New rule - forbid key types - Issue #694

[AvnerCohen]

Summary

This PR introduces a new rule, key-types, to address Issue #694. The rule allows users to forbid specific YAML type names (such as null, bool, int, float, str) from being used as keys in mappings. This helps prevent subtle bugs and cross-language compatibility issues caused by these ambiguous or error-prone key names.

Motivation

Certain YAML key names (like null, true, false, int, etc.) are valid per the YAML spec, but can cause problems in many programming languages or tools that parse YAML. This rule gives users a way to lint for and block these problematic keys, as discussed in Issue #694.

Rule Configuration

The new rule is called key-types and supports the following options (all default to false):

rules:
  key-types:
    forbid-null: true|false   # tag:yaml.org,2002:null
    forbid-bool: true|false   # tag:yaml.org,2002:bool
    forbid-int: true|false    # tag:yaml.org,2002:int
    forbid-float: true|false  # tag:yaml.org,2002:float
    forbid-str: true|false    # tag:yaml.org,2002:str

Example usage:

rules:
  key-types:
    forbid-null: true
    forbid-bool: true
    forbid-int: false
    forbid-float: false
    forbid-str: false

Implementation Notes

The rule is implemented as a single module (yamllint/rules/key_types.py) following the repo’s conventions.
The rule is registered in the rules index and added to the default config (disabled by default).
Documentation is updated in docs/rules.rst with usage and examples.
Comprehensive tests are included in tests/rules/test_key_types.py.

[coveralls]

coverage: 99.818% (+0.003%) from 99.815%
when pulling 2bc3249 on AvnerCohen:forbid_key_types
into 2698ca4 on adrienverge:master.