Matching captured metavariables with specific types

The metavariable-type operator is used to compare metavariables against their types. It utilizes the type key to specify the string representation of the type expression in the target language. For example, you can use String for Java's String type and string for Go's string type. Optionally, the language key can be used to manually indicate the target language of the type expression.

metavariable-type provides several advantages over typed metavariables. Firstly, it removes the requirement for users to memorize special syntax for defining typed metavariables in various target languages. Moreover, metavariable-type enables users to extract type expressions from the pattern expression and include them in other conditional filters for metavariables. This improves the readability of rules and promotes better organization of the code.

For instance, the following rule that identifies potentially unsafe usage of the referential equality operator when comparing String objects in Java:

rules:
  - id: no-string-eqeq
    severity: WARNING
    message: Avoid using the referential equality operator when comparing String objects
    languages:
      - java
    patterns:
      - pattern-not: null == (String $Y)
      - pattern: $X == (String $Y)

can be modified to the following rule:

rules:
  - id: no-string-eqeq
    severity: WARNING
    message: Avoid using the referential equality operator when comparing String objects
    languages:
      - java
    patterns:
      - pattern-not: null == $Y
      - pattern: $X == $Y
      - metavariable-type:
          metavariable: $Y
          type: String