Error strings should not be capitalized st1005

Explanations for all checks in Staticcheck

SA – staticcheck

The SA category of checks, codenamed staticcheck, contains all checks that are concerned with the correctness of code.

SA1 – Various misuses of the standard library

Checks in this category deal with misuses of the standard library.
This tends to involve incorrect function arguments
or violating other invariants laid out by the standard library’s documentation.

SA1000 — Invalid regular expression

Available since

2017.1

SA1001 — Invalid template

Available since

2017.1

SA1002 — Invalid format in time.Parse

Available since

2017.1

SA1003 — Unsupported argument to functions in encoding/binary

The encoding/binary package can only serialize types with known sizes.
This precludes the use of the int and uint types, as their sizes
differ on different architectures. Furthermore, it doesn’t support
serializing maps, channels, strings, or functions.

Before Go 1.8, bool wasn’t supported, either.

Available since

2017.1

SA1004 — Suspiciously small untyped constant in time.Sleep

The time.Sleep function takes a time.Duration as its only argument.
Durations are expressed in nanoseconds. Thus, calling time.Sleep(1)
will sleep for 1 nanosecond. This is a common source of bugs, as sleep
functions in other languages often accept seconds or milliseconds.

The time package provides constants such as time.Second to express
large durations. These can be combined with arithmetic to express
arbitrary durations, for example 5 * time.Second for 5 seconds.

If you truly meant to sleep for a tiny amount of time, use
n * time.Nanosecond to signal to Staticcheck that you did mean to sleep
for some amount of nanoseconds.

Available since

2017.1

SA1005 — Invalid first argument to exec.Command

os/exec runs programs directly (using variants of the fork and exec
system calls on Unix systems). This shouldn’t be confused with running
a command in a shell. The shell will allow for features such as input
redirection, pipes, and general scripting. The shell is also
responsible for splitting the user’s input into a program name and its
arguments. For example, the equivalent to

would be

exec.Command("ls", "/", "/tmp")

If you want to run a command in a shell, consider using something like
the following – but be aware that not all systems, particularly
Windows, will have a /bin/sh program:

exec.Command("/bin/sh", "-c", "ls | grep Awesome")

Available since

2017.1

SA1006 — Printf with dynamic first argument and no further arguments

Using fmt.Printf with a dynamic first argument can lead to unexpected
output. The first argument is a format string, where certain character
combinations have special meaning. If, for example, a user were to
enter a string such as

and you printed it with

it would lead to the following output:

Interest rate: 5%!(NOVERB).

Similarly, forming the first parameter via string concatenation with
user input should be avoided for the same reason. When printing user
input, either use a variant of fmt.Print, or use the %s Printf verb
and pass the string as an argument.

Available since

2017.1

SA1007 — Invalid URL in net/url.Parse

Available since

2017.1

SA1008 — Non-canonical key in http.Header map

Keys in http.Header maps are canonical, meaning they follow a specific
combination of uppercase and lowercase letters. Methods such as
http.Header.Add and http.Header.Del convert inputs into this canonical
form before manipulating the map.

When manipulating http.Header maps directly, as opposed to using the
provided methods, care should be taken to stick to canonical form in
order to avoid inconsistencies. The following piece of code
demonstrates one such inconsistency:

h := http.Header{}
h["etag"] = []string{"1234"}
h.Add("etag", "5678")
fmt.Println(h)

// Output:
// map[Etag:[5678] etag:[1234]]

The easiest way of obtaining the canonical form of a key is to use
http.CanonicalHeaderKey.

Available since

2017.1

SA1010 — (*regexp.Regexp).FindAll called with n == 0, which will always return zero results

If n >= 0, the function returns at most n matches/submatches. To
return all results, specify a negative number.

Available since

2017.1

SA1011 — Various methods in the strings package expect valid UTF-8, but invalid input is provided

Available since

2017.1

SA1012 — A nil context.Context is being passed to a function, consider using context.TODO instead

Available since

2017.1

SA1013 — io.Seeker.Seek is being called with the whence constant as the first argument, but it should be the second

Available since

2017.1

SA1014 — Non-pointer value passed to Unmarshal or Decode

Available since

2017.1

SA1015 — Using time.Tick in a way that will leak. Consider using time.NewTicker, and only use time.Tick in tests, commands and endless functions

Available since

2017.1

SA1016 — Trapping a signal that cannot be trapped

Not all signals can be intercepted by a process. Specifically, on
UNIX-like systems, the syscall.SIGKILL and syscall.SIGSTOP signals are
never passed to the process, but instead handled directly by the
kernel. It is therefore pointless to try and handle these signals.

Available since

2017.1

SA1017 — Channels used with os/signal.Notify should be buffered

The os/signal package uses non-blocking channel sends when delivering
signals. If the receiving end of the channel isn’t ready and the
channel is either unbuffered or full, the signal will be dropped. To
avoid missing signals, the channel should be buffered and of the
appropriate size. For a channel used for notification of just one
signal value, a buffer of size 1 is sufficient.

Available since

2017.1

SA1018 — strings.Replace called with n == 0, which does nothing

With n == 0, zero instances will be replaced. To replace all
instances, use a negative number, or use strings.ReplaceAll.

Available since

2017.1

SA1019 — Using a deprecated function, variable, constant or field

Available since

2017.1

SA1020 — Using an invalid host:port pair with a net.Listen-related function

Available since

2017.1

SA1021 — Using bytes.Equal to compare two net.IP

A net.IP stores an IPv4 or IPv6 address as a slice of bytes. The
length of the slice for an IPv4 address, however, can be either 4 or
16 bytes long, using different ways of representing IPv4 addresses. In
order to correctly compare two net.IPs, the net.IP.Equal method should
be used, as it takes both representations into account.

