staticcheck.io - Go static analysis

Checks

Check	Short description
S1???	Code simplifications
S1000	Use plain channel send or receive
S1001	Replace with `copy()`
S1002	Omit comparison with boolean constant
S1003	Replace with `strings.Contains`
S1004	Replace with `bytes.Equal`
S1005	Drop unnecessary use of the blank identifier
S1006	Replace with `for { ... }`
S1007	Simplify regular expression by using raw string literal
S1008	Simplify returning boolean expression
S1009	Omit redundant nil check on slices
S1010	Omit default slice index
S1011	Use a single append to concatenate two slices
S1012	Replace with `time.Since(x)`
S1016	Use a type conversion
S1017	Replace with `strings.TrimPrefix`
S1018	Replace with `copy()`
S1019	Simplify `make` call
S1020	Omit redundant nil check in type assertion
S1021	Merge variable declaration and assignment
S1023	Omit redundant control flow
S1024	Replace with `time.Until(x)`
S1025	Don't use `fmt.Sprintf("%s", x)` unnecessarily
S1028	Replace with `fmt.Errorf`
S1029	Range over the string
S1030	Use `bytes.Buffer.String` or `bytes.Buffer.Bytes`
S1031	Omit redundant nil check around loop
S1032	Replace with `sort.Ints(x)`, `sort.Float64s(x)`, `sort.Strings(x)`
SA1???	Various misuses of the standard library
SA1000	Invalid regular expression
SA1001	Invalid template
SA1002	Invalid format in `time.Parse`
SA1003	Unsupported argument to functions in `encoding/binary`
SA1004	Suspiciously small untyped constant in `time.Sleep`
SA1005	Invalid first argument to `exec.Command`
SA1006	Printf with dynamic first argument and no further arguments
SA1007	Invalid URL in `net/url.Parse`
SA1008	Non-canonical key in `http.Header` map
SA1010	`(*regexp.Regexp).FindAll` called with `n == 0`, which will always return zero results
SA1011	Various methods in the `strings` package expect valid UTF-8, but invalid input is provided
SA1012	A nil `context.Context` is being passed to a function, consider using `context.TODO` instead
SA1013	`io.Seeker.Seek` is being called with the `whence` constant as the first argument, but it should be the second
SA1014	Non-pointer value passed to `Unmarshal` or `Decode`
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
SA1016	Trapping a signal that cannot be trapped
SA1017	Channels used with `signal.Notify` should be buffered
SA1018	`strings.Replace` called with `n == 0`, which does nothing
SA1019	Using a deprecated function, variable, constant or field
SA1020	Using an invalid `host:port` pair with a `net.Listen`-related function
SA1021	Using `bytes.Equal` to compare two `net.IP`
SA1023	Modifying the buffer in an `io.Writer` implementation
SA1024	A string cutset contains duplicate characters, suggesting `TrimPrefix` or `TrimSuffix` should be used instead of `TrimLeft` or `TrimRight`
SA1025	it is not possible to use Reset's return value correctly
SA2???	Concurrency issues
SA2000	`sync.WaitGroup.Add` called inside the goroutine, leading to a race condition
SA2001	Empty critical section, did you mean to `defer` the unlock?
SA2002	Called `testing.T.FailNow` or `SkipNow` in a goroutine, which isn't allowed
SA2003	Deferred `Lock` right after locking, likely meant to defer `Unlock` instead
SA3???	Testing issues
SA3000	`TestMain` doesn't call `os.Exit`, hiding test failures
SA3001	Assigning to `b.N` in benchmarks distorts the results
