analyzers

Gopls contains a driver for pluggable, modular static analyzers, such as those used by go vet.

Most analyzers report mistakes in your code; some suggest "quick fixes" that can be directly applied in your editor. Every time you edit your code, gopls re-runs its analyzers. Analyzer diagnostics help you detect bugs sooner, before you run your tests, or even before you save your files.

This document describes the suite of analyzers available in gopls, which aggregates analyzers from a variety of sources:

• all the usual bug-finding analyzers from the go vet suite (e.g. printf; see go tool vet help for the complete list);
• a number of analyzers with more substantial dependencies that prevent them from being used in go vet (e.g. nilness);
• analyzers that augment compilation errors by suggesting quick fixes to common mistakes (e.g. fillreturns); and
• a handful of analyzers that suggest possible style improvements (e.g. simplifyrange).

To enable or disable analyzers, use the analyses setting.

In addition, gopls includes the staticcheck suite, though these analyzers are off by default. Use the staticcheck setting to enable them, and consult staticcheck's documentation for analyzer details.

appends: check for missing values after append

This checker reports calls to append that pass no values to be appended to the slice.

s := []string{"a", "b", "c"}
_ = append(s)

Such calls are always no-ops and often indicate an underlying mistake.

Default: on.

Package documentation: appends

asmdecl: report mismatches between assembly files and Go declarations

Default: on.

Package documentation: asmdecl

assign: check for useless assignments

This checker reports assignments of the form x = x or a[i] = a[i]. These are almost always useless, and even when they aren't they are usually a mistake.

Default: on.

Package documentation: assign

atomic: check for common mistakes using the sync/atomic package

The atomic checker looks for assignment statements of the form:

x = atomic.AddUint64(&x, 1)

which are not atomic.

Default: on.

Package documentation: atomic

atomicalign: check for non-64-bits-aligned arguments to sync/atomic functions

Default: on.

Package documentation: atomicalign

bools: check for common mistakes involving boolean operators

Default: on.

Package documentation: bools

buildtag: check //go:build and // +build directives

Default: on.

Package documentation: buildtag

cgocall: detect some violations of the cgo pointer passing rules

Check for invalid cgo pointer passing. This looks for code that uses cgo to call C code passing values whose types are almost always invalid according to the cgo pointer sharing rules. Specifically, it warns about attempts to pass a Go chan, map, func, or slice to C, either directly, or via a pointer, array, or struct.

Default: on.

Package documentation: cgocall

composites: check for unkeyed composite literals

This analyzer reports a diagnostic for composite literals of struct types imported from another package that do not use the field-keyed syntax. Such literals are fragile because the addition of a new field (even if unexported) to the struct will cause compilation to fail.

As an example,

err = &net.DNSConfigError{err}

should be replaced by:

err = &net.DNSConfigError{Err: err}

Default: on.

Package documentation: composites

copylocks: check for locks erroneously passed by value

Inadvertently copying a value containing a lock, such as sync.Mutex or sync.WaitGroup, may cause both copies to malfunction. Generally such values should be referred to through a pointer.

Default: on.

Package documentation: copylocks

deepequalerrors: check for calls of reflect.DeepEqual on error values

The deepequalerrors checker looks for calls of the form:

reflect.DeepEqual(err1, err2)

where err1 and err2 are errors. Using reflect.DeepEqual to compare errors is discouraged.

Default: on.

Package documentation: deepequalerrors

defers: report common mistakes in defer statements

The defers analyzer reports a diagnostic when a defer statement would result in a non-deferred call to time.Since, as experience has shown that this is nearly always a mistake.

For example:

start := time.Now()
...
defer recordLatency(time.Since(start)) // error: call to time.Since is not deferred

The correct code is:

defer func() { recordLatency(time.Since(start)) }()

Default: on.

Package documentation: defers

deprecated: check for use of deprecated identifiers

The deprecated analyzer looks for deprecated symbols and package imports.

See https://go.dev/wiki/Deprecated to learn about Go's convention for documenting and signaling deprecated identifiers.

Default: on.