Available since

2017.1

SA1023 — Modifying the buffer in an io.Writer implementation

Write must not modify the slice data, even temporarily.

Available since

2017.1

SA1024 — A string cutset contains duplicate characters

The strings.TrimLeft and strings.TrimRight functions take cutsets, not
prefixes. A cutset is treated as a set of characters to remove from a
string. For example,

strings.TrimLeft("42133word", "1234")

will result in the string "word" – any characters that are 1, 2, 3 or
4 are cut from the left of the string.

In order to remove one string from another, use strings.TrimPrefix instead.

Available since

2017.1

SA1025 — It is not possible to use (*time.Timer).Reset’s return value correctly

Available since

2019.1

SA1026 — Cannot marshal channels or functions

Available since

2019.2

SA1027 — Atomic access to 64-bit variable must be 64-bit aligned

On ARM, x86-32, and 32-bit MIPS, it is the caller’s responsibility to
arrange for 64-bit alignment of 64-bit words accessed atomically. The
first word in a variable or in an allocated struct, array, or slice
can be relied upon to be 64-bit aligned.

You can use the structlayout tool to inspect the alignment of fields
in a struct.

Available since

2019.2

SA1028 — sort.Slice can only be used on slices

The first argument of sort.Slice must be a slice.

Available since

2020.1

SA1029 — Inappropriate key in call to context.WithValue

The provided key must be comparable and should not be
of type string or any other built-in type to avoid collisions between
packages using context. Users of WithValue should define their own
types for keys.

To avoid allocating when assigning to an interface{},
context keys often have concrete type struct{}. Alternatively,
exported context key variables’ static type should be a pointer or
interface.

Available since

2020.1

SA1030 — Invalid argument in call to a strconv function

This check validates the format, number base and bit size arguments of
the various parsing and formatting functions in strconv.

Available since

2021.1

SA2 – Concurrency issues

Checks in this category find concurrency bugs.

SA2000 — sync.WaitGroup.Add called inside the goroutine, leading to a race condition

Available since

2017.1

SA2001 — Empty critical section, did you mean to defer the unlock?

Empty critical sections of the kind

are very often a typo, and the following was intended instead:

mu.Lock()
defer mu.Unlock()

Do note that sometimes empty critical sections can be useful, as a
form of signaling to wait on another goroutine. Many times, there are
simpler ways of achieving the same effect. When that isn’t the case,
the code should be amply commented to avoid confusion. Combining such
comments with a //lint:ignore directive can be used to suppress this
rare false positive.

Available since

2017.1

SA2002 — Called testing.T.FailNow or SkipNow in a goroutine, which isn’t allowed

Available since

2017.1

SA2003 — Deferred Lock right after locking, likely meant to defer Unlock instead

Available since

2017.1

SA3 – Testing issues

Checks in this category find issues in tests and benchmarks.

SA3000 — TestMain doesn’t call os.Exit, hiding test failures

Test executables (and in turn go test) exit with a non-zero status
code if any tests failed. When specifying your own TestMain function,
it is your responsibility to arrange for this, by calling os.Exit with
the correct code. The correct code is returned by (*testing.M).Run, so
the usual way of implementing TestMain is to end it with
os.Exit(m.Run()).

Available since

2017.1

SA3001 — Assigning to b.N in benchmarks distorts the results

The testing package dynamically sets b.N to improve the reliability of
benchmarks and uses it in computations to determine the duration of a
single operation. Benchmark code must not alter b.N as this would
falsify results.

Available since

2017.1

SA4 – Code that isn’t really doing anything

Checks in this category point out code that doesn’t have any meaningful effect on a program’s execution.
Usually this means that the programmer thought the code would do one thing while in reality it does something else.

SA4000 — Binary operator has identical expressions on both sides

Available since

2017.1

SA4001 — &*x gets simplified to x, it does not copy x

Available since

2017.1

SA4003 — Comparing unsigned values against negative values is pointless

Available since

2017.1

SA4004 — The loop exits unconditionally after one iteration

Available since

2017.1

SA4005 — Field assignment that will never be observed. Did you mean to use a pointer receiver?

Available since

2021.1

SA4006 — A value assigned to a variable is never read before being overwritten. Forgotten error check or dead code?

Available since

2017.1

SA4008 — The variable in the loop condition never changes, are you incrementing the wrong variable?

Available since

2017.1

SA4009 — A function argument is overwritten before its first use

Available since

2017.1

SA4010 — The result of append will never be observed anywhere

Available since

2017.1

SA4011 — Break statement with no effect. Did you mean to break out of an outer loop?

Available since

2017.1

SA4012 — Comparing a value against NaN even though no value is equal to NaN

Available since

2017.1

SA4013 — Negating a boolean twice (!!b) is the same as writing b. This is either redundant, or a typo.

Available since

2017.1

SA4014 — An if/else if chain has repeated conditions and no side-effects; if the condition didn’t match the first time, it won’t match the second time, either

Available since

2017.1

SA4015 — Calling functions like math.Ceil on floats converted from integers doesn’t do anything useful

Available since

2017.1

SA4016 — Certain bitwise operations, such as x ^ 0, do not do anything useful

Available since

2017.1

SA4017 — Discarding the return values of a function without side effects, making the call pointless

Available since

2017.1

SA4018 — Self-assignment of variables

Available since

2017.1

SA4019 — Multiple, identical build constraints in the same file

Available since

2017.1

SA4020 — Unreachable case clause in a type switch

In a type switch like the following

type T struct{}
func (T) Read(b []byte) (int, error) { return 0, nil }

var v interface{} = T{}

switch v.(type) {
case io.Reader:
    // ...
case T:
    // unreachable
}

the second case clause can never be reached because T implements
io.Reader and case clauses are evaluated in source order.