SA4???	Code that isn't really doing anything
SA4000	Boolean expression has identical expressions on both sides
SA4001	`&*x` gets simplified to `x`, it does not copy `x`
SA4002	Comparing strings with known different sizes has predictable results
SA4003	Comparing unsigned values against negative values is pointless
SA4004	The loop exits unconditionally after one iteration
SA4005	Field assignment that will never be observed. Did you mean to use a pointer receiver?
SA4006	A value assigned to a variable is never read before being overwritten. Forgotten error check or dead code?
SA4008	The variable in the loop condition never changes, are you incrementing the wrong variable?
SA4009	A function argument is overwritten before its first use
SA4010	The result of `append` will never be observed anywhere
SA4011	Break statement with no effect. Did you mean to break out of an outer loop?
SA4012	Comparing a value against NaN even though no value is equal to NaN
SA4013	Negating a boolean twice (`!!b`) is the same as writing `b`. This is either redundant, or a typo.
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
SA4015	Calling functions like `math.Ceil` on floats converted from integers doesn't do anything useful
SA4016	Certain bitwise operations, such as `x ^ 0`, do not do anything useful
SA4017	A pure function's return value is discarded, making the call pointless
SA4018	Self-assignment of variables
SA4019	Multiple, identical build constraints in the same file
SA4020	Unreachable case clause in a type switch
SA5???	Correctness issues
SA5000	Assignment to nil map
SA5001	Defering `Close` before checking for a possible error
SA5002	The empty `for` loop (`for {}`) spins and can block the scheduler
SA5003	Defers in infinite loops will never execute
SA5004	`for { select { ...` with an empty default branch spins
SA5005	The finalizer references the finalized object, preventing garbage collection
SA5006	Slice index out of bounds
SA5007	Infinite recursive call
SA6???	Performance issues
SA6000	Using `regexp.Match` or related in a loop, should use `regexp.Compile`
SA6001	Missing an optimization opportunity when indexing maps by byte slices
SA6002	Storing non-pointer values in `sync.Pool` allocates memory
SA6003	Converting a string to a slice of runes before ranging over it
SA6005	Inefficient string comparison with `strings.ToLower` or `strings.ToUpper`
SA9???	Dubious code constructs that have a high probability of being wrong
SA9001	`defer`s in `for range` loops may not run when you expect them to
SA9002	Using a non-octal `os.FileMode` that looks like it was meant to be in octal.
SA9003	Empty body in an if or else branch
SA9004	Only the first constant has an explicit type
ST1???
ST1000	Incorrect or missing package comment
ST1001	Dot imports are discouraged
ST1003	Poorly chosen identifier
ST1005	Incorrectly formatted error string
ST1006	Poorly chosen receiver name
ST1008	A function's error value should be its last return value
ST1011	Poorly chosen name for variable of type time.Duration
ST1012	Poorly chosen name for error variable
ST1013	Should use constants for HTTP error codes, not magic numbers
ST1015	A switch's default case should be the first or last case
ST1016	Use consistent method receiver names
U1???	Unused code
U1000	Unused code