Package documentation: deprecated

directive: check Go toolchain directives such as //go:debug

This analyzer checks for problems with known Go toolchain directives in all Go source files in a package directory, even those excluded by //go:build constraints, and all non-Go source files too.

For //go:debug (see https://go.dev/doc/godebug), the analyzer checks that the directives are placed only in Go source files, only above the package comment, and only in package main or *_test.go files.

Support for other known directives may be added in the future.

This analyzer does not check //go:build, which is handled by the buildtag analyzer.

Default: on.

Package documentation: directive

embed: check //go:embed directive usage

This analyzer checks that the embed package is imported if //go:embed directives are present, providing a suggested fix to add the import if it is missing.

This analyzer also checks that //go:embed directives precede the declaration of a single variable.

Default: on.

Package documentation: embed

errorsas: report passing non-pointer or non-error values to errors.As

The errorsas analysis reports calls to errors.As where the type of the second argument is not a pointer to a type implementing error.

Default: on.

Package documentation: errorsas

fillreturns: suggest fixes for errors due to an incorrect number of return values

This checker provides suggested fixes for type errors of the type "wrong number of return values (want %d, got %d)". For example:

func m() (int, string, *bool, error) {
	return
}

will turn into

func m() (int, string, *bool, error) {
	return 0, "", nil, nil
}

This functionality is similar to https://github.com/sqs/goreturns.

Default: on.

Package documentation: fillreturns

framepointer: report assembly that clobbers the frame pointer before saving it

Default: on.

Package documentation: framepointer

httpresponse: check for mistakes using HTTP responses

A common mistake when using the net/http package is to defer a function call to close the http.Response Body before checking the error that determines whether the response is valid:

resp, err := http.Head(url)
defer resp.Body.Close()
if err != nil {
	log.Fatal(err)
}
// (defer statement belongs here)

This checker helps uncover latent nil dereference bugs by reporting a diagnostic for such mistakes.

Default: on.

Package documentation: httpresponse

ifaceassert: detect impossible interface-to-interface type assertions

This checker flags type assertions v.(T) and corresponding type-switch cases in which the static type V of v is an interface that cannot possibly implement the target interface T. This occurs when V and T contain methods with the same name but different signatures. Example:

var v interface {
	Read()
}
_ = v.(io.Reader)

The Read method in v has a different signature than the Read method in io.Reader, so this assertion cannot succeed.

Default: on.

Package documentation: ifaceassert

infertypeargs: check for unnecessary type arguments in call expressions

Explicit type arguments may be omitted from call expressions if they can be inferred from function arguments, or from other type arguments:

func f[T any](T) {}

func _() {
	f[string]("foo") // string could be inferred
}

Default: on.

Package documentation: infertypeargs

loopclosure: check references to loop variables from within nested functions

This analyzer reports places where a function literal references the iteration variable of an enclosing loop, and the loop calls the function in such a way (e.g. with go or defer) that it may outlive the loop iteration and possibly observe the wrong value of the variable.

Note: An iteration variable can only outlive a loop iteration in Go versions <=1.21. In Go 1.22 and later, the loop variable lifetimes changed to create a new iteration variable per loop iteration. (See go.dev/issue/60078.)

In this example, all the deferred functions run after the loop has completed, so all observe the final value of v [<go1.22].

for _, v := range list {
    defer func() {
        use(v) // incorrect
    }()
}

One fix is to create a new variable for each iteration of the loop:

for _, v := range list {
    v := v // new var per iteration
    defer func() {
        use(v) // ok
    }()
}

After Go version 1.22, the previous two for loops are equivalent and both are correct.

The next example uses a go statement and has a similar problem [<go1.22]. In addition, it has a data race because the loop updates v concurrent with the goroutines accessing it.

for _, v := range elem {
    go func() {
        use(v)  // incorrect, and a data race
    }()
}

A fix is the same as before. The checker also reports problems in goroutines started by golang.org/x/sync/errgroup.Group. A hard-to-spot variant of this form is common in parallel tests:

func Test(t *testing.T) {
    for _, test := range tests {
        t.Run(test.name, func(t *testing.T) {
            t.Parallel()
            use(test) // incorrect, and a data race
        })
    }
}

The t.Parallel() call causes the rest of the function to execute concurrent with the loop [<go1.22].