Another example:

type T struct{}
func (T) Read(b []byte) (int, error) { return 0, nil }
func (T) Close() error { return nil }

var v interface{} = T{}

switch v.(type) {
case io.Reader:
    // ...
case io.ReadCloser:
    // unreachable
}

Even though T has a Close method and thus implements io.ReadCloser,
io.Reader will always match first. The method set of io.Reader is a
subset of io.ReadCloser. Thus it is impossible to match the second
case without matching the first case.

Structurally equivalent interfaces

A special case of the previous example are structurally identical
interfaces. Given these declarations

type T error
type V error

func doSomething() error {
    err, ok := doAnotherThing()
    if ok {
        return T(err)
    }

    return U(err)
}

the following type switch will have an unreachable case clause:

switch doSomething().(type) {
case T:
    // ...
case V:
    // unreachable
}

T will always match before V because they are structurally equivalent
and therefore doSomething()’s return value implements both.

Available since

2019.2

SA4021 — x = append(y) is equivalent to x = y

Available since

2019.2

SA4022 — Comparing the address of a variable against nil

Code such as if &x == nil is meaningless, because taking the address of a variable always yields a non-nil pointer.

Available since

2020.1

SA4023 — Impossible comparison of interface value with untyped nil

Under the covers, interfaces are implemented as two elements, a
type T and a value V. V is a concrete value such as an int,
struct or pointer, never an interface itself, and has type T. For
instance, if we store the int value 3 in an interface, the
resulting interface value has, schematically, (T=int, V=3). The
value V is also known as the interface’s dynamic value, since a
given interface variable might hold different values V (and
corresponding types T) during the execution of the program.

An interface value is nil only if the V and T are both
unset, (T=nil, V is not set), In particular, a nil interface will
always hold a nil type. If we store a nil pointer of type *int
inside an interface value, the inner type will be *int regardless
of the value of the pointer: (T=*int, V=nil). Such an interface
value will therefore be non-nil even when the pointer value V
inside is nil.

This situation can be confusing, and arises when a nil value is
stored inside an interface value such as an error return:

func returnsError() error {
    var p *MyError = nil
    if bad() {
        p = ErrBad
    }
    return p // Will always return a non-nil error.
}

If all goes well, the function returns a nil p, so the return
value is an error interface value holding (T=*MyError, V=nil).
This means that if the caller compares the returned error to nil,
it will always look as if there was an error even if nothing bad
happened. To return a proper nil error to the caller, the
function must return an explicit nil:

func returnsError() error {
    if bad() {
        return ErrBad
    }
    return nil
}

It’s a good idea for functions that return errors always to use
the error type in their signature (as we did above) rather than a
concrete type such as *MyError, to help guarantee the error is
created correctly. As an example, os.Open returns an error even
though, if not nil, it’s always of concrete type *os.PathError.

Similar situations to those described here can arise whenever
interfaces are used. Just keep in mind that if any concrete value
has been stored in the interface, the interface will not be nil.
For more information, see The Laws of
Reflection (https://golang.org/doc/articles/laws_of_reflection.html).

This text has been copied from
https://golang.org/doc/faq#nil_error, licensed under the Creative
Commons Attribution 3.0 License.

Available since

2020.2

SA4024 — Checking for impossible return value from a builtin function

Return values of the len and cap builtins cannot be negative.

See https://golang.org/pkg/builtin/#len and https://golang.org/pkg/builtin/#cap.

Example:

if len(slice) < 0 {
    fmt.Println("unreachable code")
}

Available since

2021.1

SA4025 — Integer division of literals that results in zero

When dividing two integer constants, the result will
also be an integer. Thus, a division such as 2 / 3 results in 0.
This is true for all of the following examples:

_ = 2 / 3
const _ = 2 / 3
const _ float64 = 2 / 3
_ = float64(2 / 3)

Staticcheck will flag such divisions if both sides of the division are
integer literals, as it is highly unlikely that the division was
intended to truncate to zero. Staticcheck will not flag integer
division involving named constants, to avoid noisy positives.

Available since

2021.1

SA4026 — Go constants cannot express negative zero

In IEEE 754 floating point math, zero has a sign and can be positive
or negative. This can be useful in certain numerical code.

Go constants, however, cannot express negative zero. This means that
the literals -0.0 and 0.0 have the same ideal value (zero) and
will both represent positive zero at runtime.

To explicitly and reliably create a negative zero, you can use the
math.Copysign function: math.Copysign(0, -1).

Available since

2021.1

SA4027 — (*net/url.URL).Query returns a copy, modifying it doesn’t change the URL

(*net/url.URL).Query parses the current value of net/url.URL.RawQuery
and returns it as a map of type net/url.Values. Subsequent changes to
this map will not affect the URL unless the map gets encoded and
assigned to the URL’s RawQuery.

As a consequence, the following code pattern is an expensive no-op:
u.Query().Add(key, value).

Available since

2021.1

SA4028 — x % 1 is always zero

Available since

2022.1

SA4029 — Ineffective attempt at sorting slice

sort.Float64Slice, sort.IntSlice, and sort.StringSlice are
types, not functions. Doing x = sort.StringSlice(x) does nothing,
especially not sort any values. The correct usage is
sort.Sort(sort.StringSlice(x)) or sort.StringSlice(x).Sort(),
but there are more convenient helpers, namely sort.Float64s,
sort.Ints, and sort.Strings.

Available since

2022.1

SA4030 — Ineffective attempt at generating random number

Functions in the math/rand package that accept upper limits, such
as Intn, generate random numbers in the half-open interval [0,n). In
other words, the generated numbers will be >= 0 and < n – they
don’t include n. rand.Intn(1) therefore doesn’t generate 0
or 1, it always generates 0.