S1000 – Use plain channel send or receive

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

Before:

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

After:

x := <-ch
fmt.Println(x)

Available since: 2017.1

S1001 – Replace with `copy()`

Use copy() for copying elements from one slice to another.

Before:

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

After:

copy(dst, src)

Available since: 2017.1

S1002 – Omit comparison with boolean constant

Before:

if x == true {}

After:

if x {}

Available since: 2017.1

S1003 – Replace with `strings.Contains`

Before:

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

After:

if strings.Contains(x, y) {}

Available since: 2017.1

S1004 – Replace with `bytes.Equal`

Before:

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

After:

if bytes.Equal(x, y) {}

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 – Replace with `for { ... }`

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 ` instead of " 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+\n\z`)

Available since: 2017.1

S1008 – Simplify returning boolean expression

Before:

if <expr> {
  return true
}
return false

After:

return <expr>

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:

if len(x) != 0 {}

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)
}

After:

x = append(x, y...)

Available since: 2017.1

S1012 – Replace with `time.Since(x)`

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

Before:

time.Now().Sub(x)

After:

time.Since(x)

Available since: 2017.1

S1016 – Use a type conversion

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:

var x T1
y := T2(x)

Available since: 2017.1

S1017 – Replace 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 – Replace with `copy()`

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

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

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

Available since: 2017.1

S1021 – Merge variable declaration and assignment

Before:

var x uint
x = 1

After:

var x uint = 1

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 with `time.Until(x)`

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

Before:

x.Sub(time.Now())

After:

time.Until(x)

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 the following

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

x
string(y)
z.String()

Available since: 2017.1

S1028 – Replace with `fmt.Errorf`

Before:

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

After:

fmt.Errorf(...)

Available since: 2017.1

S1029 – Range over the string

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:

for _, r := range s {}

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 never necessary to use string(buf.Bytes()) or []byte(buf.String()) – simply use the other method.

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 – Replace with `sort.Ints(x)`, `sort.Float64s(x)`, `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:

sort.Strings(x)

Available since: 2019.1

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`

Available since: 2017.1

SA1004 – Suspiciously small untyped constant in `time.Sleep`

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 ls / /tmp 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 Interest rate: 5% and you printed it with fmt.Printf(s), 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

Available since: 2017.1

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

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

Available since: 2017.1

SA1017 – Channels used with `signal.Notify` should be buffered

Available since: 2017.1

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

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

Available since: 2017.1

SA1024 – A string cutset contains duplicate characters, suggesting `TrimPrefix` or `TrimSuffix` should be used instead of `TrimLeft` or `TrimRight`

Available since: 2017.1

SA1025 – it is not possible to use Reset's return value correctly

Available since: 2019.1

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?

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

SA3000 – `TestMain` doesn't call `os.Exit`, hiding test failures

Available since: 2017.1

SA3001 – Assigning to `b.N` in benchmarks distorts the results

Available since: 2017.1

SA4000 – Boolean expression 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

SA4002 – Comparing strings with known different sizes has predictable results

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: 2017.1

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 – A pure function's return value is discarded, 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 mtching 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: Unreleased

SA5000 – Assignment to nil map

Available since: 2017.1

SA5001 – Defering `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

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

SA5006 – Slice index out of bounds

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 adding 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

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. This usually leads to using string keys and converting []bytes to strings.

Normally, a conversion of []byte to 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.

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 a Go CL 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,

for _, r := range s {}

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 How to Efficiently Compare Strings in Go.

Available since: Unreleased

SA9001 – `defer`s in `for 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

an enum
2

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

Available since: 2019.1

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.

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.

– Go Code Review Comments

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:

Available since: 2019.1
Options: initialisms

ST1005 – Incorrectly formatted error string

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

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.
– Go Code Review Comments

Available since: 2019.1

ST1006 – Poorly chosen receiver name

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.
– Go Code Review Comments

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

U1000 – Unused code

Available since: 2017.1

Documentation

Checks

S1000 – Use plain channel send or receive

S1001 – Replace with copy()

S1002 – Omit comparison with boolean constant

S1003 – Replace with strings.Contains

S1004 – Replace with bytes.Equal

S1005 – Drop unnecessary use of the blank identifier

S1006 – Replace with for { ... }

S1007 – Simplify regular expression by using raw string literal

S1008 – Simplify returning boolean expression

S1009 – Omit redundant nil check on slices

S1010 – Omit default slice index

S1011 – Use a single append to concatenate two slices

S1012 – Replace with time.Since(x)

S1016 – Use a type conversion

S1017 – Replace with strings.TrimPrefix

S1018 – Replace with copy()

S1019 – Simplify make call

S1020 – Omit redundant nil check in type assertion

S1021 – Merge variable declaration and assignment

S1023 – Omit redundant control flow

S1024 – Replace with time.Until(x)

S1025 – Don't use fmt.Sprintf("%s", x) unnecessarily

S1028 – Replace with fmt.Errorf

S1029 – Range over the string

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

S1031 – Omit redundant nil check around loop

S1032 – Replace with sort.Ints(x), sort.Float64s(x), sort.Strings(x)