The analyzer reports references only in the last statement, as it is not deep enough to understand the effects of subsequent statements that might render the reference benign. ("Last statement" is defined recursively in compound statements such as if, switch, and select.)

See: https://golang.org/doc/go_faq.html#closures_and_goroutines

Default: on.

Package documentation: loopclosure

lostcancel: check cancel func returned by context.WithCancel is called

The cancellation function returned by context.WithCancel, WithTimeout, WithDeadline and variants such as WithCancelCause must be called, or the new context will remain live until its parent context is cancelled. (The background context is never cancelled.)

Default: on.

Package documentation: lostcancel

nilfunc: check for useless comparisons between functions and nil

A useless comparison is one like f == nil as opposed to f() == nil.

Default: on.

Package documentation: nilfunc

nilness: check for redundant or impossible nil comparisons

The nilness checker inspects the control-flow graph of each function in a package and reports nil pointer dereferences, degenerate nil pointers, and panics with nil values. A degenerate comparison is of the form x==nil or x!=nil where x is statically known to be nil or non-nil. These are often a mistake, especially in control flow related to errors. Panics with nil values are checked because they are not detectable by

if r := recover(); r != nil {

This check reports conditions such as:

if f == nil { // impossible condition (f is a function)
}

and:

p := &v
...
if p != nil { // tautological condition
}

and:

if p == nil {
	print(*p) // nil dereference
}

and:

if p == nil {
	panic(p)
}

Sometimes the control flow may be quite complex, making bugs hard to spot. In the example below, the err.Error expression is guaranteed to panic because, after the first return, err must be nil. The intervening loop is just a distraction.

...
err := g.Wait()
if err != nil {
	return err
}
partialSuccess := false
for _, err := range errs {
	if err == nil {
		partialSuccess = true
		break
	}
}
if partialSuccess {
	reportStatus(StatusMessage{
		Code:   code.ERROR,
		Detail: err.Error(), // "nil dereference in dynamic method call"
	})
	return nil
}

...

Default: on.

Package documentation: nilness

nonewvars: suggested fixes for "no new vars on left side of :="

This checker provides suggested fixes for type errors of the type "no new vars on left side of :=". For example:

z := 1
z := 2

will turn into

z := 1
z = 2

Default: on.

Package documentation: nonewvars

noresultvalues: suggested fixes for unexpected return values

This checker provides suggested fixes for type errors of the type "no result values expected" or "too many return values". For example:

func z() { return nil }

will turn into

func z() { return }

Default: on.

Package documentation: noresultvalues

printf: check consistency of Printf format strings and arguments

The check applies to calls of the formatting functions such as [fmt.Printf] and [fmt.Sprintf], as well as any detected wrappers of those functions such as [log.Printf]. It reports a variety of mistakes such as syntax errors in the format string and mismatches (of number and type) between the verbs and their arguments.

See the documentation of the fmt package for the complete set of format operators and their operand types.

Default: on.

Package documentation: printf

shadow: check for possible unintended shadowing of variables

This analyzer check for shadowed variables. A shadowed variable is a variable declared in an inner scope with the same name and type as a variable in an outer scope, and where the outer variable is mentioned after the inner one is declared.

(This definition can be refined; the module generates too many false positives and is not yet enabled by default.)

For example:

func BadRead(f *os.File, buf []byte) error {
	var err error
	for {
		n, err := f.Read(buf) // shadows the function variable 'err'
		if err != nil {
			break // causes return of wrong value
		}
		foo(buf)
	}
	return err
}

Default: off. Enable by setting "analyses": {"shadow": true}.

Package documentation: shadow

shift: check for shifts that equal or exceed the width of the integer

Default: on.

Package documentation: shift

sigchanyzer: check for unbuffered channel of os.Signal

This checker reports call expression of the form

signal.Notify(c <-chan os.Signal, sig ...os.Signal),

where c is an unbuffered channel, which can be at risk of missing the signal.

Default: on.