Available since

2022.1

SA4031 — Checking never-nil value against nil

Available since

2022.1

SA5 – Correctness issues

Checks in this category find assorted bugs and crashes.

SA5000 — Assignment to nil map

Available since

2017.1

SA5001 — Deferring Close before checking for a possible error

Available since

2017.1

SA5002 — The empty for loop (for {}) spins and can block the scheduler

Available since

2017.1

SA5003 — Defers in infinite loops will never execute

Defers are scoped to the surrounding function, not the surrounding
block. In a function that never returns, i.e. one containing an
infinite loop, defers will never execute.

Available since

2017.1

SA5004 — for { select { ... with an empty default branch spins

Available since

2017.1

SA5005 — The finalizer references the finalized object, preventing garbage collection

A finalizer is a function associated with an object that runs when the
garbage collector is ready to collect said object, that is when the
object is no longer referenced by anything.

If the finalizer references the object, however, it will always remain
as the final reference to that object, preventing the garbage
collector from collecting the object. The finalizer will never run,
and the object will never be collected, leading to a memory leak. That
is why the finalizer should instead use its first argument to operate
on the object. That way, the number of references can temporarily go
to zero before the object is being passed to the finalizer.

Available since

2017.1

SA5007 — Infinite recursive call

A function that calls itself recursively needs to have an exit
condition. Otherwise it will recurse forever, until the system runs
out of memory.

This issue can be caused by simple bugs such as forgetting to add an
exit condition. It can also happen “on purpose”. Some languages have
tail call optimization which makes certain infinite recursive calls
safe to use. Go, however, does not implement TCO, and as such a loop
should be used instead.

Available since

2017.1

SA5008 — Invalid struct tag

Available since

2019.2

SA5009 — Invalid Printf call

Available since

2019.2

SA5010 — Impossible type assertion

Some type assertions can be statically proven to be
impossible. This is the case when the method sets of both
arguments of the type assertion conflict with each other, for
example by containing the same method with different
signatures.

The Go compiler already applies this check when asserting from an
interface value to a concrete type. If the concrete type misses
methods from the interface, or if function signatures don’t match,
then the type assertion can never succeed.

This check applies the same logic when asserting from one interface to
another. If both interface types contain the same method but with
different signatures, then the type assertion can never succeed,
either.

Available since

2020.1

SA5011 — Possible nil pointer dereference

A pointer is being dereferenced unconditionally, while
also being checked against nil in another place. This suggests that
the pointer may be nil and dereferencing it may panic. This is
commonly a result of improperly ordered code or missing return
statements. Consider the following examples:

func fn(x *int) {
    fmt.Println(*x)

    // This nil check is equally important for the previous dereference
    if x != nil {
        foo(*x)
    }
}

func TestFoo(t *testing.T) {
    x := compute()
    if x == nil {
        t.Errorf("nil pointer received")
    }

    // t.Errorf does not abort the test, so if x is nil, the next line will panic.
    foo(*x)
}

Staticcheck tries to deduce which functions abort control flow.
For example, it is aware that a function will not continue
execution after a call to panic or log.Fatal. However, sometimes
this detection fails, in particular in the presence of
conditionals. Consider the following example:

func Log(msg string, level int) {
    fmt.Println(msg)
    if level == levelFatal {
        os.Exit(1)
    }
}

func Fatal(msg string) {
    Log(msg, levelFatal)
}

func fn(x *int) {
    if x == nil {
        Fatal("unexpected nil pointer")
    }
    fmt.Println(*x)
}

Staticcheck will flag the dereference of x, even though it is perfectly
safe. Staticcheck is not able to deduce that a call to
Fatal will exit the program. For the time being, the easiest
workaround is to modify the definition of Fatal like so:

func Fatal(msg string) {
    Log(msg, levelFatal)
    panic("unreachable")
}

We also hard-code functions from common logging packages such as
logrus. Please file an issue if we’re missing support for a
popular package.

Available since

2020.1

SA5012 — Passing odd-sized slice to function expecting even size

Some functions that take slices as parameters expect the slices to have an even number of elements.
Often, these functions treat elements in a slice as pairs.
For example, strings.NewReplacer takes pairs of old and new strings,
and calling it with an odd number of elements would be an error.

Available since

2020.2

SA6 – Performance issues

Checks in this category find code that can be trivially made faster.

SA6000 — Using regexp.Match or related in a loop, should use regexp.Compile

Available since

2017.1

SA6001 — Missing an optimization opportunity when indexing maps by byte slices

Map keys must be comparable, which precludes the use of byte slices.
This usually leads to using string keys and converting byte slices to
strings.

Normally, a conversion of a byte slice to a string needs to copy the data and
causes allocations. The compiler, however, recognizes m[string(b)] and
uses the data of b directly, without copying it, because it knows that
the data can’t change during the map lookup. This leads to the
counter-intuitive situation that

k := string(b)
println(m[k])
println(m[k])

will be less efficient than

println(m[string(b)])
println(m[string(b)])

because the first version needs to copy and allocate, while the second
one does not.

For some history on this optimization, check out commit
f5f5a8b6209f84961687d993b93ea0d397f5d5bf in the Go repository.

Available since

2017.1

SA6002 — Storing non-pointer values in sync.Pool allocates memory

A sync.Pool is used to avoid unnecessary allocations and reduce the
amount of work the garbage collector has to do.

When passing a value that is not a pointer to a function that accepts
an interface, the value needs to be placed on the heap, which means an
additional allocation. Slices are a common thing to put in sync.Pools,
and they’re structs with 3 fields (length, capacity, and a pointer to
an array). In order to avoid the extra allocation, one should store a
pointer to the slice instead.

See the comments on https://go-review.googlesource.com/c/go/+/24371
that discuss this problem.