SA1000 – Invalid regular expression

SA1001 – Invalid template

SA1002 – Invalid format in time.Parse

SA1003 – Unsupported argument to functions in encoding/binary

SA1004 – Suspiciously small untyped constant in time.Sleep

SA1005 – Invalid first argument to exec.Command

SA1006 – Printf with dynamic first argument and no further arguments

SA1007 – Invalid URL in net/url.Parse

SA1008 – Non-canonical key in http.Header map

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

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

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

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

SA1014 – Non-pointer value passed to Unmarshal or Decode

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

SA1016 – Trapping a signal that cannot be trapped

SA1017 – Channels used with signal.Notify should be buffered

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

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

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

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

SA1023 – Modifying the buffer in an io.Writer implementation

SA1024 – A string cutset contains duplicate characters, suggesting TrimPrefix or TrimSuffix should be used instead of TrimLeft or TrimRight

SA1025 – it is not possible to use Reset's return value correctly

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

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

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

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

SA3000 – TestMain doesn't call os.Exit, hiding test failures

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

SA4000 – Boolean expression has identical expressions on both sides

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

SA4002 – Comparing strings with known different sizes has predictable results

SA4003 – Comparing unsigned values against negative values is pointless

SA4004 – The loop exits unconditionally after one iteration

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

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

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

SA4009 – A function argument is overwritten before its first use

SA4010 – The result of append will never be observed anywhere

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

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

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

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

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

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

SA4017 – A pure function's return value is discarded, making the call pointless

SA4018 – Self-assignment of variables

SA4019 – Multiple, identical build constraints in the same file

SA4020 – Unreachable case clause in a type switch

Structurally equivalent interfaces

S1001 – Replace with `copy()`

S1003 – Replace with `strings.Contains`

S1004 – Replace with `bytes.Equal`

S1006 – Replace with `for { ... }`

S1012 – Replace with `time.Since(x)`

S1017 – Replace with `strings.TrimPrefix`

S1018 – Replace with `copy()`

S1019 – Simplify `make` call

S1024 – Replace with `time.Until(x)`

S1025 – Don't use `fmt.Sprintf("%s", x)` unnecessarily

S1028 – Replace with `fmt.Errorf`

S1030 – Use `bytes.Buffer.String` or `bytes.Buffer.Bytes`

S1032 – Replace with `sort.Ints(x)`, `sort.Float64s(x)`, `sort.Strings(x)`

SA1002 – Invalid format in `time.Parse`

SA1003 – Unsupported argument to functions in `encoding/binary`

SA1004 – Suspiciously small untyped constant in `time.Sleep`

SA1005 – Invalid first argument to `exec.Command`

SA1007 – Invalid URL in `net/url.Parse`

SA1008 – Non-canonical key in `http.Header` map

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

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

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

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

SA1014 – Non-pointer value passed to `Unmarshal` or `Decode`

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

SA1017 – Channels used with `signal.Notify` should be buffered

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

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

SA1021 – Using `bytes.Equal` to compare two `net.IP`

SA1023 – Modifying the buffer in an `io.Writer` implementation

SA1024 – A string cutset contains duplicate characters, suggesting `TrimPrefix` or `TrimSuffix` should be used instead of `TrimLeft` or `TrimRight`

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

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

SA2002 – Called `testing.T.FailNow` or `SkipNow` in a goroutine, which isn't allowed

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

SA3000 – `TestMain` doesn't call `os.Exit`, hiding test failures

SA3001 – Assigning to `b.N` in benchmarks distorts the results

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

SA4010 – The result of `append` will never be observed anywhere

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

SA4015 – Calling functions like `math.Ceil` on floats converted from integers doesn't do anything useful

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

SA5001 – Defering `Close` before checking for a possible error

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

SA5004 – `for { select { ...` with an empty default branch spins

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

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

SA6005 – Inefficient string comparison with `strings.ToLower` or `strings.ToUpper`

SA9001 – `defer`s in `for range` loops may not run when you expect them to

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