Which of the following is not a valid Terraform string function?

`replace` is a valid string function that substitutes parts of a string.

`format` is a valid string function for producing formatted strings using verbs.

`join` is a valid collection function that joins elements of a list into a single string.

Terraform includes built-in functions for manipulating various data types. While `replace`, `format`, and `join` are valid string functions used for string manipulation, there is no direct `tostring` function available in Terraform for explicit type conversion.