Available since

2017.1

SA6003 — Converting a string to a slice of runes before ranging over it

You may want to loop over the runes in a string. Instead of converting
the string to a slice of runes and looping over that, you can loop
over the string itself. That is,

and

for _, r := range []rune(s) {}

will yield the same values. The first version, however, will be faster
and avoid unnecessary memory allocations.

Do note that if you are interested in the indices, ranging over a
string and over a slice of runes will yield different indices. The
first one yields byte offsets, while the second one yields indices in
the slice of runes.

Available since

2017.1

SA6005 — Inefficient string comparison with strings.ToLower or strings.ToUpper

Converting two strings to the same case and comparing them like so

if strings.ToLower(s1) == strings.ToLower(s2) {
    ...
}

is significantly more expensive than comparing them with
strings.EqualFold(s1, s2). This is due to memory usage as well as
computational complexity.

strings.ToLower will have to allocate memory for the new strings, as
well as convert both strings fully, even if they differ on the very
first byte. strings.EqualFold, on the other hand, compares the strings
one character at a time. It doesn’t need to create two intermediate
strings and can return as soon as the first non-matching character has
been found.

For a more in-depth explanation of this issue, see
https://blog.digitalocean.com/how-to-efficiently-compare-strings-in-go/

Available since

2019.2

SA9 – Dubious code constructs that have a high probability of being wrong

Checks in this category find code that is probably wrong.
Unlike checks in the other SA categories,
checks in SA9 have a slight chance of reporting false positives.
However, even false positives will point at code that is confusing and that should probably be refactored.

SA9001 — Defers in range loops may not run when you expect them to

Available since

2017.1

SA9002 — Using a non-octal os.FileMode that looks like it was meant to be in octal.

Available since

2017.1

SA9003 — Empty body in an if or else branch

Available since

2017.1

SA9004 — Only the first constant has an explicit type

In a constant declaration such as the following:

const (
    First byte = 1
    Second     = 2
)

the constant Second does not have the same type as the constant First.
This construct shouldn’t be confused with

const (
    First byte = iota
    Second
)

where First and Second do indeed have the same type. The type is only
passed on when no explicit value is assigned to the constant.

When declaring enumerations with explicit values it is therefore
important not to write

const (
      EnumFirst EnumType = 1
      EnumSecond         = 2
      EnumThird          = 3
)

This discrepancy in types can cause various confusing behaviors and
bugs.

Wrong type in variable declarations

The most obvious issue with such incorrect enumerations expresses
itself as a compile error:

package pkg

const (
    EnumFirst  uint8 = 1
    EnumSecond       = 2
)

func fn(useFirst bool) {
    x := EnumSecond
    if useFirst {
        x = EnumFirst
    }
}

fails to compile with

./const.go:11:5: cannot use EnumFirst (type uint8) as type int in assignment

Losing method sets

A more subtle issue occurs with types that have methods and optional
interfaces. Consider the following:

package main

import "fmt"

type Enum int

func (e Enum) String() string {
    return "an enum"
}

const (
    EnumFirst  Enum = 1
    EnumSecond      = 2
)

func main() {
    fmt.Println(EnumFirst)
    fmt.Println(EnumSecond)
}

This code will output

as EnumSecond has no explicit type, and thus defaults to int.

Available since

2019.1

SA9005 — Trying to marshal a struct with no public fields nor custom marshaling

The encoding/json and encoding/xml packages only operate on exported
fields in structs, not unexported ones. It is usually an error to try
to (un)marshal structs that only consist of unexported fields.

This check will not flag calls involving types that define custom
marshaling behavior, e.g. via MarshalJSON methods. It will also not
flag empty structs.

Available since

2019.2

SA9006 — Dubious bit shifting of a fixed size integer value

Bit shifting a value past its size will always clear the value.

For instance:

will always result in 0.

This check flags bit shifting operations on fixed size integer values only.
That is, int, uint and uintptr are never flagged to avoid potential false
positives in somewhat exotic but valid bit twiddling tricks:

// Clear any value above 32 bits if integers are more than 32 bits.
func f(i int) int {
    v := i >> 32
    v = v << 32
    return i-v
}

Available since

2020.2

SA9007 — Deleting a directory that shouldn’t be deleted

It is virtually never correct to delete system directories such as
/tmp or the user’s home directory. However, it can be fairly easy to
do by mistake, for example by mistakingly using os.TempDir instead
of ioutil.TempDir, or by forgetting to add a suffix to the result
of os.UserHomeDir.

Writing

d := os.TempDir()
defer os.RemoveAll(d)

in your unit tests will have a devastating effect on the stability of your system.

This check flags attempts at deleting the following directories:

• os.TempDir
• os.UserCacheDir
• os.UserConfigDir
• os.UserHomeDir

Available since

2022.1

SA9008 — else branch of a type assertion is probably not reading the right value

When declaring variables as part of an if statement (like in if foo := ...; foo {), the same variables will also be in the scope of
the else branch. This means that in the following example

if x, ok := x.(int); ok {
    // ...
} else {
    fmt.Printf("unexpected type %T", x)
}

x in the else branch will refer to the x from x, ok :=; it will not refer to the x that is being type-asserted. The
result of a failed type assertion is the zero value of the type that
is being asserted to, so x in the else branch will always have the
value 0 and the type int.

Available since

2022.1

S – simple

The S category of checks, codenamed simple, contains all checks that are concerned with simplifying code.

S1 – Code simplifications

Checks in this category find code that is unnecessarily complex and that can be trivially simplified.

S1000 — Use plain channel send or receive instead of single-case select

Select statements with a single case can be replaced with a simple
send or receive.

Before:

select {
case x := <-ch:
    fmt.Println(x)
}

After:

Available since

2017.1

S1001 — Replace for loop with call to copy

Use copy() for copying elements from one slice to another. For
arrays of identical size, you can use simple assignment.