Package documentation: sigchanyzer

simplifycompositelit: check for composite literal simplifications

An array, slice, or map composite literal of the form:

[]T{T{}, T{}}

will be simplified to:

[]T{{}, {}}

This is one of the simplifications that "gofmt -s" applies.

This analyzer ignores generated code.

Default: on.

Package documentation: simplifycompositelit

simplifyrange: check for range statement simplifications

A range of the form:

for x, _ = range v {...}

will be simplified to:

for x = range v {...}

A range of the form:

for _ = range v {...}

will be simplified to:

for range v {...}

This is one of the simplifications that "gofmt -s" applies.

This analyzer ignores generated code.

Default: on.

Package documentation: simplifyrange

simplifyslice: check for slice simplifications

A slice expression of the form:

s[a:len(s)]

will be simplified to:

s[a:]

This is one of the simplifications that "gofmt -s" applies.

This analyzer ignores generated code.

Default: on.

Package documentation: simplifyslice

slog: check for invalid structured logging calls

The slog checker looks for calls to functions from the log/slog package that take alternating key-value pairs. It reports calls where an argument in a key position is neither a string nor a slog.Attr, and where a final key is missing its value. For example,it would report

slog.Warn("message", 11, "k") // slog.Warn arg "11" should be a string or a slog.Attr

and

slog.Info("message", "k1", v1, "k2") // call to slog.Info missing a final value

Default: on.

Package documentation: slog

sortslice: check the argument type of sort.Slice

sort.Slice requires an argument of a slice type. Check that the interface{} value passed to sort.Slice is actually a slice.

Default: on.

Package documentation: sortslice

stdmethods: check signature of methods of well-known interfaces

Sometimes a type may be intended to satisfy an interface but may fail to do so because of a mistake in its method signature. For example, the result of this WriteTo method should be (int64, error), not error, to satisfy io.WriterTo:

type myWriterTo struct{...}
func (myWriterTo) WriteTo(w io.Writer) error { ... }

This check ensures that each method whose name matches one of several well-known interface methods from the standard library has the correct signature for that interface.

Checked method names include:

Format GobEncode GobDecode MarshalJSON MarshalXML
Peek ReadByte ReadFrom ReadRune Scan Seek
UnmarshalJSON UnreadByte UnreadRune WriteByte
WriteTo

Default: on.

Package documentation: stdmethods

stdversion: report uses of too-new standard library symbols

The stdversion analyzer reports references to symbols in the standard library that were introduced by a Go release higher than the one in force in the referring file. (Recall that the file's Go version is defined by the 'go' directive its module's go.mod file, or by a "//go:build go1.X" build tag at the top of the file.)

The analyzer does not report a diagnostic for a reference to a "too new" field or method of a type that is itself "too new", as this may have false positives, for example if fields or methods are accessed through a type alias that is guarded by a Go version constraint.

Default: on.

Package documentation: stdversion

stringintconv: check for string(int) conversions

This checker flags conversions of the form string(x) where x is an integer (but not byte or rune) type. Such conversions are discouraged because they return the UTF-8 representation of the Unicode code point x, and not a decimal string representation of x as one might expect. Furthermore, if x denotes an invalid code point, the conversion cannot be statically rejected.

For conversions that intend on using the code point, consider replacing them with string(rune(x)). Otherwise, strconv.Itoa and its equivalents return the string representation of the value in the desired base.

Default: on.

Package documentation: stringintconv

structtag: check that struct field tags conform to reflect.StructTag.Get

Also report certain struct tags (json, xml) used with unexported fields.

Default: on.

Package documentation: structtag

testinggoroutine: report calls to (*testing.T).Fatal from goroutines started by a test

Functions that abruptly terminate a test, such as the Fatal, Fatalf, FailNow, and Skip{,f,Now} methods of *testing.T, must be called from the test goroutine itself. This checker detects calls to these functions that occur within a goroutine started by the test. For example:

func TestFoo(t *testing.T) {
    go func() {
        t.Fatal("oops") // error: (*T).Fatal called from non-test goroutine
    }()
}

Default: on.

