go/gopls/doc/analyzers.md

Analyzers

This document describes the analyzers that gopls uses inside the editor.

A value of true means that the analyzer is enabled by default and a value of false means it is disabled by default.

Details about how to enable/disable these analyses can be found here.

Go vet suite

Below is the list of general analyzers that are used in go vet.

asmdecl

report mismatches between assembly files and Go declarations

Default value: true.

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 value: true.

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 value: true.

atomicalign

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

Default value: true.

bools

check for common mistakes involving boolean operators

Default value: true.

buildtag

check that +build tags are well-formed and correctly located

Default value: true.

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 value: true.

composite

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 value: true.

copylock

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 value: true.

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 value: true.

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 value: true.

loopclosure

check references to loop variables from within nested functions

This analyzer checks for references to loop variables from within a function literal inside the loop body. It checks only instances where the function literal is called in a defer or go statement that is the last statement in the loop body, as otherwise we would need whole program analysis.

For example:

for i, v := range s {
  go func() {
    println(i, v) // not what you might expect
  }()
}

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

Default value: true.

lostcancel

check cancel func returned by context.WithCancel is called

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

Default value: true.

nilfunc

check for useless comparisons between functions and nil

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

Default value: true.

printf

check consistency of Printf format strings and arguments

The check applies to known functions (for example, those in package fmt) as well as any detected wrappers of known functions.

A function that wants to avail itself of printf checking but is not found by this analyzer's heuristics (for example, due to use of dynamic calls) can insert a bogus call:

if false {
  _ = fmt.Sprintf(format, args...) // enable printf checking
}

The -funcs flag specifies a comma-separated list of names of additional known formatting functions or methods. If the name contains a period, it must denote a specific function using one of the following forms:

	dir/pkg.Function
	dir/pkg.Type.Method
	(*dir/pkg.Type).Method

Otherwise the name is interpreted as a case-insensitive unqualified identifier such as "errorf". Either way, if a listed name ends in f, the function is assumed to be Printf-like, taking a format string before the argument list. Otherwise it is assumed to be Print-like, taking a list of arguments with no format string.

Default value: true.

shift

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

Default value: true.

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 value: true.

structtag

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

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

Default value: true.

tests

check for common mistaken usages of tests and examples

The tests checker walks Test, Benchmark 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 value: true.

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 value: true.

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 value: true.

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 value: true.

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. This analyzer reports calls to certain functions in which the result of the call is ignored.

The set of functions may be controlled using flags.

Default value: true.

gopls suite

Below is the list of analyzers that are used by gopls.

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 value: true.

fillreturns

suggested fixes for "wrong number of return values (want %d, got %d)"

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 goreturns.

Default value: false.

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 value: false.

noresultvalues

suggested fixes for "no result values expected"

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

func z() { return nil }

will turn into

func z() { return }

Default value: true.

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.

Default value: true.

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.

Default value: true.

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.

Default value: true.

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 value: true.

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 value: true.

undeclaredname

suggested fixes for "undeclared name: <>"

This checker provides suggested fixes for type errors of the type undeclared name: <>. It will insert a new statement: <> := .

Default value: false.

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 reduce false positives it ignores:

• methods
• parameters that do not have a name or are underscored
• functions in test files
• functions with empty bodies or those with just a return stmt

Default value: false.