Before:

for i, x := range src {
    dst[i] = x
}

After:

Available since

2017.1

S1002 — Omit comparison with boolean constant

Before:

After:

Available since

2017.1

S1003 — Replace call to strings.Index with strings.Contains

Before:

if strings.Index(x, y) != -1 {}

After:

if strings.Contains(x, y) {}

Available since

2017.1

S1004 — Replace call to bytes.Compare with bytes.Equal

Before:

if bytes.Compare(x, y) == 0 {}

After:

Available since

2017.1

S1005 — Drop unnecessary use of the blank identifier

In many cases, assigning to the blank identifier is unnecessary.

Before:

for _ = range s {}
x, _ = someMap[key]
_ = <-ch

After:

for range s{}
x = someMap[key]
<-ch

Available since

2017.1

S1006 — Use for { ... } for infinite loops

For infinite loops, using for { ... } is the most idiomatic choice.

Available since

2017.1

S1007 — Simplify regular expression by using raw string literal

Raw string literals use backticks instead of quotation marks and do not support
any escape sequences. This means that the backslash can be used
freely, without the need of escaping.

Since regular expressions have their own escape sequences, raw strings
can improve their readability.

Before:

regexp.Compile("\A(\w+) profile: total \d+\n\z")

After:

regexp.Compile(`A(w+) profile: total d+nz`)

Available since

2017.1

S1008 — Simplify returning boolean expression

Before:

if <expr> {
    return true
}
return false

After:

Available since

2017.1

S1009 — Omit redundant nil check on slices

The len function is defined for all slices, even nil ones, which have
a length of zero. It is not necessary to check if a slice is not nil
before checking that its length is not zero.

Before:

if x != nil && len(x) != 0 {}

After:

Available since

2017.1

S1010 — Omit default slice index

When slicing, the second index defaults to the length of the value,
making s[n:len(s)] and s[n:] equivalent.

Available since

2017.1

S1011 — Use a single append to concatenate two slices

Before:

for _, e := range y {
    x = append(x, e)
}

for i := range y {
    x = append(x, y[i])
}

for i := range y {
    v := y[i]
    x = append(x, v)
}

After:

x = append(x, y...)
x = append(x, y...)
x = append(x, y...)

Available since

2017.1

S1012 — Replace time.Now().Sub(x) with time.Since(x)

The time.Since helper has the same effect as using time.Now().Sub(x)
but is easier to read.

Before:

After:

Available since

2017.1

S1016 — Use a type conversion instead of manually copying struct fields

Two struct types with identical fields can be converted between each
other. In older versions of Go, the fields had to have identical
struct tags. Since Go 1.8, however, struct tags are ignored during
conversions. It is thus not necessary to manually copy every field
individually.

Before:

var x T1
y := T2{
    Field1: x.Field1,
    Field2: x.Field2,
}

After:

Available since

2017.1

S1017 — Replace manual trimming with strings.TrimPrefix

Instead of using strings.HasPrefix and manual slicing, use the
strings.TrimPrefix function. If the string doesn’t start with the
prefix, the original string will be returned. Using strings.TrimPrefix
reduces complexity, and avoids common bugs, such as off-by-one
mistakes.

Before:

if strings.HasPrefix(str, prefix) {
    str = str[len(prefix):]
}

After:

str = strings.TrimPrefix(str, prefix)

Available since

2017.1

S1018 — Use copy for sliding elements

copy() permits using the same source and destination slice, even with
overlapping ranges. This makes it ideal for sliding elements in a
slice.

Before:

for i := 0; i < n; i++ {
    bs[i] = bs[offset+i]
}

After:

copy(bs[:n], bs[offset:])

Available since

2017.1

S1019 — Simplify make call by omitting redundant arguments

The make function has default values for the length and capacity
arguments. For channels, the length defaults to zero, and for slices,
the capacity defaults to the length.

Available since

2017.1

S1020 — Omit redundant nil check in type assertion

Before:

if _, ok := i.(T); ok && i != nil {}

After:

Available since

2017.1

S1021 — Merge variable declaration and assignment

Before:

After:

Available since

2017.1

S1023 — Omit redundant control flow

Functions that have no return value do not need a return statement as
the final statement of the function.

Switches in Go do not have automatic fallthrough, unlike languages
like C. It is not necessary to have a break statement as the final
statement in a case block.

Available since

2017.1

S1024 — Replace x.Sub(time.Now()) with time.Until(x)

The time.Until helper has the same effect as using x.Sub(time.Now())
but is easier to read.

Before:

After:

Available since

2017.1

S1025 — Don’t use fmt.Sprintf("%s", x) unnecessarily

In many instances, there are easier and more efficient ways of getting
a value’s string representation. Whenever a value’s underlying type is
a string already, or the type has a String method, they should be used
directly.

Given the following shared definitions

type T1 string
type T2 int

func (T2) String() string { return "Hello, world" }

var x string
var y T1
var z T2

we can simplify

fmt.Sprintf("%s", x)
fmt.Sprintf("%s", y)
fmt.Sprintf("%s", z)

to

Available since

2017.1

S1028 — Simplify error construction with fmt.Errorf

Before:

errors.New(fmt.Sprintf(...))

After:

Available since

2017.1

S1029 — Range over the string directly

Ranging over a string will yield byte offsets and runes. If the offset
isn’t used, this is functionally equivalent to converting the string
to a slice of runes and ranging over that. Ranging directly over the
string will be more performant, however, as it avoids allocating a new
slice, the size of which depends on the length of the string.