Package documentation: testinggoroutine

tests: check for common mistaken usages of tests and examples

The tests checker walks Test, Benchmark, Fuzzing and Example functions checking malformed names, wrong signatures and examples documenting non-existent identifiers.

Please see the documentation for package testing in golang.org/pkg/testing for the conventions that are enforced for Tests, Benchmarks, and Examples.

Default: on.

Package documentation: tests

timeformat: check for calls of (time.Time).Format or time.Parse with 2006-02-01

The timeformat checker looks for time formats with the 2006-02-01 (yyyy-dd-mm) format. Internationally, "yyyy-dd-mm" does not occur in common calendar date standards, and so it is more likely that 2006-01-02 (yyyy-mm-dd) was intended.

Default: on.

Package documentation: timeformat

undeclaredname: suggested fixes for "undeclared name: <>"

This checker provides suggested fixes for type errors of the type "undeclared name: <>". It will either insert a new statement, such as:

<> :=

or a new function declaration, such as:

func <>(inferred parameters) {
	panic("implement me!")
}

Default: on.

Package documentation: undeclaredname

unmarshal: report passing non-pointer or non-interface values to unmarshal

The unmarshal analysis reports calls to functions such as json.Unmarshal in which the argument type is not a pointer or an interface.

Default: on.

Package documentation: unmarshal

unreachable: check for unreachable code

The unreachable analyzer finds statements that execution can never reach because they are preceded by an return statement, a call to panic, an infinite loop, or similar constructs.

Default: on.

Package documentation: unreachable

unsafeptr: check for invalid conversions of uintptr to unsafe.Pointer

The unsafeptr analyzer reports likely incorrect uses of unsafe.Pointer to convert integers to pointers. A conversion from uintptr to unsafe.Pointer is invalid if it implies that there is a uintptr-typed word in memory that holds a pointer value, because that word will be invisible to stack copying and to the garbage collector.

Default: on.

Package documentation: unsafeptr

unusedparams: check for unused parameters of functions

The unusedparams analyzer checks functions to see if there are any parameters that are not being used.

To ensure soundness, it ignores:

• "address-taken" functions, that is, functions that are used as a value rather than being called directly; their signatures may be required to conform to a func type.
• exported functions or methods, since they may be address-taken in another package.
• unexported methods whose name matches an interface method declared in the same package, since the method's signature may be required to conform to the interface type.
• functions with empty bodies, or containing just a call to panic.
• parameters that are unnamed, or named "_", the blank identifier.

The analyzer suggests a fix of replacing the parameter name by "_", but in such cases a deeper fix can be obtained by invoking the "Refactor: remove unused parameter" code action, which will eliminate the parameter entirely, along with all corresponding arguments at call sites, while taking care to preserve any side effects in the argument expressions; see https://github.com/golang/tools/releases/tag/gopls%2Fv0.14.

Default: on.

Package documentation: unusedparams

unusedresult: check for unused results of calls to some functions

Some functions like fmt.Errorf return a result and have no side effects, so it is always a mistake to discard the result. Other functions may return an error that must not be ignored, or a cleanup operation that must be called. This analyzer reports calls to functions like these when the result of the call is ignored.

The set of functions may be controlled using flags.

Default: on.

Package documentation: unusedresult

unusedvariable: check for unused variables and suggest fixes

Default: off. Enable by setting "analyses": {"unusedvariable": true}.

Package documentation: unusedvariable

unusedwrite: checks for unused writes

The analyzer reports instances of writes to struct fields and arrays that are never read. Specifically, when a struct object or an array is copied, its elements are copied implicitly by the compiler, and any element write to this copy does nothing with the original object.

For example:

type T struct { x int }

func f(input []T) {
	for i, v := range input {  // v is a copy
		v.x = i  // unused write to field x
	}
}

Another example is about non-pointer receiver:

type T struct { x int }

func (t T) f() {  // t is a copy
	t.x = i  // unused write to field x
}

Default: on.

Package documentation: unusedwrite

useany: check for constraints that could be simplified to "any"

Default: off. Enable by setting "analyses": {"useany": true}.

Package documentation: useany