sloglint

A Go linter that ensures consistent code style when using log/slog.

Install

sloglint is part of golangci-lint, and this is the recommended way to use it.

# .golangci.yaml
linters:
  enable:
    - sloglint

Alternatively, you can download a prebuilt binary from the Releases page to use sloglint standalone.

Supported checks

For log/slog functions:

• No global logger
• Context only
• Discard handler

For log messages:

• Static message
• Message style

For log arguments:

• No mixed arguments
• Key-value pairs only
• Attributes only
• Arguments on separate lines

For log keys:

• Constant keys
• Allowed keys
• Forbidden keys
• Key naming case

The checks for log messages, arguments, and keys can also be used to analyze custom functions.

No global logger

Report the use of global loggers. Alternatively, only report the use of the slog.Default() logger.

slog.Info("a user has logged in")
// sloglint: global logger should not be used

# .golangci.yaml
linters:
  settings:
    sloglint:
      no-global: "all" # Or "default".

Context only

Report the use of functions without a context.Context. Alternatively, only report their use if a context exists within the scope of the outermost function.

slog.Info("a user has logged in")
// sloglint: InfoContext should be used instead

# .golangci.yaml
linters:
  settings:
    sloglint:
      context: "all" # Or "scope".

This check partially supports autofix.

Discard handler

Suggest using slog.DiscardHandler when possible.

slog.NewJSONHandler(io.Discard, nil)
// sloglint: use slog.DiscardHandler instead

This check is enabled by default and supports autofix.

Static message

Report dynamic log messages, such as those that are built with fmt.Sprintf.

slog.Info(fmt.Sprintf("a user with id %d has logged in", 42))
// sloglint: message should be a string literal or a constant

# .golangci.yaml
linters:
  settings:
    sloglint:
      static-msg: true

Message style

Report log messages that do not match a particular style. The supported styles are lowercased (the first letter is lowercase) and capitalized (the first letter is uppercase).

slog.Info("A user has logged in")
// sloglint: message should be lowercased

# .golangci.yaml
linters:
  settings:
    sloglint:
      msg-style: "lowercased" # Or "capitalized".

No mixed arguments

Report the use of both key-value pairs and attributes within a single function call.

slog.Info("a user has logged in", "user_id", 42, slog.String("ip_address", "192.0.2.0"))
// sloglint: key-value pairs and attributes should not be mixed

This check is enabled by default.

Key-value pairs only

Report any use of attributes as function call arguments.

slog.Info("a user has logged in", slog.Int("user_id", 42))
// sloglint: attributes should not be used

# .golangci.yaml
linters:
  settings:
    sloglint:
      kv-only: true

Attributes only

Report any use of key-value pairs as function call arguments.

slog.Info("a user has logged in", "user_id", 42)
// sloglint: key-value pairs should not be used

# .golangci.yaml
linters:
  settings:
    sloglint:
      attr-only: true

Arguments on separate lines

Report two or more arguments on the same line. A key-value pair is considered a single argument.

slog.Info("a user has logged in", "user_id", 42, "ip_address", "192.0.2.0")
// sloglint: arguments should be put on separate lines

# .golangci.yaml
linters:
  settings:
    sloglint:
      args-on-sep-lines: true

Constant keys

Report the use of string literals as log keys.

slog.Info("a user has logged in", "user_id", 42)
// sloglint: the "user_id" key should be a constant

# .golangci.yaml
linters:
  settings:
    sloglint:
      no-raw-keys: true

Allowed keys

Report the use of log keys that are not explicitly allowed.

slog.Info("a user has logged in", "id", 42)
// sloglint: the "id" key is not allowed and should not be used

# .golangci.yaml
linters:
  settings:
    sloglint:
      allowed-keys:
        - user_id

Forbidden keys

Report the use of forbidden log keys. When using the standard slog.JSONHandler or slog.TextHandler, you may want to forbid the time, level, msg, and source keys, as these will be written by the handler.

slog.Info("a user has logged in", "time", time.Now())
// sloglint: the "time" key is forbidden and should not be used

# .golangci.yaml
linters:
  settings:
    sloglint:
      forbidden-keys:
        - time
        - level
        - msg
        - source

Key naming case

Report log keys that do not match a particular naming case. The supported cases are snake_case, kebab-case, camelCase, and PascalCase.

slog.Info("a user has logged in", "user-id", 42)
// sloglint: keys should be written in snake_case

# .golangci.yaml
linters:
  settings:
    sloglint:
      key-naming-case: "snake" # Or "kebab", "camel", "pascal".

This check supports autofix.

Custom function analysis

Analyze custom functions in addition to the standard log/slog functions.

The following function properties must be specified:

1. The full name of the function, including the package, e.g. log/slog.Info. If the function is a method, the receiver type must be wrapped in parentheses, e.g. (*log/slog.Logger).Info.
2. The position of the msg string argument in the function signature, starting from 0. If there is no message in the function, a negative value must be passed.
3. The position of the args ...any argument in the function signature, starting from 0. If there are no arguments in the function, a negative value must be passed.

Here's an example for the exp/slog package, the predecessor of log/slog.

# .golangci.yaml
linters:
  settings:
    sloglint:
      custom-funcs:
        - name: "(*golang.org/x/exp/slog.Logger).InfoContext"
          msg-pos: 1
          args-pos: 2