Before:

for _, r := range []rune(s) {}

After:

Available since

2017.1

S1030 — Use bytes.Buffer.String or bytes.Buffer.Bytes

bytes.Buffer has both a String and a Bytes method. It is almost never
necessary to use string(buf.Bytes()) or []byte(buf.String()) – simply
use the other method.

The only exception to this are map lookups. Due to a compiler optimization,
m[string(buf.Bytes())] is more efficient than m[buf.String()].

Available since

2017.1

S1031 — Omit redundant nil check around loop

You can use range on nil slices and maps, the loop will simply never
execute. This makes an additional nil check around the loop
unnecessary.

Before:

if s != nil {
    for _, x := range s {
        ...
    }
}

After:

for _, x := range s {
    ...
}

Available since

2017.1

S1032 — Use sort.Ints(x), sort.Float64s(x), and sort.Strings(x)

The sort.Ints, sort.Float64s and sort.Strings functions are easier to
read than sort.Sort(sort.IntSlice(x)), sort.Sort(sort.Float64Slice(x))
and sort.Sort(sort.StringSlice(x)).

Before:

sort.Sort(sort.StringSlice(x))

After:

Available since

2019.1

S1033 — Unnecessary guard around call to delete

Calling delete on a nil map is a no-op.

Available since

2019.2

S1034 — Use result of type assertion to simplify cases

Available since

2019.2

S1035 — Redundant call to net/http.CanonicalHeaderKey in method call on net/http.Header

The methods on net/http.Header, namely Add, Del, Get
and Set, already canonicalize the given header name.

Available since

2020.1

S1036 — Unnecessary guard around map access

When accessing a map key that doesn’t exist yet, one receives a zero
value. Often, the zero value is a suitable value, for example when
using append or doing integer math.

The following

if _, ok := m["foo"]; ok {
    m["foo"] = append(m["foo"], "bar")
} else {
    m["foo"] = []string{"bar"}
}

can be simplified to

m["foo"] = append(m["foo"], "bar")

and

if _, ok := m2["k"]; ok {
    m2["k"] += 4
} else {
    m2["k"] = 4
}

can be simplified to

Available since

2020.1

S1037 — Elaborate way of sleeping

Using a select statement with a single case receiving
from the result of time.After is a very elaborate way of sleeping that
can much simpler be expressed with a simple call to time.Sleep.

Available since

2020.1

S1038 — Unnecessarily complex way of printing formatted string

Instead of using fmt.Print(fmt.Sprintf(...)), one can use fmt.Printf(...).

Available since

2020.1

S1039 — Unnecessary use of fmt.Sprint

Calling fmt.Sprint with a single string argument is unnecessary
and identical to using the string directly.

Available since

2020.1

S1040 — Type assertion to current type

The type assertion x.(SomeInterface), when x already has type
SomeInterface, can only fail if x is nil. Usually, this is
left-over code from when x had a different type and you can safely
delete the type assertion. If you want to check that x is not nil,
consider being explicit and using an actual if x == nil comparison
instead of relying on the type assertion panicking.

Available since

2021.1

ST – stylecheck

The ST category of checks, codenamed stylecheck, contains all checks that are concerned with stylistic issues.

ST1 – Stylistic issues

The rules contained in this category are primarily derived from the Go wiki and represent community consensus.

Some checks are very pedantic and disabled by default.
You may want to tweak which checks from this category run, based on your project’s needs.

ST1000 — Incorrect or missing package comment non-default

Packages must have a package comment that is formatted according to
the guidelines laid out in
https://github.com/golang/go/wiki/CodeReviewComments#package-comments.

Available since

2019.1

ST1001 — Dot imports are discouraged

Dot imports that aren’t in external test packages are discouraged.

The dot_import_whitelist option can be used to whitelist certain
imports.

Quoting Go Code Review Comments:

The import . form can be useful in tests that, due to circular
dependencies, cannot be made part of the package being tested:

package foo_test

import (
    "bar/testutil" // also imports "foo"
    . "foo"
)

In this case, the test file cannot be in package foo because it
uses bar/testutil, which imports foo. So we use the import .
form to let the file pretend to be part of package foo even though
it is not. Except for this one case, do not use import . in your
programs. It makes the programs much harder to read because it is
unclear whether a name like Quux is a top-level identifier in the
current package or in an imported package.

Available since

2019.1

Options

• dot_import_whitelist

ST1003 — Poorly chosen identifier non-default

Identifiers, such as variable and package names, follow certain rules.

See the following links for details:

• https://golang.org/doc/effective_go.html#package-names
• https://golang.org/doc/effective_go.html#mixed-caps
• https://github.com/golang/go/wiki/CodeReviewComments#initialisms
• https://github.com/golang/go/wiki/CodeReviewComments#variable-names

Available since

2019.1

Options

• initialisms

ST1005 — Incorrectly formatted error string

Error strings follow a set of guidelines to ensure uniformity and good
composability.

Quoting Go Code Review Comments:

Error strings should not be capitalized (unless beginning with
proper nouns or acronyms) or end with punctuation, since they are
usually printed following other context. That is, use
fmt.Errorf("something bad") not fmt.Errorf("Something bad"), so
that log.Printf("Reading %s: %v", filename, err) formats without a
spurious capital letter mid-message.

Available since

2019.1

ST1006 — Poorly chosen receiver name

Quoting Go Code Review Comments:

The name of a method’s receiver should be a reflection of its
identity; often a one or two letter abbreviation of its type
suffices (such as “c” or “cl” for “Client”). Don’t use generic
names such as “me”, “this” or “self”, identifiers typical of
object-oriented languages that place more emphasis on methods as
opposed to functions. The name need not be as descriptive as that
of a method argument, as its role is obvious and serves no
documentary purpose. It can be very short as it will appear on
almost every line of every method of the type; familiarity admits
brevity. Be consistent, too: if you call the receiver “c” in one
method, don’t call it “cl” in another.

