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nilcheck

About 384 words

Enforces nil checks on pointer parameters before use.

Category

Safety

What It Checks

This analyzer detects pointer parameters that are dereferenced (a field access or method call, *p, or p[i]) where they aren't known to be non-nil. A use is known to be non-nil when it comes:

  • inside an if whose condition rules nil out: if p != nil { ... }, the else of if p == nil { ... }, and && and || combinations of those
  • on the right of p != nil && or p == nil || in the same condition
  • after an if whose condition holds whenever p is nil and whose body returns, panics, exits (os.Exit, log.Fatal, t.Fatal), breaks out of a loop, or assigns p

A nil check after the use, or one whose body only logs and falls through, doesn't count. Interface and type-parameter parameters are not pointers and are not checked.

Why It Matters

Nil pointer dereferences cause panics:

panic: runtime error: invalid memory address or nil pointer dereference

Explicit nil checks provide better error messages and prevent crashes.

Examples

Bad

func ProcessUser(user *User) error {
    // Panics if user is nil!
    return user.Save()
}

Good

func ProcessUser(user *User) error {
    if user == nil {
        return errors.New("user cannot be nil")
    }
    return user.Save()
}

Bad: Multiple Usages

func UpdateProfile(user *User, profile *Profile) error {
    user.Name = profile.Name      // Both could panic!
    user.Email = profile.Email
    return user.Save()
}

Good: Check All Pointers

func UpdateProfile(user *User, profile *Profile) error {
    if user == nil {
        return errors.New("user cannot be nil")
    }
    if profile == nil {
        return errors.New("profile cannot be nil")
    }
    user.Name = profile.Name
    user.Email = profile.Email
    return user.Save()
}

Trusted Types

The analyzer skips certain types that are guaranteed non-nil by their frameworks:

  • *testing.T, *testing.B, *testing.M
  • *gin.Context, *gin.Engine
  • *cobra.Command
  • *http.Request, *http.Response
  • *zap.Logger, *log.Logger, *grpc.ClientConn, *rest.Config and a few more

It also skips parameters with names that are framework-provided by convention, such as ctx, r, w, t, cfg and opts.

Skipped Files

The analyzer skips generated files:

  • *_gen.go
  • *.pb.go
  • zz_generated*.go
  • Files in mock_* directories

Configuration

# .golint-sl.yaml
analyzers:
  nilcheck: true  # enabled by default

When to Disable

This analyzer should rarely be disabled. Nil checks are fundamental safety.

analyzers:
  nilcheck: false  # Not recommended