Deprecate a Go Function and Let go fix Migrate Every Caller With //go:fix inline

Go 1.26 introduced a new feature called go fix, which automates the process of updating deprecated API calls to their new counterparts. When a library author marks a function as deprecated, they can use a //go:fix inline directive to instruct go fix to rewrite all call sites to the new API. The directive consists of a comment on the old function, allowing the inliner to automatically update the code.

The inliner takes the body of the deprecated function and inlines it into each call site, replacing the parameters with the corresponding arguments. In the example provided, the money package in example.com/shop deprecated the FormatCents function and replaced it with a new Format function that accepts a Locale parameter. The inliner then updated all calls to FormatCents to use Format with the DefaultLocale.

This feature simplifies the process of migrating code to new APIs, reducing the burden on downstream users who would otherwise have to manually update each call site.