This document describes the analyzers that gopls uses inside the editor.
Details about how to enable/disable these analyses can be found here.
report mismatches between assembly files and Go declarations
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
check for non-64-bits-aligned arguments to sync/atomic functions
Enabled by default.
check for common mistakes involving boolean operators
Enabled by default.
check that +build tags are well-formed and correctly located
Enabled by default.
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.
Enabled by default.
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}
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
find structs that would take less memory if their fields were sorted
This analyzer find structs that can be rearranged to take less memory, and provides a suggested edit with the optimal order.
Disabled by default. Enable it by setting "analyses": {"fieldalignment": true}.
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.
Enabled by default.
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.
Enabled by default.
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
Enabled by default.
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.)
Enabled by default.
check for useless comparisons between functions and nil
A useless comparison is one like f == nil as opposed to f() == nil.
Enabled by default.
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)
}
Disabled by default. Enable it by setting "analyses": {"nilness": true}.
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.
Enabled by default.
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
}
Disabled by default. Enable it by setting "analyses": {"shadow": true}.
check for shifts that equal or exceed the width of the integer
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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
Enabled by default.
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.
Enabled by default.
check that struct field tags conform to reflect.StructTag.Get
Also report certain struct tags (json, xml) used with unexported fields.
Enabled by default.
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 }() }
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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.
Enabled by default.
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:
Disabled by default. Enable it by setting "analyses": {"unusedparams": true}.
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.
Enabled by default.
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
}
Disabled by default. Enable it by setting "analyses": {"unusedwrite": true}.
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 https://github.com/sqs/goreturns.
Enabled by default.
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
Enabled by default.
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 }
Enabled by default.
suggested fixes for “undeclared name: <>”
This checker provides suggested fixes for type errors of the type “undeclared name: <>”. It will insert a new statement: "<> := ".
Enabled by default.
note incomplete struct initializations
This analyzer provides diagnostics for any struct literals that do not have any fields initialized. Because the suggested fix for this analysis is expensive to compute, callers should compute it separately, using the SuggestedFix function below.
Enabled by default.