Available since

2019.1

ST1008 — A function’s error value should be its last return value

A function’s error value should be its last return value.

Available since

2019.1

ST1011 — Poorly chosen name for variable of type time.Duration

time.Duration values represent an amount of time, which is represented
as a count of nanoseconds. An expression like 5 * time.Microsecond
yields the value 5000. It is therefore not appropriate to suffix a
variable of type time.Duration with any time unit, such as Msec or
Milli.

Available since

2019.1

ST1012 — Poorly chosen name for error variable

Error variables that are part of an API should be called errFoo or
ErrFoo.

Available since

2019.1

ST1013 — Should use constants for HTTP error codes, not magic numbers

HTTP has a tremendous number of status codes. While some of those are
well known (200, 400, 404, 500), most of them are not. The net/http
package provides constants for all status codes that are part of the
various specifications. It is recommended to use these constants
instead of hard-coding magic numbers, to vastly improve the
readability of your code.

Available since

2019.1

Options

• http_status_code_whitelist

ST1015 — A switch’s default case should be the first or last case

Available since

2019.1

ST1016 — Use consistent method receiver names non-default

Available since

2019.1

ST1017 — Don’t use Yoda conditions

Yoda conditions are conditions of the kind if 42 == x, where the
literal is on the left side of the comparison. These are a common
idiom in languages in which assignment is an expression, to avoid bugs
of the kind if (x = 42). In Go, which doesn’t allow for this kind of
bug, we prefer the more idiomatic if x == 42.

Available since

2019.2

ST1018 — Avoid zero-width and control characters in string literals

Available since

2019.2

ST1019 — Importing the same package multiple times

Go allows importing the same package multiple times, as long as
different import aliases are being used. That is, the following
bit of code is valid:

import (
    "fmt"
    fumpt "fmt"
    format "fmt"
    _ "fmt"
)

However, this is very rarely done on purpose. Usually, it is a
sign of code that got refactored, accidentally adding duplicate
import statements. It is also a rarely known feature, which may
contribute to confusion.

Do note that sometimes, this feature may be used
intentionally (see for example
https://github.com/golang/go/commit/3409ce39bfd7584523b7a8c150a310cea92d879d)
– if you want to allow this pattern in your code base, you’re
advised to disable this check.

Available since

2020.1

ST1020 — The documentation of an exported function should start with the function’s name non-default

Doc comments work best as complete sentences, which
allow a wide variety of automated presentations. The first sentence
should be a one-sentence summary that starts with the name being
declared.

If every doc comment begins with the name of the item it describes,
you can use the doc subcommand of the go tool and run the output
through grep.

See https://golang.org/doc/effective_go.html#commentary for more
information on how to write good documentation.

Available since

2020.1

ST1021 — The documentation of an exported type should start with type’s name non-default

Doc comments work best as complete sentences, which
allow a wide variety of automated presentations. The first sentence
should be a one-sentence summary that starts with the name being
declared.

If every doc comment begins with the name of the item it describes,
you can use the doc subcommand of the go tool and run the output
through grep.

See https://golang.org/doc/effective_go.html#commentary for more
information on how to write good documentation.

Available since

2020.1

ST1022 — The documentation of an exported variable or constant should start with variable’s name non-default

Doc comments work best as complete sentences, which
allow a wide variety of automated presentations. The first sentence
should be a one-sentence summary that starts with the name being
declared.

If every doc comment begins with the name of the item it describes,
you can use the doc subcommand of the go tool and run the output
through grep.

See https://golang.org/doc/effective_go.html#commentary for more
information on how to write good documentation.

Available since

2020.1

ST1023 — Redundant type in variable declaration non-default

Available since

2021.1

QF – quickfix

The QF category of checks, codenamed quickfix, contains checks that are used as part of gopls for automatic refactorings.
In the context of gopls, diagnostics of these checks will usually show up as hints, sometimes as information-level diagnostics.

QF1 – Quickfixes

QF1001 — Apply De Morgan’s law

Available since

2021.1

QF1002 — Convert untagged switch to tagged switch

An untagged switch that compares a single variable against a series of
values can be replaced with a tagged switch.

Before:

switch {
case x == 1 || x == 2, x == 3:
    ...
case x == 4:
    ...
default:
    ...
}

After:

switch x {
case 1, 2, 3:
    ...
case 4:
    ...
default:
    ...
}

Available since

2021.1

QF1003 — Convert if/else-if chain to tagged switch

A series of if/else-if checks comparing the same variable against
values can be replaced with a tagged switch.

Before:

if x == 1 || x == 2 {
    ...
} else if x == 3 {
    ...
} else {
    ...
}

After:

switch x {
case 1, 2:
    ...
case 3:
    ...
default:
    ...
}

Available since

2021.1

QF1004 — Use strings.ReplaceAll instead of strings.Replace with n == -1

Available since

2021.1

QF1005 — Expand call to math.Pow

Some uses of math.Pow can be simplified to basic multiplication.

Before:

After:

Available since

2021.1

QF1006 — Lift if+break into loop condition

Before:

for {
    if done {
        break
    }
    ...
}

After:

Available since

2021.1

QF1007 — Merge conditional assignment into variable declaration

Before:

x := false
if someCondition {
    x = true
}

After:

Available since

2021.1

QF1008 — Omit embedded fields from selector expression

Available since

2021.1

QF1009 — Use time.Time.Equal instead of == operator

Available since

2021.1

QF1010 — Convert slice of bytes to string when printing it

Available since

2021.1

QF1011 — Omit redundant type from variable declaration

Available since

2021.1

QF1012 — Use fmt.Fprintf(x, ...) instead of x.Write(fmt.Sprintf(...))

Available since

2022.1